US20100222737A1 - High flow rate infusion unit and heat exchanger - Google Patents

Abstract

A high flow rate infusion unit includes a heating unit with opposing, electrically-operated heating plates that contact a heat exchanger to heat infusate flowing therethrough. Pressure infusers on the infusion unit are operable to provide infusate at a high flow rate. A flat, elongate heat exchanger has a laminar fluid flow path receivable between the heating plates to which heat is conducted by contact with the heating plates. A bubble trap and a valve are disposed within the heat exchanger. The bubble trap collects bubbles and vents air from infusate exiting the fluid flow path, and the valve shuts off the flow of infusate if the volume of air in the bubble trap exceeds a predefined limit.

Description

PRIORITY
• This patent application is a divisional of U.S. patent application Ser. No. 11/789,523, filed Apr. 24, 2007.
CROSS REFERENCE TO RELATED APPLICATIONS
• This patent application contains subject matter related to the following patent applications, all assigned to the assignee of this application:
• U.S. patent application Ser. No. 10/214, 966, filed Aug. 8, 2002, for “Fluid Warming Cassette with a Tensioning Rod”, published as US 2004/0026068 A1 on Feb. 12, 2004, now U.S. Pat. No. 7,232,457;
• U.S. patent application Ser. No. 10/397,942, filed Mar. 25, 2003, for “Fluid Warming Cassette and System Capable of Operation under Negative Pressure”, published as US 2004/0190885 A1 on Sep. 30, 2004, now U.S. Pat. No. 7,394,976;
• U.S. patent application Ser. No. 10/822,580, filed Apr. 12, 2004, for “Intravenous Fluid Warming Cassette with Rails and a Stiffening Member”, now U.S. Pat. No. 7,316,666;
• U.S. patent application Ser. No. 11/789,515, filed Apr. 24, 2007, for “Heat Exchanger for High Flow Rate Infusion”, published as US 2008/0267599 on Oct. 30, 2008;
• U.S. patent application Ser. No. 11/789,752, filed Apr. 24, 2007, for “Bubble Trap for High Flow Rate Infusion”, published as US 2008/0269679 on Oct. 30, 2008;
• U.S. patent application Ser. No. 12/148,719, filed Apr. 22, 2008, for “Bubble Trap for High Flow Rate Infusion”, published as US 2008/0269676 on Oct. 30, 2008;
• PCT application PCT/US2008/005198, filed Apr. 23, 2008, for “High Flow Rate Infusion Unit with Heat Exchanger”, published as WO 2008/130715 on Oct. 30, 2008; and,
• U.S. patent application Ser. No. 12/661,113 filed Mar. 11, 2010, for “Heat Exchanger for High Flow Rate Infusion”.
• The assignee of this application now owns the following issued U.S. patents containing subject matter related to the subject matter of this application: 5,807,332; 6,464,666; 6,535,689; 6,775,473; and 7,010,221.
• The assignee of this application now ownsEuropean Patent 1 159 019, granted Nov. 6, 2002 for “IV Fluid Warming System with Detection of Presence and Alignment of Cassette”, which has been validated in Germany, France, Great Britain, Ireland, and Monaco.
• See PCT application PCT/US20000/02630, filed Feb. 2, 2000 for “Pressure Tolerant Parenteral Fluid and Blood Container for a Warming Cassette”, publication WO 01/26719, 4/19/2001, filed by the assignee of this application.
BACKGROUND
• The subject matter relates to a high flow rate infusion unit that pressurizes and warm fluids for infusion into a body at pressures equal to or exceeding gravity. The subject matter also relates to a heat exchanger for a high flow rate infusion unit in which fluids are warmed by the infusion unit.
• Infusion relates to the introduction of a fluid into a body, usually, although not necessarily, into vasculature. A fluid that is infused into a body may be termed an “infusate”. Such fluids may include, for example, blood, blood products, and solutions such as saline, antibiotics, and medications.
• The combination of low operating room temperatures and the administration of anesthetics which inhibit a patient's thermoregulatory function leads to hypothermia during surgery. As is known, perioperative hypothermia can produce adverse outcomes such as surgical wound infection, extended hospitalization, and blood loss. See Sessler DI: Complications and Treatment of Mild Hypothermia. ANESTHESEOLOGY 2001; 95:531-543. Prevention or mitigation of hypothermia, particularly perioperative hypothermia, is thus a key clinical factor for successful treatment outcomes.
• Hypothermia may be accelerated by infusion of fluid, especially if the fluid is refrigerated. For example, Sessler indicates that a unit of refrigerated blood or a liter of crystalloid solution at room temperature decreases the mean body temperature of adults by approximately 0.25° C. But patients suffering from serious trauma may require rapid infusion of large amounts of fluid, which can cause a sharp and sudden loss of heat in the body core, leading to a drop in mean core body temperature. In order to prevent or mitigate infusion-caused heat loss in a trauma patient, the infusate is often heated as it is administered.
• Warming fluid prior to infusion into a human or animal body is known. See, for example the intravenous fluid warming systems and appliances described in the cross-referenced patent documents. See also the Ranger® blood/fluid warming system and products described at www.arizant.com, the web site of Arizant Healthcare Inc. The Ranger® blood/fluid warming system includes a heating appliance and a heat exchanger capable of being inserted into the heating appliance. Fluid flowing though the heat exchanger is warmed by contact between the heating appliance and heat exchanger, and then delivered intravenously to a patient. However, the disclosed systems cannot meet all rates of infusate delivery needed for treatment of trauma patients.
• The technical challenges in heating a high volume of infusate delivered at a relatively high rate, for example, at 30 liters per hour (30 L/hr), or higher, include uniform transfer of heat to the fast-flowing fluid, elimination of air from the fluid, and an infusion system construction that supports convenience and speed of operation.
• Solutions to these challenges in the prior state of the art include a known high speed infusion system that warms infusate by immersion of a heat exchanger in a warm water bath. A column of infusate flows through the heat exchanger, and the warm water bath heats the infusate as it passes through the heat exchanger. The heat, the flow pattern and high flow rate of the infusate create bubbles in the infusate, which must be removed before intravenous delivery in order to avoid formation of an air embolism in the patient being infused. This high speed infusion system includes a gas elimination device to collect bubbles from the infusate, and a clamp to halt the flow of infusate if air is detected in the infusate.
• The known high speed infusion system is constituted of an appliance with a water heating and circulation system. The heat exchanger consists of a pair of coaxial tubes, a smaller one disposed inside a larger one. The infusate flows through the annulus between the larger and smaller tube, and the heated water is circulated from the heater, through the inner tube, and back to the heater. The heat exchanger is installed in the appliance where it must be reliably coupled to an infusate flow path and to a separate hot water flow path. The gas elimination device is separate from the heat exchanger; and it is installed separately and downstream from the heat exchanger. Infusate passes through the gas elimination device into a patient line for intravenous delivery to a patient. When a predetermined amount of air is detected in the gas elimination device, a downstream clamp is activated to pinch off the patient line, thereby stopping the flow of infusate to the patient. The heat exchanger and gas elimination device are discarded after each use, and new ones must be installed each time a patient is treated.
• In this known high speed infusion system, the heat transfer mechanism poses a risk of an exchange of contaminants between the infusate and the water used to deliver heat. This may occur when the barrier between the water and the infusate is breached for some reason. The use of a warm water bath as the heat transfer mechanism requires continuous maintenance to keep the water clean and the pumping system operating with sufficient capacity. Air transported from infusate bags and bubbles generated from the infusate are collected and separated by the gas elimination device, and air is eliminated through a port in the device. At times, a large mass of collected bubbles can block the port, thereby preventing air from being vented. Then, the collected bubbles will cause detection of air that is not quickly vented, and the clamp will be activated. In such a case, the system can be restarted only after clearing or replacing the gas elimination device. In this known system, set up preceding each use requires separate handling and installation of the heat exchanger, the gas elimination device, and the length of patient line that is led through the clamp.
• There is a need for a high flow rate infusion system that effectively transfers heat to a rapidly-flowing infusate without risk of contaminant exchange between the infusate and a heat transfer fluid. Another desirable advance would reliably eliminate air from the infusate without blocking an air vent. System set up would be improved by reduction of the number of devices required to be installed each time the system is used.
SUMMARY
• A high flow rate infusion unit includes a resistive heating unit with opposing heating plates that contact a heat exchanger to conductively heat infusate flowing therethrough. Pressure infusers on the infusion unit provide infusate at a high flow rate.
• A heat exchanger has a laminar fluid flow path receivable between the heating plates to which heat is conducted by contact with the heating plates. A bubble trap and a valve are integrated with the heat exchanger. The bubble trap collects air from the infusate exiting the laminar flow path, and includes an air vent in contact with the infusate that vents the air from the bubble trap. The valve shuts off the flow of infusate if air is detected in the bubble trap.
• A heat exchanger embodiment constituted of a flat, elongate warming cassette with a fluid container defining a laminar fluid flow path is slidable in a heating unit between a seated position where the fluid container is in heat-transferring contact with heating plates and an extracted position outside of the electrical heating unit. The warming cassette includes a housing attached to the fluid container. The housing contains a bubble trap and a valve. The bubble trap is disposed in fluid communication with the laminar flow path to collect air from infusate flowing out of the fluid path. The bubble trap includes an air vent in contact with the infusate that vents air from the bubble trap. The valve, disposed in fluid communication with the bubble trap, has an open state permitting infusate to flow out of the warming cassette and a closed state blocking infusate from flowing out of the warming cassette.
• A bubble trap for collecting air from an infusate includes a flow expansion chamber to collect large bubbles, a recirculation chamber to collect large to medium bubbles, a laminar flow chamber where air is released and detected, and an outlet chamber where the infusate exits the heat exchanger.
• The infusion unit detects the presence and level of air in infusate flowing through the bubble trap and controls the flow of infusate out of the bubble trap in response to detected levels of air. Preferably, the infusion unit controls the state of the valve in response to detected levels of air in order to permit or prevent infusate to flow.
• The unification of a laminar flow path, bubble trap, and shut off valve in an integrated heat exchanger construction yields a single, easily handled appliance that simplifies setup and operation of infusate warming, bubble management, air elimination, and safety shut off for a high flow rate infusion unit.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view of a high flow rate infusion unit and a heat exchanger in an extracted position with respect to the infusion unit.
• FIG. 2 is a perspective view of the high flow rate infusion unit with the heat exchanger in a seated position with respect to the infusion unit.
• FIG. 3 is an enlarged perspective view of the top of the high flow rate infusion unit with two pressure-actuated infusers, in which one pressure infuser is opened to receive an intravenous (IV) bag.
• FIG. 4A is a side sectional view of a pressure infuser with a full IV bag mounted therein against a deflated bladder.FIG. 4B is a side sectional view of the pressure infuser ofFIG. 4A with the IV bag empty and the bladder inflated.
• FIG. 5 is an exploded view of a warming cassette, in perspective.
• FIG. 6 is a plan view of the assembled warming cassette.
• FIG. 7 is a side elevation view of an electrical heating unit of the high flow rate infusion unit with the warming cassette in the seated position with respect to heating plates of the electrical heating unit.
• FIG. 8 is an exploded top view of the electrical heating unit with the warming cassette in the seated position between the heating plates.
• FIG. 9 is a sectional view of the electrical heating unit taken along line9-9 ofFIG. 7 with the warming cassette in the seated position between the heating plates.
• FIG. 10 is an enlarged view of the front face of a housing of the warming cassette ofFIG. 6 showing a bubble trap, sensor couplers, and a valve.
• FIG. 11 is sectional view of the housing ofFIG. 10 taken along line11-11 ofFIG. 10.
• FIG. 12A is a perspective view of the front face of the housing ofFIG. 10, partially disassembled.FIG. 12B is a perspective view of the back face of the housing ofFIG. 10, partially disassembled and shown with respect to a mounting flange in the high flow rate infusion unit with sensors and an actuator partially disassembled therefrom.
• FIG. 13 is an enlarged sectional view showing details of an air vent.
• FIG. 14A is an enlarged plan view of a sensor coupler piece.FIG. 14 B is a longitudinal cross section of the sensor coupler piece.
• FIG. 15 is a partial side cross sectional view of an upper portion of the housing ofFIG. 10 showing engagement between a sensor and a sensor coupler.
• FIG. 16 is an enlarged perspective view of a bubble trap hydrophilic screen.
• FIG. 17 is an enlarged side sectional view of a valve in fluid engagement with the bubble trap in the housing ofFIG. 10.
• FIG. 18 is an enlarged perspective view of the side of the high flow rate infusion unit partially cut away to illustrate construction details.
• FIG. 19 is an enlarged perspective view of the front of the high flow rate infusion unit with the warming cassette partially inserted into the high flow rate infusion unit.
• FIG. 20 is an enlarged perspective view, partially cut away, of the front of the high flow rate infusion unit with the warming cassette seated in a heating unit thereof.
• FIG. 21 is an enlarged perspective view of the side of the high flow rate infusion unit with the warming cassette seated therein and with the warming cassette and infusion unit partially cut away to illustrate construction details.
• FIG. 22 is a block diagram representing an electronic control subsystem for the infusion unit ofFIGS. 1 and 2.
• FIG. 23 is a schematic diagram representing a pneumatic subsystem for the infusion unit ofFIGS. 1 and 2.
• FIG. 24 is a flow diagram illustrating operation of the high flow rate infusion unit in conjunction with the heat exchanger.
• FIG. 25 is a schematic diagram of a sensor in operational engagement with the bubble seen inFIG. 10.
• FIG. 26 is a schematic diagram of the bubble trap seen inFIG. 10 with a hydrophobic membrane for venting air.
• FIG. 27 is a flow diagram illustrating a logic control mechanization to control flow through the bubble trap illustrated inFIG. 26.
• FIG. 28 is a schematic diagram of a second bubble trap embodiment.
DETAILED DESCRIPTION
• In this detailed description, a high flow rate infusion unit and a heat exchanger including a laminar flow path are described. A high flow rate is a flow of infusate through a patient line at a rate that is sufficient to administer a large amount of infusate quickly to a person. For example, a high flow rate system may administer blood to a trauma patient at a rate of 30 liters per hour (30 L/hr), or higher, measured through a line connected intravenously to the patient (a “patient line”). A “laminar flow path” is a thin, relatively flat, non-sinuous space through which a sheet of infusate can flow from an inlet port to an outlet port.
• The novel designs and embodiments to be described provide a number of benefits with respect to previously known high flow rate infusion systems. Infusate is heated by direct contact between the heat generating mechanism and a heat exchanger, thereby eliminating an intermediary medium (such as water) to transport heat from a heater to the heat exchanger. This mode of heat transfer may be referred to as “dry heat” because it does not use water, or another fluid. An exemplary heat exchanger construction unifies a unidirectional, laminar flow path where infusate is heated, a bubble trap that continually vents air from the infusate, and a valve to regulate infusate flow. Bubbles are separated and collected from infusate and air is eliminated through a vent in contact with the infusate, which reduces shut downs caused by build up of bubbles, thereby ensuring uninterrupted infusate flow for longer periods of time than the previously known high flow rate infusion systems. The unified construction of the heat exchanger yields a single, easily handled appliance that simplifies setup and operation of heat exchange, air elimination, bubble entrapment, and safety shut off for a high flow rate infusion unit.
High Flow Rate Infusion System
• Refer now toFIGS. 1 and 2 which illustrate a high flow rate infusion system including a high flowrate infusion unit10 and aheat exchanger12. Theinfusion unit10 has a kiosk or tower construction with a casing including anupper section16 with dual, pressure-actuatedinfusers18, aneck20 extending from theupper section16, and apedestal22 supporting theneck20. Preferably, awheeled support base24 allows theinfusion unit10 to be easily moved or repositioned on a floor or other surface. Arack26 is supported above theupper section16 by ashaft28 slidably retained in theupper section16. Bags of infusate may be hung on therack26 as shown. The longitudinal axis of theinfusion unit10 is generally perpendicular to the surface on which it is supported. Electronics for operating theinfusion unit10 are contained in theneck20. Aheating unit27 constituted of resistively-heated plates is contained within thepedestal22. Sensors, actuators, and a pneumatic system for delivering pressurized air are distributed as needed between theneck20 and thepedestal22. Thepedestal22 has a recessedsurface portion30 where abezel32 is mounted. Thebezel32 has an elongate opening orslot34. As seen inFIG. 1, a mountingblock36 in the recessedsurface portion30 is disposed along one side of, and perpendicularly to, thebezel32.Sensors37 and38 are mounted to and extend through the body of the mountingblock36 to amajor surface42 thereof. A valve actuator40 (best seen inFIG. 12B) mounted to a rear surface of the mountingblock36 includes apiston41 that operates through themajor surface42.
• The construction of theheat exchanger12 includes a laminar flow path through which a broad sheet of infusate flows. In use, when theheat exchanger12 is installed in theinfusion unit10, the laminar flow path of the heat exchanger is sandwiched between a pair of electrically-operated heating plates, such that each side of the laminar flow path is in close heat-conducting contact with a respective one of the pair of heating plates. When the heating plates are operated, heat exchanged between the plates and the laminar flow path warms the infusate as it moves through the laminar flow path. With reference toFIGS. 1 and 2, an exemplary construction of theheat exchanger12 is illustrated. Preferably, theheat exchanger12 may be constructed as an elongate, quadrilateral, generally flat orlaminar warming cassette60. Thecassette60 includes adistal end61, afluid container62, and ahousing64 with ahand grip65. Thefluid container62 defines alaminar flow path67 of the warmingcassette60. Thecassette60 includes aninput port69 and anoutput port71, each in fluid communication with thelaminar flow path67. When viewed end on looking toward thedistal end61, the cassette has a thin, but relatively elongate aspect so as to be slidably inserted into theslot34 in thebezel32 with the fluid container sandwiched between and in heat-conducting contact with the heating plates, and slidably extracted therefrom.
• InFIG. 1, thecassette60 is shown extracted from theinfusion unit10; inFIG. 2, thecassette60 has been inserted in theinfusion unit10,distal end61 first, through theslot34 into theelectrical heating unit27, where the fluid container is disposed between and in contact with the heating plates. Preferably, when thecassette60 is inserted into theslot34, the longitudinal axis of theinfusion unit10 and a major axis of thecassette60 are generally aligned and parallel. Thus, when the cassette is received in theslot34, it is oriented to be disposed substantially vertically with respect to a surface supporting theinfusion unit10. Thecassette60 is removed from theinfusion unit10 by grasping thehand grip65 and pulling the cassette upwardly, out of theslot34. In most aspects, after infusion of a patient, a usedcassette60 is extracted from theinfusion unit10 and processed for medically safe disposal. A new,unused cassette60 is inserted into theinfusion unit10 prior to commencing infusion of another patient.
• With further reference toFIGS. 1 and 2, theinfusion unit10 is prepared for operation by placing a bag containing infusate into either or bothpressure infusers18, inserting thecassette60 into theslot34, and connecting the bag or bags to thecassette60 by IV tubing. An IV tube set such as the Y tube set73 is connected to each bag and to theinput port69 of thecassette60. The Y tube set73 is conventional and includes manually-operated means74 in each branch of the Y connected to a bag to pinch off the branch when the bag connected to it is not used. AnIV tube75 is connected to theoutput port71 of thecassette60 and is connected by known intravenous means to a patient. TheIV tube75 constitutes the “patient line” through which a flow of warmed infusate is delivered intravenously to a patient at a rate that is sufficient to administer a large amount of infusate quickly to the patient. For example, the rate may be 30 L/hr, or higher. Theinfusion unit10 is then activated by means of controls operated by a user usingcontrol panel77. ON/OFF control is afforded by way ofcontrol panel79.
• With further reference toFIGS. 1 and 2, when activation of the high flowrate infusion unit10 occurs, electrical power is applied to resistively heat the heating plates and pressurized air is introduced into an inflatable bladder in apressure infuser18. As thebladder103 inflates, it presses against the fluid-filled bag in thepressure infuser18, which forces the fluid into the IV tubing set73. The pressure against the bag is transferred to the fluid, forcing it to flow to and through thecassette60 at a rate higher than that which would result if it were flowing in response to gravity only. The infusate flows into thecassette60 through theinput port69 and therethrough into thelaminar flow path67 near thedistal end61. The infusate fans out into a thin laminar sheet and flows through thelaminar flow path67, expanding thefluid container62 so that it contacts and presses against the heating plates. The infusate continuously absorbs heat from the heating plates as it flows. As the infusate approaches thehousing64, the shape of the laminarfluid flow path67 concentrates the warmed infusate into a narrow, high speed stream that flows into thehousing64, through abubble trap80 where bubbles are separated and collected from the stream of infusate, and where air is vented through anair vent81. Passing through thebubble trap80, the narrow, high speed stream of warmed infusate flows through avalve82, out theoutput port71, into thepatient line75, through which it is administered intravenously to a patient.
• With reference toFIGS. 3,4A, and4B, eachpressure infuser18 is constructed to receive a full bag of infusate and to expel the infusate from the bag at a high rate of flow. Eachpressure infuser18 has a body constituted of arear shell93 and aninner shell95 fixed to therear shell93 and supported thereagainst byspacers97. In the space between theshells93 and95, a pneumatically controlledvalve99 and an electronically controlled, three waypneumatic valve100 are mounted to the rear surface of theinner shell95. Aport101 extending through theinner shell95 connects thevalve99 to aninflatable bladder103 supported on the front surface of theinner shell95. Eachpressure infuser18 has adoor105 that swings on ahinge107 mounted to the body of thepressure infuser18. Eachdoor105 is held shut by an elongatereleasable latch109 mounted to the body of apressure infuser18. A pair ofspring retainers111 is mounted to the body of eachpressure infuser18 so as to extend into the space between a door and an inner shell. The springs support bags when the doors open and aid door opening.
• With further reference toFIGS. 3 and 4A, thedoor105 of apressure infuser18 is opened, and a full bag B of infusate with a lower port P is placed in thepressure infuser18, such that the port P extends downwardly through a gap between thedoor105 and the body of thepressure infuser18. The bag B is retained against the front surface of theinner shell95 by the pair ofspring retainers111 and by closing and latching thedoor105. Preferably, the bag B has a construction that is conventional for IV bags, although the design may be customized to accommodate other design requirements. For a conventional construction, the bag B is connected to one line of the Y tube set73 by a spike on the end of the line that penetrates the bottom of the bag B through the port P. With reference toFIGS. 4A and 4B, infusate is pressurized and forced from the bag B, through the port P, when thebladder103 is inflated by pressurized air provided by the two-way valve99 through theport101. Pressurized infusate flows out of the bag B through the port P into theline73a, and therethrough to theheat exchanger12. When the bag B is emptied, the setting of thevalve99 is reversed, and thebladder103 is deflated by venting air from the bladder through theport101. The empty bag B may then be removed from thepressure infuser18 and replaced by another full bag.
• The flow rate of the infusion system just described is established by, among other parameters, the viscosity of the infusate, the pressure capacity of the pressure infusers18, and the resistance to fluid flow. Infusate viscosity varies according to the nature of the fluid being infused. The rate of inflation of thebladders103 and the relative sizes of thebladders103 and infusate bags are the principal determinants of pressure capacity. The broad laminar flow path in thefluid container62 reduces flow resistance, compared to previous heat exchanger designs based on a flat cassette, by elimination of curves, bends, and abrupt changes in flow direction. Tubing can be selected to provide a range of flow resistance appropriate to the other factors and the desired flow rate. Preferably, the high flow rate infusion system ofFIGS. 1 and 2 administers blood to a trauma patient at a rate of 30 liters per hour (30 L/hr), or higher, when the pressure infusers18 are operated to pump infusate by inflation of thebladders103. Of course, the infusate bags may be connected to aheat exchanger12 installed in theinfusion unit10 for flow of infusate through the infusion system at a lower pressure. In fact, infusate will flow without activating the pumping operation of the pressure infusers18 at all, in which case, infusate will flow through the system by gravity. Thus, the infusion system ofFIGS. 1 and 2 can provide warmed infusate at flow rates in the range of from 0 to at least 30 L/hr; in some instances, the infusion system can provide warmed infusate at a maximum flow rate exceeding 70 L/hr.
Heat Exchanger
• Infusate expelled from a pressure-activated infusate bag travels at a high flow rate through tubing connecting the bag to the heat exchanger in which it is warmed for administration to a patient. The heat exchanger is exemplified by a warming cassette construction adapted for use in theinfusion unit10 ofFIG. 1. In this description of the warming cassette, the term “heat exchanger” is used to denote the warming cassette, even though heat exchange occurs through the fluid container and is only one function of the warming cassette. The warmingcassette60 has an integrated construction that unites a heat exchanger in the form of thefluid container62, with a bubble trap and shut-off valve disposed in thehousing64. This construction enables the heat exchanger, bubble trap, and shut-off valve to be installed in and removed from theinfusion unit10 in a single step. With reference toFIGS. 5 and 6, thefluid container62 is a thin, quadrilaterally-shaped, fluid-tight pouch150 formed by joining coextensive sheets of flexible plastic material together by a pattern of fluid-resistant seals151 around the periphery of thepouch150. Two semi-rigidplastic rails153 and155 are positioned between the coextensive sheets and between elements of theseals151 just inside of and parallel to theelongate edges157 of thepouch150. Therails153 and155 are sealed to the sheets of flexible plastic material by fluid-resistant seals152, near the ends of the rails. Thelaminar flow path67 is positioned between therails153 and155 and has aninlet160 and anoutlet161. Therail153 has a straw like construction with acentral passageway162 that opens through oneend163 of therail153 and extends to agroove164 terminated in a shortlongitudinal slot165 near the opposing end. Theslot165 opens through the side surface of the rail into theinlet160 to laminarfluid flow path67. Therail155 has a shortcentral passageway162 that opens into theoutlet161 through a shortlongitudinal slot166 and runs from there to and through oneend167 of therail155. Preferably, thehousing64 is formed by molding plastic to yield two rigid complementarily-shaped pieces that are joined together over ends163 and167 of therails153 and155 and the nearshort edge168 of thepouch150. Together, thehousing64 and therails153,155 form a generally quadrilateral frame on which thepouch150 is supported.
• Many materials and processes may be used to construct the warmingcassette60. For example, with reference toFIGS. 5 and 6, we assemble thefluid container62 from sheets of laminated material which include a layer of polyethylene material on a layer of polyester; we use rails made of molded polyethylene; and we assemble thehousing64 using pieces made of a molded acrylic, polycarbonate, or blended medical grade plastic such as Cyrolite®. The sheets are oriented with the polyethylene layers facing and the rails are disposed between the polyethylene layers in the orientations seen inFIGS. 5 and 6. Theseals151 and152 in this case may be formed by heat applied through the polyester layers. Because of difficulty in sealing the polyethylene rails to the polycarbonate housing, we usecompliant sleeves169 and170 made of polyvinyl chloride (PVC) to attach theends163 and167 of therails153 and155 to the housing. In this regard, thesleeves169 and170 are contained within thehousing64 and their outside surfaces are sealed with solvent to complementary structures in the housing. Barbs formed on theends163 and167 of therails153 and155 mechanically seat against the interior surfaces of thesleeves169 and170, attaching therails153 and155 to thehousing64 in the positions shown inFIG. 6.
• Infusate flow through the warmingcassette60 is shown inFIG. 6. Theend163 acts as theinput port69 of the warmingcassette60. Infusate enters the warming cassette through a tube (not shown) in fluid communication with theend163, flows through thecentral passageway162 in therail153 and exits therail153 through theslot165. Infusate flows through theinlet160 wherefrom it fans out into a broad thin sheet that extends across thelaminar flow path67 that flows toward thehousing64. As the sheet of infusate approaches thehousing64, it is funneled toward theoutlet161 by thecurve172 formed by the contour of theseal151. The infusate flows out of thelaminar flow path67 through theoutlet161 into the short passageway of therail155 via theslot166. The infusate flows out of the short passageway of therail155 through theend167 and into thebubble trap80 in thehousing64 of the warmingcassette60. The infusate flows through thebubble trap80 and thevalve82, to and out of theoutput port71.
Heating Unit
• With reference now toFIGS. 7,8,20, and21, the warmingcassette60 is shown inserted into aheating unit180 supported in theinfusion unit10. Theheating unit180 includes twoopposed heating plates182 and184 that define a narrow laminar space within which thefluid container62 is seated. Preferably, theheating plates182 and184 are formed of low thermal resistance aluminum anodized with a hard coat. Theheating plates182 and184 conduct heat generated by a pair of resistance heaters on the outside surfaces of the heating plates. Onesuch heater183 is best seen inFIG. 20. Theresistance heaters183 may comprise, for example, laminated silicone resistance heaters, or equivalents thereof. Theheating plates182 and184, with theheaters183 mounted to the outside surfaces thereof, are conventionally mounted in thepedestal22 of theinfusion unit10. As seen inFIGS. 7-9, theheating plate182 has elongateparallel grooves186 near its lateral edges which face opposing elongateparallel grooves188 in theheating plate184. As best seen inFIGS. 8 and 9, the facinggrooves186,188 form elongate parallel channels that accommodate therails153 and155 and guide the warmingcassette60 to and from correct seating as it slides between theheating plates182 and184. As seen inFIGS. 8 and 9, the warmingcassette60 has a thin, but relatively elongate aspect (when viewed distal end on) so as to be slidably inserted through theslot34 in thebezel32 with thefluid container62 sandwiched between and in heat-conducting contact with theheating plates182 and184, and slidably extracted therefrom. As seen inFIG. 9, thelaminar flow path67 is sandwiched between and in close abutting contact with theheating plates182 and184 when the warmingcassette60 is installed in the infusingunit10, thereby eliminating the need for an intermediary medium to transport heat to the warmingcassette60.
• InFIG. 7, a pair of opposing shallowtransverse channels190 formed in the surfaces of the heating plates that face thefluid container62 run from an edge of the heating plate surface. Corresponding ends of the channels are near the location of theslot165 in therail153 when thecassette60 is seated in theelectrical heating unit180. The pressure of infusate flowing out of theslot165 forces opposing strips of thefluid container62 into conformance with thechannels190, thereby forming an input manifold through which infusate can spread into thelaminar flow path67. Similarly, a pair of opposing shallowtransverse channels192 formed in the surfaces of the heating plates that face thefluid container62 cause the formation of an output manifold in thefluid container62 that channels infusate out of thelaminar flow path67 into theslot166 in therail155.
• FIG. 7 also shows monitoring and extraction elements of theheating plates182 and184. A pair of opposing throughholes193 and194 are formed in theheating plates182 and184 for positioning heat sensors (not seen) in the pair of opposing shallowtransverse channels192 that cause the formation of an input manifold in thefluid container62. At these opposing locations, the temperature of infusate flowing out of the warming cassette may be measured. Two pairs of opposing throughholes195 are formed in theheating plates182 and184 for channeling jets of pressurized air against the sides of the fluid cassette in order to dislodge the warming cassette from theheating plates182 and184. In this regard, when the flow of infusate ceases, a sheet of infusate fills thefluid container62, pressing the sides of the fluid container against the opposing surfaces of theheating plates182 and184. Surface tension and friction between thefluid container62 andheating plates182 and184 can make it difficult to dislodge warmingcassette60. Jets of pressurized air through theholes195 force infusate out of thefluid container62, thereby breaking the surface tension and reducing the friction, making it easier to extract the warmingcassette60.
• With reference toFIGS. 6 and 9, important benefits of the warming cassette construction will be appreciated. The dry heat mode of warming infusate shown in these figures eliminates the need for a fluid such as water to transport heat to the infusate. At the same time, the broad, unidirectional laminar flow through the heat exchanger that is constrained between the heating plates minimizes flow path resistance by eliminating successive curves and reverses in the direction of flow. Presuming a maximum width of the laminar flow path that is dictated by design constraints, it is, of course, possible to reduce flow resistance further by increasing the spacing between the heating plates, but this also reduces the rate of heat transfer from the heating plates to the infusate. Thus, there are optimal balances between fluid flow and heat transfer that can be achieved for various applications of the warming cassette construction illustrated and described above.
Bubble Trap and Shut Off Valve
• For the purposes of the following explanation, thehousing64 has a front face, seen inFIGS. 10 and 12A, that is visible to an operator when the warmingcassette60 is installed in theinfusion unit10, and a rear face, seen inFIG. 12B that faces the mountingblock36 when the warmingcassette60 is installed in theinfusion unit10. With reference toFIG. 10, after infusate has been warmed in the heat exchanger, thebubble trap80 separates and collects air and bubbles from the infusate as it streams in a flow path (a “trap flow path”) through thebubble trap80, and vents air through a vent. If a threshold level of air is detected in thebubble trap80, thevalve82 closes, thereby stopping the flow of warmed infusate to the patient line. Preferably, thebubble trap80 andvalve82 are integral parts of thehousing64. That is to say, the molding process with which the components of thehousing64 is made forms the structural components of thebubble trap80 andvalve82 in the housing components, so that the components of thebubble trap80 are assembled and contained within thehousing64 when the molded halves of the housing are joined. This construction is preferred, but should not be limiting. For example, a bubble trap can be constructed separately and placed within thehousing64 as the housing is assembled.
• Thebubble trap80 includes a trap flow path designed for high flow rates, that is, flow rates of 30 L/hr, and higher. Preferably, the bubble trap operates with fluid flow rates in the range from 0 to 70 L/hr, or higher, through the trap flow path. The trap flow path is constructed to separate bubbles from the infusate in a succession of stages. The stages include, in sequence, a flow velocity reduction chamber (“reduction chamber”), a separation chamber, a laminar flow chamber, and an outlet chamber.
• With reference toFIG. 10, thereduction chamber200 is in fluid communication with the central passageway of therail155, so that infusate flows out of theend167 of therail155 into thereduction chamber200. Thereduction chamber200 has a hook-shaped cross section that increases in width from theend202 to the top portion of the hook. When the warming cassette is oriented vertically in theinfusion unit10 as shown inFIG. 2, theend202 is in the bottom of thebubble trap80. In this case, the top portion of the hook bends downwardly at203 to theseparation chamber205. In some aspects, abaffle206 may be provided to channel infusate flow into theseparation chamber205. With reference toFIGS. 10 and 11, the trap flow path narrows substantially in the transition from theseparation chamber205 to thelaminar flow chamber207. As best seen inFIG. 11, thelaminar flow chamber207 has a narrow cross section with an outer side64oin the front face of thecasing64 and an opposinginner side64iin the rear face of thecasing64. Referring toFIGS. 10,13, and16, a disc-shapedhydrophobic membrane209 is welded to the inside surface of the outer side64o, spaced apart fromapertures210 through the outer side64o. When infusate flows through the bubble trap, thehydrophobic membrane209 is continually in contact with the infusate as it flows through thelaminar flow chamber207. Advantageously, the accelerated flow of infusate through the laminar flow chamber keeps bubbles from sticking to the surface of, and clogging, thehydrophobic membrane209. As best seen inFIGS. 10 and 12B, first andsecond sensor couplers37A and38A are supported on the outside surface of theinner side64i. Presume that the warmingcassette60 is oriented vertically in theinfusion unit10 as shown inFIG. 2. In this case, as shown inFIG. 10, thehydrophobic membrane209 is positioned above and upstream of bothsensor couplers37A and38A, and thefirst sensor coupler37A is positioned above thesecond sensor coupler38A. As seen inFIGS. 10 and 11, the trap flow path transitions at214 to theoutlet chamber215. As best seen inFIG. 10, infusate flows out of theoutlet chamber215 through a thimble-shapedhydrophilic screen217 into ashort riser219 by which it enters one side of acircular valve chamber220 that is in fluid communication with theoutput port71. Preferably, theoutlet chamber215 is widened with respect to the laminar flow path in order to reduce flow velocity of the infusate through thehydrophilic screen217 so that bubbles will not be pushed through the screen and can rise up off to the air pocket forming in the top portions of thebubble trap80.
• In some instances, thehousing64 may be transparent in order to enable an operator to see and judge bubble trap operation through theseparation chamber205; in these instances, as best seen inFIG. 11, thehousing64 may bulge outwardly at208 thereby to enable the operator to clearly see the infusate level in thebubble trap80. For example, the operator may view the cascade of infusate flowing over thebend203 to visually ascertain infusate flow and judge the flow rate.
• With reference toFIG. 10, infusate flows into thebubble trap80 from theend202 of thereduction chamber200. Presume that the warming cassette is oriented vertically in theinfusion unit10 as shown inFIG. 2. In this case, as thebubble trap80 is primed with infusate through theend202, the infusate wells up from the bottom of the bubble trap, thus ensuring that it does not form a free jet as it enters the bubble trap. As infusate flows through thereduction chamber200, the increasing width of the reduction chamber expands and slows the infusate stream. The slowed infusate stream rises in the hook shape of thereduction chamber200 and flows over thebend203, cascading from the upper portion of thereduction chamber200 into theseparation chamber205. If used, thebaffle206 is positioned to confine the cascading infusate stream downwardly, through a channel along thewall211, into the bottom of theseparation chamber205. The cascade of infusate into theseparation chamber205 enters the widest portion of thebubble trap80, but encounters the sharp reduction in cross section in the transition to thelaminar flow chamber207, which sets up a recirculating flow pattern in theseparation chamber205. The narrow cross section in thelaminar flow chamber207 accelerates the infusate and forces it once more into a sheet that traverses thelaminar flow chamber207 across the hydrophobic membrane and past thesensor couplers37A and38A. The laminar stream of infusate enters theoutlet chamber215, where it is funneled into theshort riser219, which narrows and further accelerates the infusate flow and turns it toward thevalve chamber220 from which the infusate stream flows out of the warming cassette through theoutput port71.
• InFIG. 10, as the infusate flows through the trap flow path, the buoyancies of air boluses and large bubbles in the infusate pull them from the infusate stream as the stream flow slows through thereduction chamber200. These large-diameter bubbles are collected in thereduction chamber200. Thus, for example, bubbles222 having diameters in the range of 1 to 3 mm, and larger, will separate from the stream of infusate and rise to be collected in the hooked upper portion of thereduction chamber200. As the large bubbles rise and collect, they burst, which causes anair pocket223 to form. As the infusate stream turns at thebend203 and cascades into theseparation chamber205, bubbles remaining in the stream are circulated in the eddy of infusate in the separation chamber. This lengthens the dwell time of bubbles in theseparation chamber205, thereby increasing the likelihood that they will rise and burst, adding to the air pocket in thereduction chamber200. Some small (1 mm diameter, for example) bubbles may be entrained into the eddy in theseparation chamber205 from foam at the border between an air pocket and the infusate; these bubbles tend to remain trapped in the eddy without passing to thelaminar flow chamber207. As the infusate stream passes through thelaminar flow chamber207 to theoutlet chamber215, very small bubbles remaining in the infusate are prevented by thehydrophilic screen217 from leaving theoutlet chamber215. These small bubbles stick to the surface of thescreen217, but are not drawn through. Over time, multiple bubbles coalesce on thehydrophilic screen217, forming larger bubbles with enough buoyancy to lift off the hydrophilic screen and rise to the top of thebubble trap80. Air expelled with infusate from a bag may also enter the trap. As air accumulates in the top of thebubble trap80, it is vented from the trap through theair vent81 by thehydrophobic membrane209 and theapertures210. If the level of collected air in the bubble trap reaches thesensor couplers37A and38A thevalve82 is closed and infusate is stopped from flowing out of the warmingcassette60.
• Thehydrophobic membrane209 provides preferential flow of gases over liquids and therefore draws air from thebubble trap80 and releases it to the ambient atmosphere. Thus, thehydrophobic membrane209 serves as a vent through which air is eliminated from thebubble trap80. A representative construction of the hydrophobic membrane is shown inFIG. 13, wherein a 2-3 mil thick hydrophobic membrane constituted of a polymer material, preferably an expanded polytetrafluoroethylene (ePTFE)disc225 having a nominal 0.45 micron pore size has apolyester nonwoven backing227. We have obtained such a hydrophobic membrane from W.L. Gore & Associates. Themembrane209 has a disc-like shape and may be glued, bonded, or welded directly to the inner surface of the outer side64o, with thepolyester backing227 in contact with the inner surface and the hydrophobic material facing thelaminar chamber207. The outer side64oof the laminar flow chamber is outwardly contoured to provide acylindrical ledge229 on its inner surface to position and support themembrane209, and acylindrical vent chamber230 to collect air passed through themembrane209 from thebubble trap80.Intermittent ridges232 in thechamber230 support themembrane209 against the pressure of infusate flowing through thebubble trap80, but do not impede the circulation of air in thevent chamber230. Vent holes210 (best seen inFIGS. 10 and 12A) permit air to pass from thebubble trap80, through the outer side64o, to the ambient atmosphere. With reference toFIGS. 12A and 13, an umbrella-shapedsilicone check valve234 is mounted on the outer surface of the outer side64oby acentral mounting hole236. Theouter rim238 of thecheck valve234 covers theopenings210. When the pressure of the air collected in thevent chamber230 exceeds atmospheric pressure, theouter rim238 yields and collected air passes through theopenings210 to the ambient atmosphere.
• With reference toFIGS. 12B and 15, thesensors37 and38 sense the level of fluid (air and infusate, for example) and enable the detection of air in thebubble trap80 for the purpose of controlling the flow of infusate. In some aspects, thesensors37 and38 may operate ultrasonically. In these instances, accurate sensing requires suppression of an echo reflected from an impedance mismatch such as solid/air transition at the rear face of thehousing64, which faces thesensors37 and38. Thesensor couplers37A and38A mounted to the housing eliminate reflections of transmitted ultrasonic pulses from the rear face of thehousing64. A representative construction of thesensor couplers37A and38A is shown inFIGS. 14A and 14B. Thesensor couplers37A and38A may be formed in apiece240 of a relatively soft, but durable material that has a high transmissivity at ultrasonic wavelengths. Thepiece240 has a flat, planarfront surface242 and arear surface244 on which domes246 may be formed to increase coupling effectiveness. Thedomes246 constitute thesensor couplers37A and38A. Thefront surface242 of thepiece240 is adhered, bonded, or welded to the outside surface of theinner side64i, adjacent thelaminar flow chamber207. Presume the warmingcassette60 is seated in theinfusion unit10 as shown inFIG. 2; then, as seen inFIG. 15, the faces of thesensors37 and38 are in intimate pressing contact with thesensor couplers37A and38A. The material of which the sensor couplers are constructed minimizes or eliminates reflection of transmitted ultrasonic pulses from the outside surface of the rear face of thehousing64 and passes echoes reflected from the inside surface of the front face of thehousing64. It is advantageous to have thesensor couplers37A and38A mounted to thehousing64 because the material of which the apertures are made can be less durable than if mounted to the mountingblock36 or thesensors37 and38. This is because thepiece240 has to undergo only a single use that occurs when the warming cassette is inserted in theinfusion unit10. Thedomes246 formed on thepiece240 allow the material of which it is formed to displace more easily when in response to sensor contact, which makes the material appear even softer than if thesensors37 and38 displaced a flat planar surface. We use asensor coupler piece240 formed of injection-molded thermo-plastic elastomer (TPE) 5.175 mm thick, 30 durometer, shore A.
• A representative construction of thehydrophilic screen217 that filters small bubbles from the infusate path in theoutlet chamber215 of thebubble trap80 is shown inFIG. 16. Thehydrophilic screen217 is constituted of a 263micron nylon mesh250 supported on a moldedplastic support252. We have obtained such a screen from GVS Filter Technology, Rome, Italy. The hydrophilic screen has anopen end254, and an opposite end (not visible inFIG. 16) which may be closed by an element of the moldedplastic support252. Referring again toFIG. 10, theopen end254 of the resulting thimble-like structure is glued, bonded, welded, or fitted to the outlet structure of theoutlet chamber215, in fluid communication with the inlet to theriser219.
• With reference toFIGS. 12B, and17, thevalve82 includes thecircular valve chamber220, avalve membrane260, and aseating ring262. Thevalve membrane260 is disposed over a second side of thecircular valve chamber220 and held thereto by theseating ring262. When thepiston41 is retracted, thevalve82 is open; to close thevalve82, theactuator40 is activated, which throws thepiston41 against thevalve membrane260, forcing the membrane against theopen end264 of theriser219. This prevents infusate from flowing into thecircular valve chamber220 and out of theoutput port71. Preferably, thevalve membrane260 may be formed of silicone or any other durable, flexible material that is compatible with blood. We have obtained such a silicone valve membrane from Liquid Molding Systems, Midland, Mich. Alternately, thevalve82 could be constituted of a rigid, electromechanically-actuated valve, such as a quarter-turn stopcock.
Heat Exchanger Installation and Retention
• Use and operation of the high flow rate infusion unit are simplified by an interlocking mechanical interface between the infusion unit and heat exchanger that enables an operator to quickly and easily install the heat exchanger, bubble trap, and shut off valve in a single act. By sliding the heat exchanger into position between the heating plates, the operator positions the laminar flow path for heat exchange, locates the bubble trap for monitoring by thesensors37 and38, and orients the shut off valve for operation.
• Considering the exemplary embodiment of the heat exchanger, when the warming cassette is installed in the high flow rate infusion unit, various elements of the warmingcassette60 and theinfusion unit10 cooperate to seat the warming cassette and to enable the infusion unit to control the flow of infusate. In this regard, with reference toFIGS. 18 and 19, thehousing64 engages the mountingblock36 and rests on thebezel32. The warming cassette is thus retained in place against the mountingblock36, and supported by the mountingblock36 and thebezel32, when installed. In this position, thefluid container62 is aligned in operable engagement with the heating plates, thesensor couplers37A and38A are aligned in operable engagement with thesensors37 and38, and thevalve membrane260 is aligned in operable engagement with thepiston41.
• With reference toFIG. 19, a warmingcassette60 is partially installed in theinfusion unit10, with itsdistal end61 having been received in theslot34 and its rails in thecircular enlargements270. As downward pressure is exerted on the warmingcassette60, thehousing64 moves toward thebezel32. Construction details of thebezel32 are shown inFIG. 18. Thebezel32 is fixedly mounted on flat planar area of the recessedsurface portion30, oriented transversely to thepedestal22. The mountingblock36 is fixedly mounted in the recessedsurface portion30, disposed substantially perpendicularly to and abutting an inside edge of thebezel32. Theslot34 in thebezel32 is oriented transversely to thepedestal22 and in parallel with themajor surface42 of the mountingblock36. Theslot34 is aligned with the narrow laminar space between the heating plates and includes a diamond shaped, oval, or round enlargement at each end to accommodate the rails of a warming cassette. Each circular enlargement is aligned with the elongate parallel channels formed by the elongate parallel grooves of the heating plates (SeeFIGS. 8 and 9). One suchcircular enlargement270 is seen inFIG. 18. Thebezel32 is therefore constructed to receive a warming cassette, distal end first, in theslot34, with the rails of the warming cassette received in thecircular enlargements270 so as to guide the fluid container of the warming cassette into the narrow laminar space between the heating plates for seating therebetween. As is evident fromFIG. 18, thebezel32 forms a raised frame to support thehousing64, and includes aforward edge271 that slopes downwardly and away from theslot34.
• As seen inFIGS. 12B and 18, athin flange272 projects from anedge273 of the mountingblock36; the front surface of theflange272 forms a portion of themajor surface42. Asensor274 is mounted adjacent the rear side of theflange272, on theedge273. Preferably, thesensor274 is an inductive proximity sensor. Twotabs275 protrude outwardly in opposite directions from the bottoms of the lateral edges of the mountingblock36. Onetab275 is seen inFIG. 12B; its opposite is seen inFIG. 19. The rear side of theflange272 has a recess with a projectingnotch276 near theedge273.FIG. 15 shows that theflange272 is wedge-shaped in itsupper extent277. A pair of retaining pins is fixedly mounted in opposing relationship to the opposing sides of the recessedsurface portion30 of theinfusion unit pedestal22. One of the retaining pins278 can be seen on one of the opposingsides279 inFIG. 18. As seen inFIGS. 12B,18 and19, a slopedelongate trench280 with rounded ends in themajor surface42 surrounds the locations of thesensors37 and38, which protrude beyond the plane of themajor surface42, toward thehousing64. When a warming cassette is installed, the rear face of thehousing64 is slightly separated from themajor surface42. When the housing engages and latches to the mountingblock36, thesensor couplers37A and38A on the rear face of the cassette housing align with and contact the faces of thesensors37 and38, and retain the sensors in engagement while the warming cassette is installed in the infusion unit.FIGS. 12B and 18 also show the actuator40 mounted to the back of the mountingblock36 aligned with a through thehole282 through which thepiston41 is moved back and forth.
• Acutout286 with anupper edge288 in the rear face of thehousing64 is visible inFIG. 12B. Thecutout286 is shaped to accommodate the shape and dimensions of the mounting blockmajor surface42. As best seen inFIGS. 12B and 15,inner side64iof thehousing60 is inset from thecutout286.FIG. 12B shows aslot290 in theupper edge288 and ametallic strip291 mounted in the housing adjacent theslot290.FIG. 12B also shows an upperflexible tab292 formed in theupper edge288. Two spaced-apart flexible tabs inset from theupper edge288 are formed in the lower portion of thehousing64. One of thetabs293 is seen inFIG. 12B.
• With reference toFIG. 19, the warmingcassette60 is installed in theinfusion unit10 by orienting the rear face of thehousing64 to face theinfusion unit10 and then sliding thedistal end61 into thebezel slot34, with therails153 and155 received in thecircular enlargements270. With reference toFIG. 15, as the warmingcassette60 slides home, theupper edge288 of thehousing cutout286 engages and slides along the wedge-shapedupper extent277 on the back of the mounting block'sflange272, and (as shown inFIG. 20) the front face of the housing slides along the inner sides of the retaining pins278. Theinner side64iis inset from thecutout286 and spaced by a small gap from themajor surface42 of the mountingblock36. As theupper edge288 of the housing cutout approaches theledge273, theslot290 in theupper edge288 aligns with and accommodates thesensor274 on the mountingblock36, andmetallic strip291 is located near thesensor274. With reference toFIG. 12B, theflexible tab292 in theupper edge288 aligns with and latches to the projectingnotch276 on the back of theflange272, and further movement of the warmingcassette60 is stopped when the cutoutupper edge288 meets theledge273 of the mountingblock36, and the lower edge of thehousing64 meets the upper surface of thebezel32. The warming cassette is now installed in the infusion unit10 (as shown inFIG. 20), with thefluid container62 seated between and in contact with the heating plates (seeFIGS. 7-9), and with thesensor couplers37A and38A aligned and in contact with thesensors37 and38, and thevalve membrane260 aligned with the piston41 (seeFIGS. 12B,20, and21). The warming cassette is guided by thepins278 into retention in the installed position by engagement between theupper edge288 and the rear of theflange272, engagement between thetabs293 and thetabs275, and locking of theflexible latch292 to thenotch276. The warming cassette is released by disengaging the flexible latch from the notch while pulling upwardly on thehousing64.
• Audible and tactile feedback indicating that the warming cassette is completely seated is provided to an operator by the latching action of thetab292 and the stopping of thehousing64 by theledge273. As best seen inFIGS. 20 and 21, the lower front and side edges of thehousing64 surround and shroud thebezel slot34 so that thehousing64 shrouds theslot34, enclosing and covering it to prevent fluid that might reach thebezel32 from leaks in infusate bags, IV lines, or thehousing64 from flowing thereinto.
Infusion Unit Subsystems
• The high flow rate infusion unit includes an electronic control subsystem with input, logic, and output elements that receive command and sensor inputs, process the inputs to set or change the control configuration of the unit during operation, and produce outputs that implement the current control configuration. The electronic control subsystem is assembled from conventional electrical, electronic, and electro-mechanical components mounted conventionally by means of printed circuit boards and structural elements in the neck and pedestal of the infusion unit. The electronic control subsystem is illustrated inFIG. 22.
• InFIG. 22, the electronic control system (“control subsystem”)300 includes acontroller302 having at least five logic blocks labeled #1-#5. Preferably, thecontroller302 is assembled using discrete components conventionally mounted to one or more circuit boards. However, thecontroller302 may also be assembled from programmable and/or programmed elements including general or special purpose processors, programmable logic arrays, and other equivalent components. Inputs to thecontroller302 are received from apower supply304 and abattery pack306. The power supply operates conventionally, converting AC mains power to various DC power outputs. The battery pack provides standby DC power to operate thecontroller302 and control subsystem components in the event that operation of thepower supply304 is interrupted. AC mains power is provided to operate theheaters183 through apower relay308 and a solid state relay (SSR)310. Both relays must be closed in order for AC power to reach theheaters183. Opening either relay will interrupt the supply of AC power to theheaters183, thereby causing the interruption of heat supplied to infusate flowing through a warming cassette seated between theheating plates182 and184. An operator interface311 (including thecontrol panels77,79 inFIGS. 1 and 2) provides means by which an operator can input commands and means to output information to the operator.
• With further reference toFIG. 22,logic block #1 of thecontroller302 executes a fail safe control function based upon comparison of a temperature measured by athermistor312 with a threshold temperature to turn off power to theSSR310. Thethermistor312 measures a temperature of theheating plate182. If the measured temperature should exceed the threshold temperature, thelogic block #1 generates signals to open therelay308, thereby blocking the provision of AC power to theSSR310 and thus to theheaters183.
• With further reference toFIG. 22,logic block #2 of thecontroller302 mediates a temperature control function that is based upon a set point temperature and an input from a resistance temperature detector (RTD)314 that measures a temperature of theheating plate184. In this regard, a temperature-influenced resistance measured by theRTD314 is provided to acontroller316 and converted to a temperature value by the controller. Thecontroller316 executes a temperature control function to maintain the measured temperature at a set point value by turning theSSR310 on and off as needed to keep the measured temperature at the set point temperature. Control signals produced by thecontroller316 are passed to theSSR310.
• With reference toFIG. 7, thethermistor312 is mounted in thehole194 in theheating plate182, and theRTD314 is mounted in thehole193 in theheating plate184, opposite the thermistor. As is evident from the figure, thethermistor312 andRTD314 are located in thetransverse channels192 formed in the surfaces of the heating plates that cause the formation of a manifold in thefluid container62 that channels infusate from thelaminar flow path67 into theslot166 in therail155. Thus, with accounting for heat transfer through the fluid container, thethermistor312 and theRTD314 effectively measure the temperature of the heated infusate as it enters the bubble trap. Thus, thecontroller316 operates to maintain the temperature of warmed infusate flowing into thebubble trap80 at the set point. Inlogic block #1, if the temperature of warmed infusate flowing into thebubble trap80 as measured by thethermistor312 exceeds the threshold temperature (which preferably is the sum of the set point temperature and a predetermined safety margin), the power relay is signaled to shut off AC power to theSSR310. For example, we have used a set point temperature of 42° C., and a threshold temperature of 46° C.
• InFIG. 22,logic block #3 responds to activation of arelease button320 by an operator signaling that a warming cassette is to be extracted from the infusion unit. As seen inFIG. 18, therelease button320 is located on the top of thepedestal22, adjacent the recessedsurface portion30. Preferably, the release button is a manually operated, push button switch, although it may also be embodied as a pressure activated electronic switch or a touch screen icon. Throughlogic block #3, activation of therelease button320 assists in releasing a warming cassette from engagement with the infusion unit by dislodging the warming cassette from the warmingplates182 and184 and withdrawing thepiston41 from contact with thevalve82. In this regard, the warming cassette may be dislodged by activating an electronically controlledpneumatic valve322 to release one or more jets of pressurized air which pass through theholes195 in theheating plates182 and184 seen inFIG. 7. The piston position is determined by the conditions of electronically controlledpneumatic valves324 and326.
• Logic block #4 of thecontroller302 seen inFIG. 22 monitors thesensor274 seen inFIG. 12B. When thesensor274 senses close proximity of the metallic strip291 (as would occur when the housing was seated on the mounting block36), it produces a signal interpreted as confirming the presence of a warming cassette properly aligned with and seated in theinfusion unit10. Alternately, when thesensor274 senses close proximity of themetallic strip291, the signal produced may be interpreted as confirming the presence of thebubble trap80 and proper alignment of thevalve82 with theactuator40 in the infusion unit. With reference toFIGS. 18 and 22, in some aspects, aphotosensor328 may be provided on the mountingblock36 to provide an initial indication of the presence of thehousing64 near the mounting block, following which thesensor274 will respond to close proximity of themetallic strip291 to provide an indication that the housing has been properly seated on the mounting block in the manner previously explained. In this case, concurrent outputs from thesensors274 and328 is interpreted as confirming correct installation of a warming cassette with its fluid container seated between the heating plates.Logic block #4 also provides control signals for activating an electronically controlledpneumatic valve332 that controls pressure in an air reservoir (not seen).
• Logic block #5 of thecontroller302 seen inFIG. 22 receives and processes signals output by theultrasonic sensors37 and38 that indicate the presence of a fluid (air or infusate) in the bubble trap, and signals output by aHall effect sensor330 in theactuator40 that indicates the position of thepiston41. As an additional safety measure,logic block #5 provides control signals for activating the ON/OFF function of the pressure inf users.
• The high flow rate infusion unit includes a pneumatic subsystem with elements that receive signals from theelectronic control subsystem300 indicating the control configuration of the unit during operation, and respond to the inputs by setting or changing the operational pneumatic configuration. The pneumatic subsystem also includes sensors that provide signals to theelectronic control subsystem300. The pneumatic subsystem is assembled from conventional pneumatic components mounted conventionally by means of structural elements in the neck and pedestal of the infusion unit.
• With reference toFIG. 23, thepneumatic subsystem360 includes amain distribution channel370. Pressurized air is provided to thedistribution channel370 fromdual pumps373, operating in parallel, viacheck valves375. The dual pump configuration is preferred for enhanced performance under normal operating conditions and also for safety reasons. Both pumps operate while the infusion unit is warming infusate; if either pump fails during infusion, the remaining pump has the capacity to carry on the operations necessary to keep the pneumatic subsystem operating.
• With further reference toFIG. 23, pressurized air in thedistribution channel370 flows to the electronically controlled three way valves in the pressure infusers18 through apressure regulator376; pressurized air in thedistribution channel370 flows through acheck valve378 to the electronically controlledvalves324 and326 which are preferably three way valves; and pressurized air in thedistribution channel370 flows to the electronically controlledvalve322 which is preferably a three way valve. The valve configuration in the pressure infusers18 is not limiting; many other configurations may be used.
• With reference toFIGS. 4A,4B, and23, logic block #5 (FIG. 22) allows the pressure infusers18 to turn ON/OFF, or not. In this regard, each threeway valve100 in the pressure infusers18 is controlled electronically vialogic block #5 in thecontroller302 to connect either the ambient atmosphere or thedistribution channel370 to its associateddump valve99. When apressure infuser18 is operated, its threeway valve100 is operated to connect thedistribution channel370 to the associateddump valve99, which causes pressurized air to inflate the associatedbladder103, thereby forcing infusate from a bag B in the pressure infuser. When the bag B is empty, when infusion is completed, or in other appropriate circumstances, thevalve100 is operated to connect the ambient atmosphere to the associateddump valve99, which causes pressurized air in the associatedinflated bladder103 to flow out of the dump valve to the atmosphere, thereby deflating thebladder103.
• With reference toFIGS. 22, and23, pressurized air flows through thecheck valve378 into thereservoir382. The threeway valve332 is controlled electronically vialogic block #4 to connect the output of thereservoir382 to either the ambient atmosphere or the inputs of thevalves324 and326. Preferably, theactuator40 is a double acting pneumatic piston actuator conventionally operated by pressurized air provided by thevalves324 and326. Thevalves324 and326 are operated 180° out of phase bylogic block #5 to position thepiston41 at an extended position against thevalve membrane260, which closes thevalve82, or a retracted position away from thevalve membrane260, which opens thevalve82. When thepumps373 are turned off and therelease button320 is operated, the states of the threeway valves324 and326 are configured bylogic block #3 for withdrawal of thepiston41 to the retracted position and pressurized air from thepumps373 and in thereservoir382 is provided to the three way valves to move the piston to the retracted position. If thepiston41 is in the retracted position and the warmingcassette60 is extracted when thepumps373 are turned off and therelease button320 is operated, the state of the threeway valve332 is set to vent the contents of thereservoir382 to the ambient atmosphere.
• With reference toFIGS. 7,22, and23, the threeway valve322 is controlled electronically vialogic block #3 in thecontroller302 to connect either the ambient atmosphere or thedistribution channel370 to theholes195 in theheating plates182 and184. When therelease button320 is operated, the threeway valve322 is configured to connect thedistribution channel370 to theholes195, thereby jetting pressurized air therethrough which breaks away surface tension and pushes fluid out of thefluid container62. Otherwise, thevalve322 is configured to connect the ambient atmosphere to, or to close, theholes195.
Method of Operation
• The high flowrate infusion unit10 with theheat exchanger12 illustrated inFIGS. 1 and 2 may be operated according to a method shown in the flow diagram ofFIG. 24. With reference toFIGS. 1,2, and24, the method ofoperation380 preferably initiates from a power onstate382 in which power is initially supplied to theinfusion unit10, with or without a heat exchanger12 (for example, the warming cassette60) installed. During initiation of operation, electronics, pneumatics, and logic are tested. If anomalies are found, the method exits to afailure mode384, where one or more status indicators are provided on the infusion unit's operator interface. With power successfully turned on, the method checks for installation of the heat exchanger at386. If no heat exchanger is installed, the method of operation ensures that thepiston41 is retracted at387 and operation suspends at385 until a heat exchanger, installed with its planar flow path seated in the heating unit, is detected. With a heat exchanger installed, correct operation of thepiston41 is validated at388 by, for example, three successive oscillations between the retracted and extended positions. Failure at388 causes the method to exit to afailure mode384. Otherwise, themethod380 branches to concurrently executing air management and heater control loops.
• In this explanation, air management is based on a test for the presence of a fluid such as infusate or air in the bubble trap. InFIGS. 2 and 24, during theair management loop390, thebubble trap80 is checked (by thesensors37 and38, for example) for the presence of air at391 In this regard, it is preferred that, when aheat exchanger12 is installed, it will be connected to an infusate bag for priming. Preferably, but not necessarily, the bag will be located in apressure infuser18. Theheat exchanger12 will be primed by gravitational flow of infusate to and through the laminar flow path. Air will be expelled through the bubble trap vent, permitting the priming infusate to flow into and fill thebubble trap80. While theheat exchanger12 is being primed, air will be detected and theair management loop390 will transition through392 and393, keeping thevalve82 closed and disabling the pneumatic subsystem from inflating theinfusion bladders103 shown inFIGS. 4A and 4B (by configuration of the threeway valves100, for example). When thebubble trap80 has been filled to a level at which the sensors no longer detect air, theair management loop390 will transition through394 and395, opening thevalve82 and enabling operator action for starting the pneumatic subsystem to inflate theinfusion bladders103. That is to say, thecontroller302 will open thevalve82, but will not initiate inflation. Instead an operator is prompted by an alarm or other indication to use theoperator interface311 to activate a pressure infuser. In this regard, the operator will input a command via theinterface311 causing a pressure infuser to activate. Thereafter, during infusion, theair management loop390 operates in response to the presence or absence of air in the bubble trap by taking the appropriate transition from391. In thetransition391,394,395,391, no action is required at394 if thevalve82 is open or at395 if the pressure infusers are enabled. When a bag of infusate has been emptied or is near empty in one pressure infuser, the operator reconfigures the Y tube set73 to stream infusate from a full bag in the other pressure infuser. Using theinterface311, the operator will stop operation of the pressure infuser with the empty bag and start operation of the other pressure infuser. In response to the stop/start indications from the operator, the controller302 (FIG. 22) operates the threeway valves100 to deflate thebladder103 in the stopped pressure infuser and to inflate the bladder in the pressure infuser with the full bag. To continue infusion, the operator replaces the empty bag in the stopped pressure infuser with a full one.
• An important safety feature of the air control loop is realized in closing thevalve82 and stopping infusion when air is detected. If thevalve82 should leak under the pressure of the infusate when closed, air might pass with leaking infusate through the closed but leaking valve. Deflating the active bladder relieves the pressure on the closed valve, thereby reducing, if not eliminating the risk of air leaking through the closed valve.
• With reference toFIGS. 22 and 24, theheater control loop400 is initiated at402 by initiating thecontroller316, turning on theheaters183, and bringing the heating unit to the set point temperature. If turn on fails to execute properly, the method exits to afailure mode384. After successful turn on, control of heating plate temperature for set point operation is implemented by operation of theset point controller316. While the heating plates operate, thecontroller302 continuously checks the fail safe control function at404. If the threshold temperature is exceeded, the heating plates are turned off at406 and afailure mode384 is entered.
• With reference toFIG. 24, the method of operation continuously checks the status of all infusion unit processes during all operations. Failure modes are dealt with as appropriate to the particular circumstances of failure. In most instances, thecontroller302 responds to a failure mode by deflating thebladders103 in the pressure infusers18, closing thevalve82, and providing audible and visual indicators via theinterface311. Operator action, such as selection of an “OFF” button or condition to turn the infusion unit off when an infusion is terminated and system operation is to be ceased will trigger power off status. In some instances an operator may also select an “OFF” button or condition when a heat exchanger is not installed in theinfusion unit10. For these cases, and in other appropriate circumstances, once power on has been successfully completed, the method ofoperation380 continuously monitors a power off test at410. If a power off condition is active, the method terminates all currently active processes, including the air management and heater control loops, and transitions to412, testing whether a heat exchanger is installed in theinfusion unit10. If a heat exchanger is not installed, the method ensures that thepiston41 is retracted at413, and then completes action by transitioning to a power off state at415 wherein all processes are terminated and power is turned off. If a heat exchanger is detected at412, themethod380 ensures that thevalve82 is closed and the pressure infusers are disabled (if not already turned off by the operator) at416 so that infusate flow to the patient line and to the heat exchanger is stopped. When the release button is activated at418, the method retracts thepiston41 at419 and dislodges the heat exchanger at420. In this regard, for the warming cassette embodiment, dislodging at420 includes operating the pneumatic subsystem to jet compressed air through theholes195 to disengage thefluid container62 from theheating plates182 and184. The method then transitions to the power off state at415.
Air Sensing and Management
• Preferably, air is sensed in the bubble trap by one or more sensors mounted in theinfusion unit10; preferably, at least two such sensors are used in order to provide redundancy, operational hysteresis, and a rich logical control mechanism for air management. We have used ultrasonic sensors that operate like sonar devices by transmitting and receiving pulses of ultrasonic energy. In particular, each of thesensors37 and38 may comprise a ceramic pulse echo sensor embedded potted, or screw mounted in a respective hole through the mountingblock36. In operation, each sensor sends out an ultrasonic pulse through a medium, and detects an echo of the pulse reflected back to the sensor off of an impedance mismatch, such as occurs at a solid/air interface. One source of such sensors is the Zevex Applied Technology Division, Salt Lake City, Utah.
• As seen inFIGS. 12B and 25, the sensors protrude through themajor surface42 of the mountingblock36 and face the rear face389 of thehousing60, in contact with thesensor couplers37A and38A formed on thematerial piece240. Presume that thesensor37 emits a pulse of ultrasonic energy. The sensor pulse enters thecoupler37A, and travels through thematerial piece240 and therear face430. Because of the insignificant difference in impedance between the sensor coupler and housing materials, no echo is produced by the outside surface of therear face430. If the level of infusate is above the position of thesensor coupler37A, the pulse travels through infusate in the housing to thefront face431, and an echo is produced by the solid/air discontinuity at the outside surface of the front face. The front face echo travels back, through the infusate, the rear face, and thematerial piece240 and is detected by thesensor37. If, however, the level of infusate is below the position of thesensor coupler37A, the transmitted pulse meets an impedance discontinuity at the solid/air interface between the rear face of the housing and air in the bubble trap, and an echo is produced by therear face431. The rear face echo travels through thematerial piece240 and is detected by thesensor37. Manifestly, the elapsed time to detect the front face echo is longer than that for the rear face echo. Thesensor37 provides a signal indicative of the elapsed time on aconductor432 to thecontroller302. The signal is interpreted as indicating the absence or presence of air (or, conversely, the presence or absence of infusate) in thebubble trap80. The consequence of the difference in elapsed time is that absence of a rear face echo is interpreted as the presence of infusate (or, conversely, as the absence of air), while detection of a rear face echo is interpreted as the presence of air (or as the absence of infusate). Logic provided in the sensor utilizes a pulse window beginning with the transmission of a pulse having a width wide enough for a pulse to travel to and from the front face. An echo received within the pulse window is interpreted as indicating the presence of infusate (or the absence of air); no echo received within the pulse window is interpreted as indicating the presence of air (or the absence of infusate). Thesensor38 operates identically. This sensor arrangement provides a single point of sensor contact for transmitting and receiving.
• Preferably, air management in the bubble trap is based upon venting air through a hydrophobic membrane in contact with infusate flowing through the bubble trap. InFIG. 26, thetransition214 between the laminar flow andoutlet chambers207,215 includes a downwardlyangled wall440. Thesensors37 and38 have fields of view through thesensor couplers37A and38A into thelaminar flow chamber207. Thelevel line442 is centered in the field of view of thesensor37, and thelevel line444 is centered in the field of view of thesensor38. Thelevel line442 passes through the lower quadrant of thehydrophobic membrane209, and thelevel line444 is parallel to thelevel line442, below thehydrophobic membrane209, but above theriser219 through which infusate flows to thevalve82 and then to theoutput port71. As air collects in a pocket in the upper reaches of the bubble trap, the border between the air pocket and infusate moves down the downwardlyangled wall440; when the border moves downwardly across thehydrophobic membrane209, air is vented from the air pocket through the membrane. When the border between the air pocket and infusate is above alevel line442 or444, thesensor37 or38 associated with the respective level line senses fluid; when the border is below alevel line442 or444, thesensor37 or38 associated with the respective level line senses air. An advantage of the sensor locations is that the increased velocity of the laminar sheet of infusate through thelaminar flow chamber207 sweeps bubbles from the fields of view of thesensors37 and38. This reduces the risk of eithersensor37 or38 producing false level indications in response to bubbles.
• The preferred air management logic control mechanization for the sensors disposed with respect to the bubble trap as inFIG. 26 is shown inFIG. 27; this logic represents an adaptation of theair control loop360 ofFIG. 24 for the case of two sensors. The logic ofFIG. 27 controls the state of thevalve82 and enablement of the pressure infusers18 according to whether thesensors37 and38 report the presence of infusate or air in the bubble trap. Initially, the heat exchanger is primed at450, when thefluid container62 and thebubble trap80 are empty. Thesensors37 and38 both report the presence of air at460 and461, satisfying the test at462. Thevalve82 is closed and the pressure infusers18 are disabled at463. The logic loops through460,461,462 and463 until eithersensor38 or37 reports the presence of infusate (or, conversely, no air). When the presence of infusate is reported at460 or461 thevalve82 is opened and the operator is given an indication to activate inflation of a bladder in apressure infuser18 at464. Then both sensors are monitored for air. When both sensors report air, thevalve82 is closed and the operating pressure infuser is deactivated at463, and the logic again loops until infusate is reported by either or both sensors as previously mentioned.
• When thevalve82 is closed in response to the test at462, the bubble trap is again primed with infusate, which will rise in the bubble trap, first passing thelower sensor38. In some aspects, the logic ofFIG. 27 may utilize a time delay to the negative exit of the test at461, thereby prolonging the closure of thevalve82 while the bubble trap primes. In these instances, the use of two sensors provides hysteresis in the operation of thevalve82.
• Other air sensing and management configurations for thebubble trap80 are possible. One such configuration, shown inFIG. 28 as an adaptation of thebubble trap80, uses a second solenoid drivenvalve470 to isolate theair vent81 in anair chamber471 in order to keep thehydrophobic membrane209 dry. If either of thesensors37 and38 senses infusate, thevalve82 is open. If bothsensors37 and38 sense air, thevalve82 is closed. If either of thesensors37 and472 senses the presence of infusate, thevalve470 remains closed. If bothsensors37 and472 sense the presence of air, thevalve470 is opened. The pressure of infusate flowing into thebubble trap80 from the fluid container forces the air into theair chamber471 where it is vented through thehydrophobic membrane209. The level of infusate rises as air exits into theair chamber471, and thevalves82 and470, respectively, open and close when thesensors37,38, and472 once again sense the presence of infusate. A third ultrasonic sensor to sense the contents of thebubble trap80 throughcoupler location472 may be included in order to provide greater redundancy, a larger degree of hysteresis, and a richer functional set than the twosensors37 and38. One additional function realized by the addition of a third sensor is to open thevalve470 at some intermediate infusate level while holding open thevalve82 in order to vent air while continuing to deliver infusate to a patient.
• Although a high flow rate infusion unit and a heat exchanger for a high flow rate infusion unit have been described with reference to a number of embodiments, it should be understood that various modifications can be made without departing from the principles of this specification, which are limited only by the following claims.

Claims (37)

1. An infusion system, comprising

an infusion unit with heating plates, a slot opening to space between the heating plates, and a mounting block near the opening;

a heat exchanger including a fluid container receivable through the slot for seating in contact with the heating plates, a housing attached to the fluid container and receivable on the mounting block, a laminar flow path in the fluid container, a bubble trap in the housing in fluid communication with the laminar flow path and a valve in the housing in fluid communication with the bubble trap;

the infusion unit including at least one air sensor positioned to be adjacent the bubble trap when the housing is received on the mounting block;

the infusion unit including a valve actuator positioned to be adjacent the valve when the housing is received on the mounting block; and

means in the infusion unit for pressurizing bags of infusate.

2. The infusion system ofclaim 1, wherein the at least one air sensor includes two ultrasonic sensors mounted to the mounting block, the infusion system further including two sensor couplers mounted to the housing to be in contact with the sensors when the housing is received on the mounting block.

3. The infusion system ofclaim 2, wherein the valve includes a membrane mounted to the housing, and the valve actuator includes a piston for acting through the mounting block against the membrane when the housing is received on the mounting block.

4. The infusion system ofclaim 1, further comprising means acting between the mounting block and the housing to latch the housing to the infusion unit, and means on the infusion unit for retaining the housing adjacent the mounting block.

5. The infusion system ofclaim 1, wherein the housing shrouds the opening when the housing is received on the mounting block.

6. The infusion system ofclaim 5, further comprising means acting between the mounting block and the housing to latch the housing to the infusion unit.

7. The infusion system ofclaim 6, further comprising means on the infusion unit for guiding the housing to a seating position adjacent the mounting block.

8. The infusion system ofclaim 7, wherein the at least one air sensor includes two ultrasonic sensors mounted to the mounting block, the infusion system further including two sensor couplers mounted to the housing to be in contact with the sensors when the housing is received on the mounting block.

9. The infusion system ofclaim 6, wherein the at least one air sensor includes two ultrasonic sensors mounted to the mounting block, the infusion system further including two sensor couplers mounted to the housing to be in contact with the sensors when the housing is received on the mounting block.

10. The infusion system ofclaim 6, wherein the valve includes a membrane mounted to the housing, and the valve actuator includes a piston for acting through the mounting block against the membrane when the housing is received on the mounting block.

11. The infusion system ofclaim 1, the infusion unit further including means connected to the at least one air sensor for detecting air in the bubble trap.

12. The infusion system ofclaim 11, the infusion unit further including means connected to the valve actuator for closing the valve when air is detected in the bubble trap and for opening the valve when air is not detected in the bubble trap.

13. The infusion system ofclaim 11, the infusion unit further including means for sensing presence of the heat exchanger.

14. The infusion system ofclaim 13, wherein the valve includes a valve membrane mounted to the housing and the valve actuator includes a piston for being moved to a retracted position away from the valve membrane or an extended position against the valve membrane, the infusion unit further including means connected to the valve actuator for moving the piston to the retracted position when no heat exchanger presence is detected, and for moving the piston to the extended position when heat exchanger presence is detected.

15. The infusion system ofclaim 14, the means connected to the valve actuator further for moving the piston to the extended position to close the valve when air is detected in the bubble trap and for moving the piston to the retracted position to open the valve when air is not detected in the bubble trap.

16. The infusion system ofclaim 1, wherein the infusion unit includes means for releasing a heat exchanger from the infusion unit.

17. The infusion system ofclaim 16, wherein the means for releasing include means for pneumatically releasing a fluid container from between the heating plates.

18. The infusion system ofclaim 3, wherein the infusion unit includes a release button and means responsive to the release button for releasing a fluid container from between the heating plates and for moving the piston away from the membrane.

19. The infusion system ofclaim 1, wherein the infusion unit includes a manually operable release element and means for releasing a heat exchanger from the infusion unit in response to operation of the release element.

20. The infusion system ofclaim 19, wherein the valve actuator includes a piston for engaging the valve, the means for releasing include means for releasing the piston from engagement with the valve and means for pneumatically releasing a fluid container from between the heating plates.

21. A high flow rate infusion unit for use with a heat exchanger including a fluid container with a laminar flow path, a bubble trap in fluid communication with the laminar flow path, and a valve in fluid communication with the bubble trap, the high flow rate infusion unit comprising:

a casing;

opposed heating plates disposed in the casing;

an opening through the casing for seating the fluid container between and in contact with the heating plates;

a mounting block on the casing near the opening for supporting the heat exchanger while the fluid container is seated;

pressure infusers mounted on the casing;

sensor means acting through the mounting block for sensing fluid in the bubble trap;

valve actuator means acting through the mounting block for opening and closing the valve; and

means in the casing for pressurizing a bag of infusate in either pressure infuser.

22. The infusion unit ofclaim 21, the sensor means including at least one transmit/receive sensor.

23. The infusion unit ofclaim 21, further including control means responsive to sensing air in the bubble trap for causing the valve actuator means to close the valve and for causing the means for pressurizing to release pressure on the bag of infusate.

24. The infusion unit ofclaim 21, wherein the heating plates are vertically disposed in the casing, further including opposing pairs of elongate grooves in facing surfaces of the heating plates, each pair defining a respective elongate channel.

25. The infusion unit ofclaim 24, further including rounded ends of the opening, each aligned with a respective elongate channel.

26. The infusion unit ofclaim 21, further comprising means for latching the heat exchanger to the mounting block and means on the casing for retaining the bubble trap adjacent the mounting block.

27. The infusion unit ofclaim 21, further including a major surface on the mounting block facing the opening, wherein the sensor means include at least one ultrasonic sensor mounted in the mounting block.

28. The infusion unit ofclaim 21, wherein the valve actuator means includes a piston for acting through the mounting block against a valve membrane in the valve.

29. The infusion unit ofclaim 21, further including means connected to the sensor means for detecting air in the bubble trap.

30. The infusion unit ofclaim 29, further including means connected to the valve actuator means for closing the valve when air is detected in the bubble trap and for opening the valve when no air is detected in the bubble trap.

31. The infusion unit ofclaim 29, the infusion unit further including means for sensing presence of the bubble trap and proper alignment of the valve with the valve actuator.

32. The infusion unit ofclaim 31, wherein the valve actuator means include a piston for being moved to a retracted position away from the valve or an extended position against the valve, further including means connected to the valve actuator means for moving the piston to the retracted position when no heat exchanger presence is detected, and for moving the valve to the extended position when heat exchanger presence is detected.

33. The infusion unit ofclaim 32, the means connected to the valve actuator means further for moving the piston to the extended position to close the valve when air is detected in the bubble trap and for moving the piston to the retracted position to open the valve when air is not detected in the bubble trap.

34. The infusion unit ofclaim 21, further including means for releasing the heat exchanger from the infusion unit.

35. The infusion ofclaim 34, wherein the means for releasing include means for pneumatically releasing the fluid container from between the heating plates.

36. The infusion unit ofclaim 21, further including a manually operable release element and means for releasing a heat exchanger from the infusion unit in response to operation of the release element.

37. The infusion system ofclaim 36, wherein the means for releasing include means for releasing the actuator from engagement with the valve and means for pneumatically releasing the fluid container from the unit.

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