US20110022013A1 - Negative pressure assisted tissue treatment system - Google Patents

Abstract

A system for stimulating the healing of tissue comprises a porous pad positioned within a wound cavity, and an airtight dressing secured over the pad, so as to provide an airtight seal to the wound cavity. A proximal end of a conduit is connectable to the dressing. A distal end of the conduit is connectable to a negative pressure source, which may be an electric pump housed within a portable housing, or wall suction. A canister is positioned along the conduit to retain exudates suctioned from the wound site during the application of negative pressure. A first hydrophobic filter is positioned at an opening of the canister to detect a canister full condition. A second hydrophobic filter is positioned between the first filter and the negative pressure source to prevent contamination of the non-disposable portion of the system by exudates being drawn from the wound. An odor filter is positioned between the first and second hydrophobic filters to aid in the reduction of malodorous vapors. A securing means is supplied to allow the portable housing to be secured to a stationary object, such as a bed rail or intravenous fluid support pole. A means for automated oscillation of pressure over time is provided to further enhance and stimulate the healing of an open wound. A means for varying pump drive frequency and a means for managing a portable power supply are provided to increase battery life and improve patient mobility.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 11/364,264, filed Feb. 28, 2006, which is a continuation of U.S. patent application Ser. No. 09/939,166 filed Aug. 24, 2001, now U.S. Pat. No. 7,004,915, which claims priority to International Application No. PCT/US2002/027070, filed Aug. 23, 2002, which designated the United States, all of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This invention relates generally to tissue treatment systems. More particularly this invention relates to vacuum assisted treatment systems that aid in the healing of open wounds.
• 2. Description of Related Art
• Vacuum induced healing of open wounds has recently been popularized by Kinetic Concepts, Inc. of San Antonio, Tex., by its commercially available V.A.C.® produce line. The vacuum induced healing process has been described in commonly assigned U.S. Pat. No. 4,969,880 issued on Nov. 13, 1990 to Zamierowski, as well as its continuations and continuations-in-part, U.S. Pat. No. 5,100,396, issued on Mar. 31, 1992, U.S. Pat. No. 5,261,893, issued Nov. 16, 1993, and U.S. Pat. No. 5,527,293, issued Jun. 18, 1996, the disclosures of which are incorporated herein by this reference. Further improvements and modifications of the vacuum induced healing process are also described in U.S. Pat. No. 6,071,267, issued on Jun. 6, 2000 to Zamierowski and U.S. Pat. Nos. 5,636,643 and 5,645,081 issued to Argenta et al. on Jun. 10, 1997 and Jul. 8, 1997, respectively, the disclosures of which are incorporated by reference as through fully set forth herein. Additional improvements have also been described in U.S. Pat. No. 6,142,982, issued on May 13, 1998 to Hunt, et al.
• In practice, the application to a wound of negative gauge pressure, commercialized by Assignee or its parent under the designation “Vacuum Assisted Closure” (or “V.A.C.®”) therapy, typically involves the mechanical-like contraction of the wound with simultaneous removal of excess fluid. In this manner, V.A.C.® therapy augments the body's natural inflammatory process while alleviating many of the known intrinsic side effects, such as the production of edema caused by increased blood flow absent the necessary vascular structure for proper venous return. As a result, V.A.C.® therapy has been highly successful in the promotion of wound closure, healing many wounds previously through largely untreatable.
• The frequency at which negative pressure is applied to the wound, as well as the frequency of the pressure change over time, ahs a direct impact on the rate of wound healing. A variation of pressure change over time, not provided by current vacuum assisted therapy devices, is thought to significantly increase the rate of wound healing. Similarly, a rapid return to normal activities for the patient receiving wound therapy, may also improve the rate of wound healing, as increased physical activity is often accompanied by increased vascular circulation, which in turn leads to improved blood flow at the wound site. One barrier to a return to normal activities is limited battery life, which is a result of the electrical power required to power existing vacuum assisted wound therapy systems. Additionally, frequent inspection of the wound site is required in order to ensure the wound is not becoming infected. However, a rapid return to normal activities must not preclude the precautions that must be utilized during use of vacuum assisted therapy to prevent inadvertent spillage of wound exudates from the canister, or entry of the wound exudates into the pumping mechanism.
• Additional limitations are associated with the use of fixed frequency oscillating pumps in the prior art. Such limitations are the result of the size of the pump required to maintain the desired negative pressure at the wound site, and/or a reduction in battery life due to the power required to operate the oscillating pumps. Oscillating pumps, as known in the art, are typically designed for limiting operating conditions. For example, to maximize low pressure flow rate at a fixed frequency. Typically the mass and/or stiffness of various components are altered to change the resonant frequency of the pump under the design operating conditions. If the pressure across the pump increases, the stiffness of the system is increased by back pressure across the diaphragm of the oscillating pump. The resonant frequency of the pump changes and the fixed frequency drive is not driving the pump at the optimum frequency. As a result, flow rate drops quickly and the capability of the pump to drive air at high pressure is limited. Accordingly, in order to provide increased flow rate at higher pressures requires either a sacrifice in flow rate at low pressures, or a pump of significantly greater size, when utilizing a fixed frequency oscillating pump.
• For the foregoing reasons, there is a need for a vacuum assisted wound treatment system that is capable of automated pressure change over time. Additionally, there is a need for a more efficient vacuum assisted wound treatment system, that allows the patient more mobility, while reducing the risk of exudate spillage or pump contamination.
• It is therefore an object of the present invention to provide a vacuum assisted wound treatment system that provides a means for increasing the stimulation of cellular growth by a variation of pressure over time.
• A further object is to provide a system that is capable of extended operation in the absence of an alternate current power supply.
• An additional object of the present invention is to provide a sanitary and cost effective means for sampling fluids drawn from the wound site without necessitating removal of the canister, or disturbing of the wound site.
• Still another object of the present invention is to provide a vacuum assisted wound therapy device that can be secured to an object so as to reduce the likelihood of disturbance to the device, while still allowing convenient placement for its operation.
BRIEF SUMMARY OF THE INVENTION
• In accordance with the foregoing objects, the present invention generally comprises a porous pad for insertion substantially into a wound site and a wound drape for air-tight sealing enclosure of the pad at the wound site. A distal end of the a tube is connected to the dressing in order to provide negative pressure at the wound site. A fluid sampling port is provided on the tube to allow for sampling of wound fluids being drawn through the tube from the wound site. A source of negative pressure is in communication with a proximal end of the tube. A collection canister is removably connected to the tube for collection of fluid removed from the wound during the application of negative pressure. A first filter is incorporated into an opening of the canister, and a second filter is positioned between the canister and the source of negative pressure. As the source of negative pressure may be an electric pump, supplied by alternating or direct current, a power management device, and its associated power management protocol, is incorporated to maximize battery life when the unit is being supplied by direct current. A clamping mechanism is utilized to secure the system to a stationary object, such as a bed rail, or pole, such as that used to suspend a container of intravenous fluid.
• The pad, comprised of a foam having relatively few open cells in contact with the areas upon which cell growth is to be encouraged so as to avoid unwanted adhesions, but having sufficiently numerous open cells so that drainage and negative pressure therapy may continue unimpaired, is placed in fluid communication with a vacuum source for promotion of fluid drainage, as known in the art. The porous pad of the present invention may be comprised of polyvinyl alcohol foam. The fluid communication may be established by connecting a tube to a dressing, such as that described in International Application WO 99/13792, entitled “Surgical Drape and Suction Heads for Wound Treatment,” the disclosure of which is incorporated herein.
• Upon placement of the pad, an airtight seal is formed over the wound site to prevent vacuum leakage. Such a seal may be provided by placing a drape over the wound, such that the drape adheres to the healthy skin surrounding the wound site, while maintaining an airtight seal over the wound itself.
• A conduit or tube is placed in fluid communication with the foam pad, its distal end communicating with a fluid drainage canister which is in fluid communication with a vacuum source. A constant or intermittent negative pressure therapy is conducted as described in the prior art. Alternatively, the negative pressure is varied over time, so as to further stimulate cell growth, which in turn may shorten the healing process. The negative pressure induced on the wound adjusts to meet a varying target pressure, which oscillates between a target maximum and target minimum pressure.
• Flow rate of a variable displacement pump, used in accordance with the present invention, is maximized over a pressure range by varying the drive frequency of the pump. The optimum drive frequency is continuously adjusted by a system that periodically or continuously monitors the pressure across the pump to determine the optimum drive frequency for that pressure. Pump performance is thereby improved over variable displacement pumps utilized in the prior art, without increasing pump size or weight. Similarly, pump performance of a typical variable displacement pump can be achieved with a smaller pump, which in turn reduces the size and weight of the overall system in order to improve ease of use and portability for the patient. An alternative negative pressure source, such as a fixed displacement pump, sometimes referred to as a positive displacement pump, may also be utilized.
• The power management system is utilized to maximize battery life when the present invention is being supplied with electric power under direct current. The power management system comprises deactivation of a backlight to a display terminal, or touch screen liquid crystal display (LCD) control panel, after a predetermined interval. Battery life is further extended when the power management system prevents electric power from reaching an electric motor until the targeted power setting is actually large enough to activate the motor. In such an instance, the motor is utilized to provide negative pressure by driving an electric pump as known in the art.
• The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention as will be described. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the following Detailed Description of the Invention, which includes the preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
• These and other features and advantages of the invention will now be described with reference to the drawings of certain preferred embodiments, which are intended to illustrate and not to limit the invention, and wherein like reference numbers refer to like components, and in which:
• FIG. 1 is a schematic block diagram of a tissue treatment system utilized in accordance with the present invention.
• FIG. 2A is a perspective view of a fluid sampling port utilized in accordance with the present invention.
• FIG. 2B is a perspective view of an alternative embodiment of a fluid sampling port utilized in accordance with the present invention.
• FIG. 3A is a perspective view of the back portion of a pump housing utilized in accordance with the present invention.
• FIG. 3B is a perspective view of the front portion of a pump housing utilized in accordance with the present invention.
• FIGS. 4A and 4B are flow charts representing the preferred steps in the implementation of a power management system utilized in accordance with the present invention.
• FIG. 5 is a flow chart illustrating the preferred steps in the implementation of pulse therapy utilized in accordance with the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
• Although those of ordinary skill in the art will readily recognize many alternative embodiments, especially in light of the illustrations provided herein, this detailed description is exemplary of the preferred embodiment of the present invention, the scope of which is limited only by the claims that are drawn hereto.
• The present invention is a vacuum assisted system for stimulating the healing of tissue.
• Referring now toFIG. 1 in particular, there is illustrated the primary components of a system that operates in accordance with the present invention. Thepresent invention10 includes afoam pad11 for insertion substantially into awound site12 and awound drape13 for sealing enclosure of thefoam pad11 at thewound site12. Thefoam pad11 may be comprised of a polyvinyl alcohol (PVA) open cell polymer material, or other similar material having a pore size sufficient to facilitate wound healing. A pore density of greater than 38 pores per linear inch is preferable. A pore density of between 40 and 45 pores per linear inch is most preferable. Such a pore density translates to a pore size of approximately 400 microns.
• Addition of an indicating agent, such as crystal violet, methylene blue, or similar agents known in the art causes a color change in thefoam11 when in the presence of a bacterial agent. As such, a user or health care provider can easily and readily ascertain if an infection is present at thewound site12. It is contemplated that the indicating agent may also be placed in line of theconduit16, between thewound site12 and thecanister18. In such a configuration (not shown), the presence of bacterial contaminants in thewound site12, could be easily and readily ascertained without disturbing the wound bed, as there would be a nearly immediate color change as bacterially infected wound exudates are drawn from thewound site12 and through theconduit16 during application of negative pressure.
• It is also contemplated that thefoam pad11 may be coated with a bacteriostatic agent. Addition of such an agent, would serve to limit or reduce the bacterial density present at thewound site12. The agent may be coated or bonded to thefoam pad11 prior to insertion in the wound site, such as during a sterile packaging process. Alternatively, the agent may be injected into thefoam pad11 after insertion in thewound site12.
• After insertion into thewound site12 and sealing with thewound drape13, thefoam pad11 is placed in fluid communication with avacuum source14 for promotion of fluid drainage and wound healing, as known to those of ordinary skill in the art. Thevacuum source14 may be a portable electrically powered pump, or wall suction as commonly provided in medical care facilities.
• According to the preferred embodiment of the present invention, thefoam pad11, wounddrape13, andvacuum source14 are implemented as known in the prior art, with the exception of those modifications detailed further herein.
• Thefoam pad11 preferably comprises a highly reticulated, open-cell polyurethane or polyether foam for effective permeability of wound fluids while under suction. Thepad11 is preferably placed in fluid communication, via a plastic or likematerial conduit16, with acanister18 and avacuum source14. A firsthydrophobic membrane filter20 is interposed between thecanister18 and thevacuum source14, in order to prevent wound exudates from contaminating thevacuum source14. Thefirst filter20 may also serve as a fill-sensor forcanister18. As fluid contacts thefirst filter20, a signal is sent to thevacuum source14, causing it to shut down. The wound drape13 preferably comprises an elastomeric material at least peripherally covered with a pressure sensitive adhesive for sealing application over thewound site12, such that a vacuum seal is maintained over thewound site12. Theconduit16 may be placed in fluidic communication with thefoam11 by means of anappendage17 that can be adhered to thedrape13.
• According to the preferred method of the present invention, a secondhydrophobic filter22 is interposed between thefirst filter20 and thevacuum source14. The addition of thesecond filter22 is advantageous when thefirst filter20 is also used as a fill sensor for thecanister18. In such a situation, thefirst filter20 may act as a fill sensor, while thesecond filter22 further inhibits contamination of wound exudates into thevacuum source14. This separation of functions in a safety device and a control (or limiting) device, allows for each device to be independently engineered. Anodor vapor filter23, which may be a charcoal filter, may be interposed between thefirst filter20 and thesecond filter22, in order to counteract the production of malodorous vapors present in the wound exudates. In an alternate embodiment (not shown), theodor vapor filter23 may be interposed between the secondhydrophobic filter23 and thevacuum source14. Asecond odor filter15 may be interposed between thevacuum source14 and anexternal exhaust port25, in order to further reduce the escape of malodorous vapors from the present system. A further embodiment allows for first20 andsecond filters22 to be incorporated as an integral part of thecanister18 to ensure that thefilters20,22, at least one of which are likely to become contaminated during normal use, are automatically disposed of in order to reduce the exposure of the system to any contaminants that may be trapped by thefilters20 and22.
• A means for sampling fluids may also be utilized by providing aresealable access port24 from theconduit16. Theport24 is positioned between thedistal end16aof theconduit16 and the proximal end16bof theconduit16. Theport24, as further detailed inFIGS. 2A and 2B, is utilized to allow for sampling of fluids being suctioned from thewound site12. Although theport24 is shown as an appendage protruding from theconduit16, it is to be understood that a flush mounted port (not shown) will serve an equivalent purpose. Theport24 includes aresealable membrane26 that after being punctured, such as by a hypodermic needle, the seal is maintained. Various rubber-like materials known in the art for maintaining a seal after puncture can be utilized.
• The process by which wound fluids are sampled, utilizing the present invention, comprises penetrating themembrane26 with afluid sampler28, such as a hypodermic needle or syringe. Thesampler28 is inserted through themembrane26 and into theport24 until it is in contact with wound fluids flowing through theinner lumen30 of theconduit16. As illustrated inFIG. 2B, and further described in U.S. Pat. No. 6,142,982, issued to Hunt, et al. on May 13, 1998, and whose reference is incorporated herein as though fully set forth, theinner lumen30 may be surrounded by one or moreouter lumens31. Theouter lumens31 may serve as pressure detection conduits for sensing variations in pressure at thewound site12. In an alternative embodiment (not shown), the outer lumen orlumens31 may act as the negative pressure conduit, while theinner lumen30 may act as the pressure detection conduit. In the present invention, thefluid sampling port24, communicates only with theinner lumen30, so as not to interfere with pressure detection that may be conducted by theouter lumens31. In an alternate embodiment (not shown) in which theouter lumen31 serves as the negative pressure conduit, thefluid sampling port24 communicates with theouter lumen31.
• Thevacuum source14 may consist of a portable pump housed within ahousing32, as illustrated inFIGS. 3A and 3B. Ahandle33 may be formed or attached to thehousing32 to allow a user to easily grasp and move thehousing32.
• According to the preferred embodiment of the present invention, a means for securing thehousing32 to a stationary object, such as an intravenous fluid support pole for example, is provided in the form aclamp34. Theclamp34, which may be a G-clamp as known in the art, is retractable, such that when not in use is in a stored position within arecess36 of thehousing32. Ahinging mechanism38 is provided to allow theclamp34 to extend outward from thehousing32, to up to a 90 degree angle from its stored position. An alternative embodiment (not shown) allows theclamp34 to be positioned at up to a 180 degree angle from its stored position. Thehinging mechanism38 is such that when theclamp34 is fully extended, it is locked in position, such that thehousing32 is suspended by theclamp34. A securingdevice40, such as a threaded bolt, penetrates through anaperture42 of theclamp34, to allow theclamp34 to be adjustably secured to various stationary objects of varying thickness.
• Alternatively, the securingdevice40, may be comprised of a spring actuated bolt or pin, that is capable of automatically adjusting to various objects, such as intravenous fluid support poles, having varying cross-sectional thickness.
• The present invention also allows for management of a power supply to thevacuum source14, in order to maximize battery life when the present invention is utilizing a direct current as its power supply. In the preferred embodiment, as illustrated in the flow chart ofFIG. 4A, amotor control44 determines if the actual pressure is less than or equal to atarget pressure46. If the actual pressure is less than the target pressure, a tentative motor drive power required to reach the target pressure is calculated48. If the tentative motor drive power required to reach the target pressure is greater or equal to thestall power49, the tentative motor drive power is actually applied to themotor50. If the actual pressure is greater than the target pressure, the tentative motor drive power is decreased and a determination is made as to whether additional power is needed to overcome thestall power52. If it is determined that the tentative power is inadequate to overcome the stall power, the tentative power is not supplied to themotor54. If the tentative power is adequate to overcome the stall power, the tentative power is actually applied to themotor50. Themotor control44 functions as a closed loop system, such that the actual pressure is continuously measured against the predetermined target pressure. The advantage of such a system is that it prevents power from being supplied to the motor when it is not necessary to maintain the target pressure specified for V.A.C.® therapy. Accordingly, battery life is extended because power is not needlessly used to power the motor when it is not necessary.
• Battery life is further extended, as illustrated in the flow chart shown inFIG. 4B, by providing a means, such as an integrated software program in a computer processor, for automatically disengaging a backlight of thevisual display19 of the present invention10 (as seen inFIG. 3B). User input ofinformation55, such as target pressure desired, or duration of therapy, activates57 a backlight of thevisual display19 shown inFIG. 3B.User input55 may also be simply touching thevisual display19, which may be a touch activated or a pressure sensitive screen as known in the art. Activation of analarm55 may also activate57 the backlight of thedisplay19. An alarm may be automatically activated if an air leak is detected at thewound site12. Such a leak may be indicated by a drop of reduction in pressure being detected at thewound site12. The backlight remains active until a determination is made as to whether a preset time interval has elapsed58. If the time interval has not elapsed, the backlight remains active57. If the time interval has elapsed, the backlight is automatically extinguished59, until such time as the user inputs additional information, or an alarm is sounded55.
• Referring now back toFIG. 1, battery life is further extended by means of a variable frequencypump drive system80, when thepump14, used in accordance with the present invention, is an oscillating pump. Thepump drive system80 consists of apressure sensor82, acontrol system84, and a variablefrequency drive circuit86. In the preferred embodiment thepressure sensor82 measures the pressure across the pump, which is relayed to thecontrol system84. Thecontrol system84 determines the optimum drive frequency for thepump14 given the pressure measured and relayed by thepressure sensor82. The optimum drive frequency for thepump14 may be determined by thecontrol system84 either repeatedly or continuously. Thecontrol system84 adjusts the variablefrequency drive circuit86 to drive the pump at the optimum frequency determined by thecontrol system84.
• The use of the variable frequencypump drive system80 allows the pressure of thepump14 to be maximized. In tests on sample oscillating pumps, the maximum pressure achieved was doubled by varying the drive frequency by only 30%. Additionally, thesystem80 maximizes flow rate over the extended frequency range. As a result, performance of thepump14 is significantly improved over existing fixed frequency drive system pumps without increasing the pump size or weight. Consequently, battery life is further extended, thus giving the user greater mobility by not having to be tethered to a stationary power source. Alternatively, a similar performance level to the prior art fixed frequency drive system pumps can be achieved with a smaller pump. As a result, patient mobility is improved by improving the portability of the unit.
• The preferred embodiment also increases the stimulation of cellular growth by oscillating the pressure over time, as illustrated in the flow chart ofFIG. 5. Such as oscillation of pressure is accomplished through a series of algorithms of a software program, utilized in conjunction with a computer processing unit for controlling the function of the vacuum source or pump. The program is initialized when a user, such as a health care provider, activates the pulsing mode of thepump60. The user then sets a target pressure maximum peak value and a target pressureminimum peak value62. The software then initializes the pressure direction to “increasing”63. The software then enters a software control loop. In this control loop, the software first determines if the pressure is increasing64.
• If the actual pressure is increasing intest64, a determination is then made as to whether a variable target pressure is still less than themaximum target pressure70. If the variable target pressure is still less than the maximum target pressure the software next determines whether the actual pressure has equaled (risen to) the ascendingtarget pressure66. If the actual pressure has attained the ascending target pressure, the software increments the variable target pressure by oneinterval68. Otherwise, it refrains from doing so until the actual pressure has equaled the ascending target pressure. If the variable target pressure has reached the maximum target pressure in thetest block70 the software sets the pressure direction to “decreasing”69 and the variable target pressure beings to move into the downward part of its oscillatory cycle.
• The interval may be measured in mmHg or any other common unit of pressure measurement. The magnitude of the interval is preferably in the range of about 1 to 10 mmHg, according to the preference of the user.
• If the actual pressure is decreasing intest64, a determination is then made as to whether the variable target pressure is still greater than theminimum target pressure74. If the variable target pressure is still greater than the minimum target pressure the software next determines whether the actual pressure has attained (fallen to) to the descendingtarget pressure76. If the actual target pressure has equaled the descending target pressure the software decrements the variable target pressure by oneinterval72. Otherwise it refrains from doing so until the actual pressure has equaled the descending target pressure. If the variable target pressure has reached the minimum target pressure in the test ofblock74, the software sets the pressure direction to “increasing”73 and the variable target pressure beings to move into the upward part of its oscillatory cycle. This oscillatory process continues until the user de-selects the pulsing mode.
• While the invention has been described herein with reference to certain preferred embodiments, these embodiments have been presented by way of example only, and not to limit the scope of the invention. Accordingly, the scope of the invention should be identified only in accordance with the claims that follow.

Claims (15)

1. A system for stimulating healing of tissue at a wound site comprising:

a porous pad;

a pump for delivering an actual pressure to apply as negative pressure to the wound site through said porous pad; and

a controller for varying the negative pressure by determining whether the actual pressure is increasing or decreasing and comparing the actual pressure to a target pressure and, when the actual pressure has equaled the target pressure, moving the target pressure toward (a) a maximum target pressure if the actual pressure is increasing, or (b) a minimum target pressure if the actual pressure is decreasing.

2. The system ofclaim 1, wherein the controller moves the target pressure by incrementing the target pressure toward the maximum target pressure when the actual pressure is increasing and equal to or greater than the target pressure.

3. The system ofclaim 2, wherein the actual pressure is increasing and said controller is adapted to compare the target pressure to the maximum target pressure and begin decreasing the actual pressure when the target pressure is equal to or greater than the maximum target pressure.

4. The system ofclaim 3, wherein said controller is further adapted to decrement the target pressure toward the minimum target pressure when the actual pressure is equal to or less than the target pressure.

5. The system ofclaim 4, wherein said controller is further adapted to compare the target pressure to the minimum target pressure and begin increasing the actual pressure when the target pressure is equal to or less than the minimum target pressure, whereby the negative pressure oscillates over time between the maximum target pressure and the minimum target pressure.

6. The system ofclaim 1, wherein the controller decrements the target pressure toward the minimum target pressure when the actual pressure is decreasing and equal to or less than the target pressure.

7. The system ofclaim 6, wherein the actual pressure is decreasing and said controller is adapted to compare the target pressure to the minimum target pressure and begin increasing the actual pressure when the target pressure is equal to or less than the minimum target pressure.

8. The system ofclaim 7, wherein said controller is further adapted to increment the target pressure toward the maximum target pressure when the actual pressure is equal to or greater than the target pressure.

9. The system ofclaim 8, wherein said controller is further adapted to compare the variable target pressure to the maximum target pressure and begin decreasing the actual pressure when the target pressure is equal to or greater than the maximum target pressure, whereby the negative pressure oscillates over time between the maximum target pressure and the minimum target pressure.

10. A system for stimulating healing of tissue at a wound site comprising:

a porous pad;

a pump for delivering an actual pressure to apply as negative pressure to the wound site through said porous pad; and

a controller for varying the negative pressure by comparing the actual pressure to a target pressure and, (a) if the actual pressure is increasing, incrementing the target pressure toward a maximum target pressure when the actual pressure is equal to or greater than the target pressure, or (b) if the actual pressure is decreasing, decrementing the target pressure toward a minimum target pressure when the actual pressure is equal to or less than the target pressure.

11. The system ofclaim 10, wherein the actual pressure is increasing and said controller is further adapted to compare the target pressure to the maximum target pressure and begin decreasing the actual pressure when the target pressure is equal to or greater than the maximum target pressure.

12. The system ofclaim 10, wherein the actual pressure is decreasing and said controller is further adapted to compare the target pressure to the minimum target pressure and begin increasing the actual pressure when the target pressure is equal to or less than the minimum target pressure.

13. The system ofclaim 10, wherein said controller further adapted to decrease the actual pressure when the target pressure is equal to or greater than the maximum target pressure and increase the actual pressure when the target pressure is equal to or less than the minimum target pressure, whereby the negative pressure oscillates over time between the maximum target pressure and the minimum target pressure.

14. A system for stimulating healing of tissue at a wound site comprising:

a porous pad;

a pump for delivering an actual pressure to apply as negative pressure to the wound site through said porous pad; and

a controller for varying the negative pressure by determining whether the actual pressure is increasing or decreasing and comparing the actual pressure to a target pressure, and (a) if the actual pressure is increasing and equal to or greater than the target pressure, incrementing the target pressure toward a maximum target pressure, or (b) if the actual pressure is decreasing and equal to or less than the target pressure, decrementing the target pressure toward a minimum target pressure.

15. The system ofclaim 14, wherein said controller is further adapted to (a) begin decreasing the actual pressure when the target pressure is equal to or greater than the maximum target pressure, or (b) begin increasing the actual pressure when the target pressure is equal to or less than the minimum target pressure, whereby the negative pressure oscillates over time between the maximum target pressure and the minimum target pressure.

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