US20090087328A1 - Pulse generating device - Google Patents

Abstract

A pulse generator has a rigid body with a resilient flow conduit passing therethrough. Pressure is periodically selectively applied to the interior of the body which compresses the flow conduit and induces a pressure pulse in the fluid contained therein. The pulse generator may be used as a stand alone pump or in concert with a continuous flow pump to provide a pulsatile output flow having customized waveform characteristics. The pulse generator may be used with medical equipment such as a heart-lung bypass machine or a dialysis machine.

Description

BACKGROUND OF THE INVENTION
• This invention relates generally to fluid circulation and pumps, and more particularly to a method for applying a pressure pulse to a fluid.
• Medical procedures such as open heart surgery, heart transplants, and kidney dialysis require equipment which extracts a patient's blood, treats the blood by processes such as filtering, oxygenation, and the like, and returns the blood to the patient's body. Such equipment uses pumps with essentially constant pressure output to circulate the patient's blood through the treatment equipment. Unfortunately, this kind of flow is much different from the flow provided by a patient's heart. It has been found that this constant-pressure flow can have undesirable side effects including brain disorders and limited or reduced circulation, especially in flow restrictive areas. This places undesirable limits on the usage of this type of equipment.
BRIEF SUMMARY OF THE INVENTION
• Accordingly, it is an object of the invention to provide a method and apparatus for creating a pulsating flow in a fluid.
• It is another object of the invention to creating a pulsating flow in a bodily fluid while avoiding shear stress and cell damage in the fluid.
• It is another object of the invention to create a preselected pressure profile in a flowing fluid.
• These and other objects are met by the present invention, which according to one aspect provides a pulse generator for fluid flow, including: a substantially rigid body defining a pressure cavity; a resilient flow conduit extending through the pressure cavity and arranged to carry a first fluid therein; and a pressure inlet communicating with the pressure cavity for introducing a second fluid into the pressure cavity, such that the first fluid may be exposed to a time-varying pressure signal introduced through the pressure inlet without mixing of the first and second fluids.
• According to another aspect of the invention, the flow conduit is generally S-shaped.
• According to another aspect of the invention, the flow conduit has a four-sided cross-sectional shape defined by spaced-apart walls extending between radiused corners.
• According to another aspect of the invention, the flow conduit has a cross-sectional shape including a pair of spaced-apart lobes connected by a pinched waist.
• According to another aspect of the invention, the flow conduit has a cross-sectional shape including a pair of spaced-apart lobes connected by a pinched waist.
• According to another aspect of the invention, the flow conduit has a cross-sectional shape including a plurality of spaced-apart, radially-extending lobes.
• According to another aspect of the invention, the flow conduit has a cross-sectional shape including: a double-walled, generally C-shaped portion; and a plurality of spaced-apart, a flexible bridge connecting free ends of the C-shaped portion; wherein the C-shaped portion and the flexible bridge cooperate to define a central opening.
• According to another aspect of the invention, the pressure cavity is shaped such that a pressure signal applied thereto will propagate symmetrically therein.
• According to another aspect of the invention, a pulse generator for fluid flow includes: a substantially rigid housing which defines an open cavity, the housing including an inlet and outlet communicating with the cavity; a flexible diaphragm which closes off the cavity; and an actuator operably connected to the diaphragm and adapted to selectively move the diaphragm from a first position in which the pulse generator encloses a first fluid volume, and a second position in which the pulse generator encloses a second fluid volume less than the first fluid volume.
• According to another aspect of the invention, the diaphragm forms a portion of a torus.
• According to another aspect of the invention, the pulse generator further includes a programmable controller operably connected to the actuator.
• According to another aspect of the invention, a fluid circulation system includes: a circulation pump for a first fluid; a first pulse generator disposed in series flow with the circulation pump, the first pulse generator including: a substantially rigid first body defining a first pressure cavity; a resilient first flow conduit extending through the first pressure cavity and arranged to carry the first fluid therein; and a first pressure inlet communicating with the first pressure cavity for receiving a second fluid into the first pressure cavity, such that the first fluid may be exposed to a pressure signal received through the first pressure inlet without mixing of the first and second fluids; and a control mechanism operably connected to the first pressure inlet and adapted to introduce a pressure signal into the first pressure cavity so as to impress a predetermined time-varying pressure profile into the first fluid through the first flow conduit.
• According to another aspect of the invention, the circulation pump is a type generating a substantially constant output pressure in the first fluid.
• According to another aspect of the invention, the control mechanism includes: a source of pressurized fluid; and a valve operable to selectively transfer the pressurized fluid to the first pressure inlet.
• According to another aspect of the invention, the first pulse generator has a displacement capacity, and in which the control mechanism is adapted to control the circulation pump and the first pulse generator such that the first pulse generator operates in a preselected displacement range within the displacement capacity.
• According to another aspect of the invention, the fluid circulation system further includes: a second pulse generator disposed in series flow with the circulation pump and the first pulse generator, the second pulse generator including: a substantially rigid second body defining a second pressure cavity; a resilient second flow conduit extending through the second pressure cavity and arranged to carry the first fluid therein; and a second pressure inlet communicating with the second pressure cavity for receiving a second fluid into the second pressure cavity, such that the first fluid may be exposed to a pressure signal received through the second pressure inlet without mixing of the first and second fluids; wherein the control mechanism is operable to selectively pressurize the second pulse generator to a degree so as to substantially close off flow through the second flow conduit.
• According to another aspect of the invention the fluid circulation system further includes equipment for treatment of a bodily fluid connected in series flow therewith.
• According to another aspect of the invention, a fluid circulation system includes: a first pulse generator including: a substantially rigid first body defining a first pressure cavity; a resilient first flow conduit extending through the first pressure cavity and arranged to carry the first fluid therein; and a first pressure inlet communicating with the first pressure cavity for receiving a second fluid into the first pressure cavity, such that the first fluid may be exposed to a pressure signal received through the first pressure inlet without mixing of the first and second fluids; and a second pulse generator connected in series flow with the first pulse generator, and including: a substantially rigid second body defining a second pressure cavity; a resilient second flow conduit extending through the second pressure cavity and arranged to carry the first fluid therein; and a second pressure inlet communicating with the second pressure cavity for receiving a second fluid into the second pressure cavity, such that the first fluid may be exposed to a pressure signal received through the second pressure inlet without mixing of the first and second fluids. A control mechanism is operably connected to the first and second pressure inlets and adapted to: cyclically pressurize one of the first pulse generator so as to impress a predetermined time-varying pressure profile into the first fluid; and cyclically pressurize the second pulse generator to a degree so as to substantially close off flow through the second flow conduit in coordination with the pressurization of the first pulse generator, such that the first fluid is moved through the fluid circulation system in a single direction.
• According to another aspect of the invention, the fluid circulation system further includes: a third pulse generator connected in series flow with the first and second pulse generators, and including: a substantially rigid third body defining a third pressure cavity; a resilient third flow conduit extending through the third pressure cavity and arranged to carry the first fluid therein; and a third pressure inlet communicating with the third pressure cavity for receiving a second fluid into the third pressure cavity, such that the first fluid may be exposed to a pressure signal received through the third pressure inlet without mixing of the first and second fluids; and a control mechanism operably connected to the third pressure inlet and adapted to selectively pressurize the third pulse generator to a degree so as to substantially close off flow through the third flow conduit.
• According to another aspect of the invention, the fluid circulation system further includes equipment for treatment of a bodily fluid connected in series flow therewith.
• According to another aspect of the invention, a method of generating a fluid pulse includes: passing a first fluid through a resilient first flow conduit arranged to carry the first fluid therein; and while the fluid is in the first flow conduit, introducing a pressurized second fluid into a first pressure cavity defined by a substantially rigid first body surrounding the first flow conduit, as to impress a predetermined time-varying pressure profile into the first fluid through the first flow conduit.
• According to another aspect of the invention, the method includes passing the first fluid through a circulation pump connected in series flow relationship with the first flow conduit so as to impress a substantially constant pressure component in the first fluid.
• According to another aspect of the invention, the method includes: passing the first fluid through a resilient second flow conduit arranged to carry the first fluid therein; introducing a pressurized second fluid into a second pressure cavity defined by a substantially rigid second body surrounding the second flow conduit; wherein the second flow conduit is cyclically compressed to a degree so as to substantially close off flow through the second flow conduit in coordination with the pressurization of the fluid in the first flow conduit, such that the first fluid is moved through the first and second flow conduits in a single direction.
• According to another aspect of the invention, the first fluid is a bodily fluid, which is passed through equipment for treatment thereof.
• According to another aspect of the invention, the first fluid is heated or cooled as it passes through the first flow conduit.
• According to another aspect of the invention, the second fluid is at a lower pressure than the first fluid during at least a portion of the pressure profile.
BRIEF DESCRIPTION OF THE DRAWINGS
• The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
• FIG. 1 is a schematic view of a prior art heart-lung bypass apparatus;
• FIG. 2 is a schematic view of a heart-lung bypass apparatus incorporating a pulse generator constructed in accordance with the present invention;
• FIG. 3 is a cross-sectional view of a first embodiment of a pulse generator;
• FIG. 4 is a cross-sectional view of a first variation of a flow conduit for use with the pulse generator ofFIG. 3;
• FIG. 5 is a cross-sectional view of another variation of a flow conduit;
• FIG. 6 is a cross-sectional view of another variation of a flow conduit;
• FIG. 7 is a cross-sectional view of yet another variation of a flow conduit;
• FIG. 8 is a schematic graph of a flow characteristic of the pulse generator in operation;
• FIG. 9 is a perspective, cut-away view of an alternative pulse generator;
• FIG. 10A is a graph illustrating the behavior of a pulse generator in a first condition;
• FIG. 10B is a graph illustrating the behavior of a pulse generator in a second condition;
• FIG. 10C is a graph illustrating the behavior of a pulse generator in a third condition;
• FIG. 11 is a schematic view of a heart-lung bypass apparatus illustrating an alternate control apparatus and method:
• FIG. 12 illustrates a portion of a flow circulation system incorporating multiple pulse generators;
• FIG. 13 illustrates a portion of an alternate flow circulation system incorporating multiple pulse generators; and
• FIG. 14 is a cross-sectional view of yet another variation of a flow conduit.
DETAILED DESCRIPTION OF THE INVENTION
• Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,FIG. 1 depicts a prior art heart-lung bypass apparatus10 comprisingtreatment equipment12 and acirculation pump14. Thetreatment equipment12 typically includes facilities for removing carbon dioxide and waste products, and supplying oxygen. Thecirculation pump14 is most often a positive-displacement pump of the type referred to as a “peristaltic” pump. Theapparatus10 is connected to a patient “P” by aninlet line16 and anoutlet line18.
• In operation, blood circulates in a closed loop from the patient P to thetreatment equipment12 through thecirculation pump14 and back to the patient P. While efforts are made to select the flow rate and pressure of thecirculation pump14 to meet the patient's requirements, It has been found that the constant pressure flow from thepump14 can result in unwanted side effects such as brain disorders (e.g. cognitive dysfunction).
• FIG. 2 depicts a heart-lung bypass apparatus10′ constructed according to the present invention. It should be noted that the present invention is equally applicable to other types of medical equipment, or any other fluid flow application requiring a pulsating flow. Theapparatus10′ is similar to theprior art apparatus10 and includestreatment equipment12′ and acirculation pump14′, and is connected to a patient “P” by aninlet line16′ and anoutlet line18′. Theapparatus10′ also includes apulse generator20 connected downstream of thepump14. Thepulse generator20, which is described in detail below, receives blood flow from thepump14 and applies a cyclic pressure pulse thereto, so that the patient receives a pulsating flow which mimics the flow characteristics of the patient's heart. The pressure pulse is provided by a control andpressurization apparatus22, including for example, a source of pressurized air such as the illustratedair tank24, and asolenoid valve26 controlled by a programmable electronic controller or computer of a known type, illustrated schematically at28.
• FIG. 3 illustrates a first embodiment of thepulse generator20 in more detail. Thepulse generator20 includes a relativelyrigid body30 which defines apressure cavity32 therein. Apressure inlet34 is formed in thebody30. Aflow conduit36 having aninlet38 and anoutlet40 passes through thebody30. Thepressure cavity32 is shaped so that a pressure signal applied thereto will be uniformly directed and will propagate symmetrically (with reduced pressure waveform reverberations). It is noted that the term “pressure signal” is used generally to refer to a fluid pressure having a preselected value, be that constant or time-varying, and that the absolute value of the pressure signal may be greater or lesser than the pressure within the flow conduit. In other words, a partial vacuum, relative to the fluid in theflow conduit36, may be used as the pressure signal.
• The interior of theflow conduit36 is isolated from thepressure cavity32 such that blood flowing therethrough will be subject to the prevailing pressure (i.e. positive pressure or vacuum) in thepressure cavity32, but no mixing of the blood with the fluid in thepressure cavity32 will take place. The path of theflow conduit36 is chosen to expose a selected surface area to thepressure cavity32 while minimizing the overall dimensions of thepulse generator20. In the illustrated example, theflow conduit36 is generally “S”-shaped, but it could also be straight, curved, coiled, looped or helical depending upon the particular application.
• Theflow conduit36 is constructed of bio-compatible material of a known type, such as a medical-grade elastomer. The cross-section of theflow conduit36 may be a simple circular shape as in ordinary tubing, or it may be a more complex shape. The mechanical response of theflow conduit36 may be customized so that it exhibits the desired qualities in terms of volume, resilience, collapsibility, expansibility, capacitance, and restitution.
• FIG. 4 illustrates an example of aflow conduit136 having a box-section shape withwalls138 extending betweenradiused corners140, whileFIG. 5 illustrates anotherflow conduit236 with awall238 defining a pair of spaced-apartlobes240 connected by apinched waist242.
• FIGS. 6 and 7 illustrate twovariations336 and336′ of yet another flow conduit in which a closed wall is curved into a “C”-shape. The free ends of the “C” are connected by aflexible bridges338 and338′, respectively. In the variation shown inFIG. 6, the working fluid (e.g. blood) flows through thecentral opening340. The C-shaped portion may be of a hollow, double-wall construction as shown, or the wall structure may be customized by including a solid (e.g. honeycomb or web), liquid, or gas filler, or by making the C-shaped portion a relatively thick solid wall.
• In the variation shown inFIG. 7, working fluid flow through the double-walled C-shaped portion of theflow conduit336′, and thecentral opening340′ serves as a chamber that can act as a resilient “gas spring” to control recovery rates. It can also be provided with a time-varying pressure signal to further customize its response.
• FIG. 14 illustrates yet anotherflow conduit736 in which a wall defines a plurality of spaced-apart, radially-extendinglobes738. Thelobes738 have rounded outer ends740 and are interconnected at their inner ends by concave-curvedtransitional sections742. This results in a generally star-shapedcentral opening744.
• It should be generally noted the cross-sectional geometry of any of the described tube sections (radii, wall thicknesses, material properties) can be modified to achieve desired collapse and restoration properties. The tube sections perform as a collapsible beam and structural engineering techniques (2D and 3D) can therefore be applied to advantageously customize the design and dimensional parameters.
• Referring again toFIG. 2, thepulse generator20 operates in series flow with thecirculation pump14. Thepulse generator20 receives the blood or other working fluid. A cyclic fluid pressure pulse (signal) is provided by the control andpressurization apparatus22 through theinlet38. As the pressure increases in thebody30, it compresses theflow conduit36 and the fluid inside, increasing the fluid's pressure. When the pressure pulse is terminated, the flow decreases in pressure. A vacuum component or negative pressure signal may then be applied subsequent to the positive pressure pulse. This may be used to assist in restoration of theflow conduit36 to its original state in between positive pressure pulses. It should be noted that the operating characteristics ofcirculation pump14 prevent any substantial backflow in this apparatus Therefore, no check valves are required for thepulse generator20.
• FIG. 8 illustrates an example of the flow characteristics that can be obtained. The dashedline42 represents the essentially constant pressure output of thecirculation pump14, while thesolid line44 represents the total pressure after the fluid passes through thepulse generator20. Appropriate feedback signals are provided to thecontroller28, representative of the output of theapparatus10′. In the illustrated example, the flow has a pulsating pressure withpeaks46 occurring at regular intervals. A quasi-square-wave flow characteristic is shown; however, by careful selection of the properties of thebody30,flow conduit36, and the pressure pulses, almost any wave shape desired can be obtained. This allows theapparatus10′ to closely simulate the flow characteristics of the patient's heart or to generate specific preferred waveforms as determined by the physician or technician involved in a particular procedure. It is thought that this will eliminate or reduce undesirable affects, including brain disorders, normally associated with heart-lung bypass equipment.
• Heating or cooling of bypass blood flow is sometimes done during heart bypass surgery. In the prior art, this is performed with a separate heat exchanger. It should be noted that it is entirely practical to use the pressure transmission fluid chamber, therigid body30 as a heat transfer chamber such that the fluid flowing through theflow conduit36 is heated or cooled depending on the procedure requirement. With this configuration, it would be possible to consolidate thepulse generator20 and heat exchanger into one device, thereby reducing complexity and reducing the amount of blood required outside the patient's body to fill the system while also reducing blood-wetted surface of theapparatus10′.
• Thepulse generator20 has a finite capacity for pulse generation. In other words, the pressure, volume, and total work input capacity are each limited by the pulse generator's construction and power source. Thecirculation pump14′ typically has a substantially constant output pressure and flow at a given input RPM. The flow demands from the patient on thebypass apparatus10′ may vary during the course of a medical procedure. Using the feedback control described above, the behavior of the pulse generator20 (i.e. its stroke length, acceleration, and velocity) will vary over a wide range to result in the desired total output flow characteristics.
• FIGS. 10A,10B, and10C are graphs depicting the total displacement range of thepulse generator20 between upper and lower limits noted as “H” and “L”, respectively, and illustrating the pulse generator's behavior under different conditions, assuming a fixed input RPM to thecirculation pump14′. For example, if the patient demand should be relatively high, thepulse generator20 will have to operate in a range “R1” near the upper end of its displacement capacity, as shown inFIG. 10A. if the patient demand should be relatively low, thepulse generator20 will have to operate in a range “R2” near the lower end of its displacement capacity, as shown inFIG. 10B. If the patient demand is near nominal, thepulse generator20 will operate in a “nominal” range “R3” near the center its displacement capacity, as shown inFIG. 10C.
• Thepulse generator20 can be forced to operate in a the nominal range R3 in order to provide a good margin of stroke or pulse volume on the compression cycle and a good margin of recovery volume on the expansion cycle, and thus achieve more consistent overall performance.
• This is achieved by including both thecirculation pump14′ and thepulse generator20 in a control loop.FIG. 11 depicts a heart-lung bypass apparatus110 which is substantially identical to theapparatus10′ described above except for the method of control. Theapparatus110 includestreatment equipment112, acirculation pump114, inlet andoutlet lines116 and118 connected to a patient “P”, apulse generator120 and a control andpressurization apparatus122, including a programmable electronic controller or computer of a known type, illustrated schematically at128. Feedback signals (e.g. pressure, volume) representative of the total flow output to the patient are provided to thecontroller128 bysensors130, and feedback signals indicative of the range of operation of the pulse generator120 (e.g. displacement) are provided to thecontroller128 bysensors132. Thecontroller128 is operatively connected to both thecirculation pump114 and thepulse generator120.
• Control of the total flow output to the patient P is as described for theapparatus10′ described above. However, thecontroller128 also monitors thepulse generator120 to determine if it is operating in its desired nominal range R3 (seeFIG. 10C). If not, the output of thecirculation pump114 is changed to achieve the desired pulse generator behavior, by modifying the input RPM of thecirculation pump114 or other appropriate means. For example, if thepulse generator120 is operating in a “high” range R1 to satisfy patient demand, then the flow of thecirculation pump114 would be increased so that thepulse generator120 operation shifts back down to the desired range R3.
• FIG. 9 illustrates a second embodiment of apulse generator420, which may be substituted for the pulse generators described above, in more detail. Thepulse generator420 includes a hollow, open-endedrigid housing422 with aninlet424 and anoutlet426. Aflexible diaphragm428 seals off thehousing422. Thehousing422 and thediaphragm428 are constructed of bio-compatible materials such as medical-grade plastics. Thediaphragm428 may have a partial toroidal shape as illustrated or other shape effective to minimize or eliminate lost motion (e.g. later movement) as thediaphragm428 moves through its working range. Thediaphragm428 may also include one or more radially-extending reinforcing ribs (not shown) on its inner or outer surface (or both). The ribs, if used, stiffen the diaphragm and help it to resist compressive loads, which would occur if a vacuum were applied to thediaphragm428. Apiston430 has afirst end432 connected to thediaphragm428 and asecond end434 connected to a known type of linear electric motor or othersuitable actuator436. Inward motion of thediaphragm428 increases the pressure in the fluid flowing through thepulse generator420, which outward motion decreases the pressure.
• Thelinear motor436 is driven by aprogrammable controller438 of a known type such as a PLC or general-purpose computer. Thecontroller438 is able to control the displacement, velocity, and acceleration of thepiston430 so as to obtain selected flow characteristics as described above, including positive pressure and/or vacuum pulses. The control loop described above may also be applied when using thepulse generator420. It should be noted thehousing422 chamber bottom may be shaped similar (such as the top section of a torus) to the preshaped membrane in order to better disperse fluid impulse energy and reduce system volume requirements.
• Thepulse generators20 have been described as separate units for use with circulation pumps. They may also be used as stand-alone units to provide check-valve or pumping functions by scaling, connecting, and/or combining them appropriately.
• For example,FIG. 12 illustrates a portion of a flow circulation system comprising acirculation pump514 of a type such as a peristaltic pump, afirst pulse generator520A, asecond pulse generator520B, and a control andpressurization unit522. The pulse generators520 are substantially identical in their operation to thepulse generators20 described above. They may have individual housings as described above, or they may take the form of chambers within a singlerigid housing524. In this configuration, thefirst pulse generator520A is activated so that it acts as a check valve during the output pulse cycle to prevent back flow from thesecond pulse generator520B. That is, the flow channel therein (not shown) is deflected to a degree that it is closed or nearly closed to flow therethrough when thesecond pulse generator520B is discharging. It does so in a way that limits the localized acceleration/deceleration of the blood flow and/or reduces the shear stress of the blood flow within the blood flow tube.
• FIG. 13 illustrates a portion of an alternate flow circulation system comprising afirst pulse generator620A, asecond pulse generator620B, an optionalthird pulse generator620C, and a control andpressurization unit622. The pulse generators620 are substantially identical in their operation to thepulse generators20 described above. They may have individual housings as described above, or they may take the form of chambers within a singlerigid housing624. In this configuration, thefirst pulse generator620A is activated so that it acts as a check valve during the output pulse cycle to prevent back flow from thesecond pulse generator620B. That is, the flow channel therein (not shown) is deflected to a degree that it is closed or nearly closed to flow therethrough when thesecond pulse generator620B is discharging. If thethird pulse generator620C is used, it is also activated in sequence with the first andsecond pulse generators620A and620B. For example, thethird pulse generator620C would be “open” when thesecond pulse generator620B is discharging. In this system, the pulse generators620 act as a pump and do not require a separate circulation pump to achieve the desired blood flow.
• The arrangements illustrated inFIGS. 12 and 13 have several advantages over flow systems using conventional check valves. Their operation induces low levels of stress to the blood to prevent the formation of mircobubbles during the suction cycle. A relatively small blood volume is required for suitable performance. For example, the individual chambers may having volume flow capacities from about 30 ml to about 150 ml. They also produce low thrombogenic response and low levels of platelet activation compared to conventional check valves. They also avoid fluttering of blood pressure (i.e. “hammering”) of flow in the outlet tubing.
• The pulse generators described herein have the ability to generate a blood pulse that can reside on top of a steady pump pulse during medical procedures such as heart transplant, kidney dialysis, and the like. This has the potential to relieve steady pressure induced brain disorders and to allow procedures to last longer, such as when physicians wish to keep the patient on a bypass system for several weeks to allow the patient's heart to rest and restore itself.
• The foregoing has described a pulse generator and a method of creating a pulsating flow in a fluid. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.

Claims (26)

1. A pulse generator for fluid flow, comprising:

(a) a substantially rigid body defining a pressure cavity;

(b) a resilient flow conduit extending through the pressure cavity and arranged to carry a first fluid therein; and

(c) a pressure inlet communicating with the pressure cavity for introducing a second fluid into the pressure cavity, such that the first fluid may be exposed to a time-varying pressure signal introduced through the pressure inlet without mixing of the first and second fluids.

2. The pulse generator ofclaim 1 in which the flow conduit is generally S-shaped.

3. The pulse generator ofclaim 1 in which the flow conduit has a four-sided cross-sectional shape defined by spaced-apart walls extending between radiused corners.

4. The pulse generator ofclaim 1 in which the flow conduit has a cross-sectional shape including a pair of spaced-apart lobes connected by a pinched waist.

5. The pulse generator ofclaim 1 in which the flow conduit has a cross-sectional shape including a pair of spaced-apart lobes connected by a pinched waist.

6. The pulse generator ofclaim 1 in which the flow conduit has a cross-sectional shape including a plurality of spaced-apart, radially-extending lobes.

7. The pulse generator ofclaim 1 in which the flow conduit has a cross-sectional shape including:

(a) a double-walled, generally C-shaped portion; and a plurality of spaced-apart, a flexible bridge connecting free ends of the C-shaped portion;

(b) wherein the C-shaped portion and the flexible bridge cooperate to define a central opening.

8. The pulse generator ofclaim 1 wherein the pressure cavity is shaped such that a pressure signal applied thereto will propagate symmetrically therein.

9. A pulse generator for fluid flow, comprising:

(a) a substantially rigid housing which defines an open cavity, the housing including an inlet and outlet communicating with the cavity;

(b) a flexible diaphragm which closes off the cavity; and

(c) an actuator operably connected to the diaphragm and adapted to selectively move the diaphragm from a first position in which the pulse generator encloses a first fluid volume, and a second position in which the pulse generator encloses a second fluid volume less than the first fluid volume.

10. The pulse generator ofclaim 9 in which the diaphragm forms a portion of a torus.

11. The pulse generator ofclaim 9 further including a programmable controller operably connected to the actuator.

12. A fluid circulation system, comprising:

(a) a circulation pump for a first fluid;

(b) a first pulse generator disposed in series flow with the circulation pump, the first pulse generator including:

(i) a substantially rigid first body defining a first pressure cavity;

(ii) a resilient first flow conduit extending through the first pressure cavity and arranged to carry the first fluid therein; and

(iii) a first pressure inlet communicating with the first pressure cavity for receiving a second fluid into the first pressure cavity, such that the first fluid may be exposed to a pressure signal received through the first pressure inlet without mixing of the first and second fluids; and

(c) a control mechanism operably connected to the first pressure inlet and adapted to introduce a pressure signal into the first pressure cavity so as to impress a predetermined time-varying pressure profile into the first fluid through the first flow conduit.

13. The fluid circulation system ofclaim 12 in which the circulation pump is a type generating a substantially constant output pressure in the first fluid.

14. The fluid circulation system ofclaim 12 in which the control mechanism includes:

(a) a source of pressurized fluid; and

(b) a valve operable to selectively transfer the pressurized fluid to the first pressure inlet.

15. The fluid circulation system ofclaim 12 in which the first pulse generator has a displacement capacity, and in which the control mechanism is adapted to control the circulation pump and the first pulse generator such that the first pulse generator operates in a preselected displacement range within the displacement capacity.

16. The fluid circulation system ofclaim 12 further including:

(a) a second pulse generator disposed in series flow with the circulation pump and the first pulse generator, the second pulse generator including:

(i) a substantially rigid second body defining a second pressure cavity;

(iii) a resilient second flow conduit extending through the second pressure cavity and arranged to carry the first fluid therein; and

(b) a second pressure inlet communicating with the second pressure cavity for receiving a second fluid into the second pressure cavity, such that the first fluid may be exposed to a pressure signal received through the second pressure inlet without mixing of the first and second fluids;

wherein the control mechanism is operable to selectively pressurize the second pulse generator to a degree so as to substantially close off flow through the second flow conduit.

17. The fluid circulation system ofclaim 12 further comprising equipment for treatment of a bodily fluid connected in series flow therewith.

18. A fluid circulation system, comprising:

(a) a first pulse generator including:

(i) a substantially rigid first body defining a first pressure cavity;

(ii) a resilient first flow conduit extending through the first pressure cavity and arranged to carry the first fluid therein; and

(iii) a first pressure inlet communicating with the first pressure cavity for receiving a second fluid into the first pressure cavity, such that the first fluid may be exposed to a pressure signal received through the first pressure inlet without mixing of the first and second fluids; and

(b) a second pulse generator connected in series flow with the first pulse generator, and including:

(i) a substantially rigid second body defining a second pressure cavity;

(ii) a resilient second flow conduit extending through the second pressure cavity and arranged to carry the first fluid therein; and

(iii) a second pressure inlet communicating with the second pressure cavity for receiving a second fluid into the second pressure cavity, such that the first fluid may be exposed to a pressure signal received through the second pressure inlet without mixing of the first and second fluids; and

(c) a control mechanism operably connected to the first and second pressure inlets and adapted to:

(i) cyclically pressurize one of the first pulse generator so as to impress a predetermined time-varying pressure profile into the first fluid; and

(ii) cyclically pressurize the second pulse generator to a degree so as to substantially close off flow through the second flow conduit in coordination with the pressurization of the first pulse generator, such that the first fluid is moved through the fluid circulation system in a single direction.

19. The fluid circulation system ofclaim 18 further comprising:

(a) a third pulse generator connected in series flow with the first and second pulse generators, and including:

(i) a substantially rigid third body defining a third pressure cavity;

(ii) a resilient third flow conduit extending through the third pressure cavity and arranged to carry the first fluid therein; and

(iii) a third pressure inlet communicating with the third pressure cavity for receiving a second fluid into the third pressure cavity, such that the first fluid may be exposed to a pressure signal received through the third pressure inlet without mixing of the first and second fluids; and

(b) a control mechanism operably connected to the third pressure inlet and adapted to selectively pressurize the third pulse generator to a degree so as to substantially close off flow through the third flow conduit.

20. The fluid circulation system ofclaim 18 further comprising equipment for treatment of a bodily fluid connected in series flow therewith.

21. A method of generating a fluid pulse, comprising:

(a) passing a first fluid through a resilient first flow conduit arranged to carry the first fluid therein; and

(b) while the first fluid is in the first flow conduit, introducing a pressurized second fluid into a first pressure cavity defined by a substantially rigid first body surrounding the first flow conduit, as to impress a predetermined time-varying pressure profile into the first fluid through the first flow conduit.

22. The method ofclaim 21, further comprising:

passing the first fluid through a circulation pump connected in series flow relationship with the first flow conduit so as to impress a substantially constant pressure component in the first fluid.

23. The method ofclaim 21, further comprising:

(a) passing the first fluid through a resilient second flow conduit arranged to carry the first fluid therein;

(b) introducing a pressurized second fluid into a second pressure cavity defined by a substantially rigid second body surrounding the second flow conduit;

wherein the second flow conduit is cyclically compressed to a degree so as to substantially close off flow through the second flow conduit in coordination with the pressurization of the fluid in the first flow conduit, such that the first fluid is moved through the first and second flow conduits in a single direction.

24. The method ofclaim 21, wherein the first fluid is a bodily fluid, further comprising passing the first fluid through equipment for treatment thereof.

25. The method ofclaim 21, further comprising heating or cooling the first fluid as it passes through the first flow conduit.

26. The method ofclaim 21, wherein the second fluid is at a lower pressure than the first fluid during at least a portion of the pressure profile.

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