Device for controlling leaks in reduced pressure treatment systems

Abstract

In one exemplary embodiment, a system for stimulating tissue healing at a wound site is disclosed. The system includes a dressing including a porous pad and a drape for covering the pad at the wound site and maintaining negative pressure at the wound site, a source of negative pressure for application to the wound site including a pump and an electric motor for generating pump pressure, and a first pressure sensor for sensing the pump pressure. The system further includes a controller coupled to the first pressure sensor and the electric motor, and a regulator coupled to the controller for inducing leakage into a treatment environment of the system and for maintaining a minimum level of fluid flow within the dressing.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 062,218, filed August 6, 2020, which is incorporated herein by reference in its entirety.

[0002] The present invention as recited in the accompanying claims relates generally to tissue treatment systems and, more particularly, but not exclusively, to leak control in negative pressure wound treatment systems. [Background technology]

[0003] Clinical studies and clinical practice have shown that reducing pressure in the vicinity of a tissue site can enhance and accelerate the growth of new tissue at the tissue site. While the applications of this phenomenon are numerous, it has proven particularly advantageous for treating wounds. Regardless of the cause of the wound, whether traumatic, surgical, or otherwise, proper care of the wound is critical to the outcome. Treating wounds or other tissues with reduced pressure may generally be referred to as "negative pressure therapy," but is also known by other names, including, for example, "negative pressure wound therapy," "reduced pressure therapy," "vacuum therapy," and "vacuum-assisted closure." Negative pressure therapy can provide many benefits, including epithelial and subcutaneous tissue migration, improved blood flow, and minimal tissue deformation at the wound site. Collectively, these benefits can increase granulation tissue development and reduce healing time.

[0004] While the clinical benefits of negative pressure therapy are widely known, the cost and complexity of negative pressure therapy can be factors that limit its application, and the development and operation of negative pressure systems, negative pressure components, and negative pressure processes continue to present significant challenges to manufacturers, healthcare providers, and patients. Summary of the Invention

[0005]

[0006] Novel and useful systems, devices, and methods for maintaining negative pressure in low-leak and high-leak conditions in a negative pressure therapy environment are set forth in the accompanying claims. Exemplary embodiments are also provided to enable those skilled in the art to make and use the claimed subject matter.

[0006] For example, reduced pressure delivered by a reduced pressure treatment system to a tissue site, such as an incision or wound, may need to be appropriately controlled to increase the effectiveness of the reduced pressure treatment. The reduced pressure treatment system may include a pump for delivering the reduced pressure, a wound dressing positioned adjacent to the wound, and a drape covering both and providing a confined environment from the pump to the confined environment for providing reduced pressure treatment. However, leakage may occur in the dressing and other components of the reduced pressure treatment system, for example, leakage between the drape and the tissue site, i.e., system leakage. System leakage may be high or low, depending on the fluid flow rate within the system, particularly near the dressing. In a low system leakage, it may be necessary to monitor and control the flow rate to ensure that it does not fall below a minimum value, i.e., minimum flow rate, to maintain effective treatment of the wound at the tissue site. If the flow rate falls below the minimum flow rate, the system may be configured to increase the flow rate by inducing leakage of air from the external environment into the treatment environment of the dressing to maintain effective treatment of the wound.

[0007] In one exemplary embodiment, a system for promoting wound healing at a tissue site includes a dressing configured to be placed at the tissue site and adapted to be covered by a drape to form a therapeutic environment isolated from the external environment for maintaining a wound pressure (WP) at the tissue site. The system may further include a negative pressure source including a pump adapted to generate a pump pressure (PP) and further adapted to be fluidly coupled to the porous pad to apply a negative pressure to the tissue site. The system may further include a flow sensor having an input fluidly coupled between the pump and the porous pad and an output for providing a flow signal representative of a fluid flow rate (FR) indicative of leakage between the pump and the porous pad. The system may further include a controller having an input coupled to the output of the flow sensor and an output. In some embodiments, the controller may be configured to determine the flow rate (FR) based on the flow signal and then compare the measured flow rate (FR) to a minimum flow rate (MinFR). The controller may be further configured to generate a low leakage signal at the output of the controller when the flow rate (FR) is less than the minimum flow rate (MinFR). The system may further include a regulator coupled to the output of the controller that may be adapted to increase the flow rate (FR) by increasing leakage between the pump and the porous pad in response to the occurrence of a low leakage signal.

[0008] In some exemplary embodiments, the flow sensor may comprise a first pressure sensor having a first input for sensing a pump pressure (PP) and a first output for providing a signal indicative of the pump pressure (PP), and a second pressure sensor having a second input for sensing a wound pressure (WP) and a second output for providing a signal indicative of the wound pressure (WP). The controller may be electrically coupled to the first output of the first pump and the second output of the second pump and may be further configured to determine the flow rate (FR) based on a difference between the pump pressure (PP) and the wound pressure (WP).

[0009] In one exemplary embodiment, a method for stimulating wound healing at a tissue site may include placing a porous pad at the tissue site and covering the porous pad with a drape to form a therapeutic environment isolated from the external environment to maintain a wound pressure (WP) at the tissue site. The method may further include applying a negative pressure to the porous pad using a pump to generate a pump pressure (PP) for applying the negative pressure to the tissue site. The method may further include determining a fluid flow rate (FR) between the pump and the porous pad indicative of a leak, providing a flow rate signal, and then comparing the flow rate (FR) to a minimum flow rate (MinFR). If the measured flow rate (FR) is less than the minimum flow rate (MinFR), the method further includes generating a low leak signal and, in response to the generation of the low leak signal, increasing the flow rate (FR) by increasing leakage between the pump and the porous pad.

[0010] The objects, advantages and preferred modes of making and using the claimed subject matter will be best understood by referring to the accompanying drawings in conjunction with the following detailed description of illustrative embodiments. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a functional block diagram of an example embodiment of a reduced pressure treatment system according to the present disclosure, including a controller coupled to a pump motor and a pump capable of providing hybrid control of pressure delivered to a tissue site and control of leakage in a reduced pressure treatment system. [Figure 2] FIG. 2 is a graph illustrating the stall voltage characteristics of a pump motor that may be used in the reduced pressure treatment system of FIG. 1, where the x-axis represents the vacuum pressure load on the pump motor and the y-axis represents the stall voltage. [Figure 3]3 is a graph illustrating pressure control of a motor-driven system according to an example of an exemplary embodiment. The x-axis represents time in minutes (min) and / or seconds (sec). The y-axis represents pressure generated by a pump in Torr (mmHg) varying over time in continuous control mode and intermittent mode, which may be used in the reduced pressure treatment system of FIG. 1. [Figure 4] 4 is a graph illustrating pressure control of a motor drive system according to an example of an exemplary embodiment. The x-axis represents time in minutes (min) and / or seconds (sec). The y-axis represents pressure generated by the pump in Torr (mmHg), comparing a manipulated variable, such as wound pressure (WP) at or at the tissue site, to a controlled variable, such as pump pressure (PP), for use in a PID controller and / or bang-bang controller. [Figure 5A] 5A is a graph illustrating pressure control of a bang-bang controller according to an example embodiment. The x-axis represents time in seconds (sec) and the y-axis represents pressure in Torr (mmHg) generated by the pump as it changes over time in continuous control mode. The pressure control of the bang-bang controller is affected by the head pressure generated by the reduced pressure treatment system of FIG. 1 shown in FIG. 5A, which is greater than the head pressure shown in FIG. 5B, which is smaller. [Figure 5B] 5B is a graph illustrating pressure control of a bang-bang controller according to an example embodiment. The x-axis represents time in seconds (sec) and the y-axis represents pressure in Torr (mmHg) generated by the pump as it changes over time in continuous control mode. The pressure control of the bang-bang controller is affected by the head pressure generated by the reduced pressure treatment system of FIG. 1 shown in FIG. 5A, which is greater than the head pressure shown in FIG. 5B, which is smaller. [Figure 6]6 is a graph illustrating pressure control of a PID controller according to an example embodiment. The x-axis represents time in seconds (sec) and the y-axis represents the pressure produced by the pump in Torr (mmHg) as it varies over time in continuous control mode. Here, the horizontal time scale is substantially the same as the horizontal time scale shown in FIG. 5B to compare the pressure control with that of a bang-bang controller. [Figure 7] FIG. 7 is a flowchart illustrating a process or treatment loop that may be stored on the controller of FIG. 1 for controlling reduced pressure at a tissue site, according to an example embodiment, including a treatment algorithm for selecting an appropriate pump pressure control for controlling reduced pressure at a tissue site. [Figure 8] FIG. 8 is a flow chart illustrating a process or flow loop for controlling gas leakage from an external environment into a treatment environment of a reduced pressure treatment system, according to an example of an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following description of exemplary embodiments provides information to enable one skilled in the art to make and use the subject matter recited in the appended claims, but may omit certain details already known in the art. Accordingly, the following detailed description is to be construed as illustrative and not limiting.

[0013] Exemplary embodiments may also be described herein with reference to the spatial relationships between or orientation of various elements as shown in the accompanying drawings. Generally, such relationships or orientations are in a frame of reference that is consistent with or relative to a patient in a position to receive treatment. However, those skilled in the art will understand that this frame of reference is not a strict requirement and is merely for convenience of explanation.

[0014] FIG. 1 is a simplified functional block diagram of an exemplary embodiment of a reduced pressure treatment system 100 capable of providing negative pressure treatment in accordance with the present disclosure. More specifically, the treatment system 100 can be used to control which pump pressure control is utilized to deliver an appropriate amount of reduced pressure to a tissue site 105. The tissue site 105 can be any bodily tissue of a human, animal, or other organism, including bone tissue, adipose tissue, muscle tissue, dermal tissue, vascular tissue, connective tissue, cartilage tissue, tendons, ligaments, or any other tissue. The tissue site 105 can include wounds, diseased tissue, or defective tissue, although the tissue site can also include healthy tissue that is not wounded, diseased, or defective. Application of reduced pressure to the tissue site 105 can be used to promote drainage of exudates and other fluids from the tissue site 105 and to promote the growth of additional tissue. If the tissue site 105 is a wound site, the growth of granulation tissue and the removal of exudates and bacteria promotes wound healing. The application of reduced pressure to non-wounded or non-defective tissue, including healthy tissue, can be used to promote the growth of tissue that can be harvested and transplanted to another tissue location.

[0015] The reduced pressure applied to the tissue site 105 may be supplied by a reduced pressure source 110. The reduced pressure source 110 may be any type of manually, mechanically, or electrically operated pump. Non-limiting examples of reduced pressure source 110 include devices that are powered by stored energy and can generate reduced pressure. Examples of these stored energy, reduced pressure sources include, but are not limited to, pumps powered by piezoelectric energy, spring energy, solar energy, kinetic energy, energy stored in a capacitor, combustion, and energy generated by a Stirling cycle or similar cycle. Still other devices and processes that may be used or included in the reduced pressure source 110 include syringes, lead screws, ratchets, clockwork-driven devices, pendulum-driven devices, manual generators, osmotic processes, thermal heating processes, and processes in which vacuum pressure is generated by condensation. In another embodiment, reduced pressure source 110 can include a pump 112 that provides negative or reduced pressure, i.e., pump pressure (PP), to tissue site 105, which can be driven by a motor 114 electrically coupled to a controller 170, also referred to as a system controller, which is a component of reduced pressure treatment system 100. Motor 114 can be, for example, a direct current motor powered by a DC power source, such as a battery (not shown). Preferably, pump 112 uses a small amount of power and can operate for extended periods on a single battery charge, such as, for example, a diaphragm pump.

[0016] In one exemplary embodiment, reduced pressure source 110 comprises a DC motor 114 powered by a battery, i.e., applied power. Applied power can be defined as a current or voltage applied to the motor, i.e., an "applied voltage" (V A ) can be varied to control the speed of the motor. A) can be varied, for example, by modulating the voltage with a square wave and varying the duty cycle of the square wave to control the speed of the DC motor 114. Reduced pressure source 110 also includes a pump 112 that applies reduced pressure or vacuum to tissue site 105. As a result, pump 112 represents a load on DC motor 114, and as a result, the applied voltage (V) supplied to DC motor 114 increases as treatment dictates that increased reduced pressure at tissue site 105 is required. A ) is increased to achieve the target reduced pressure at the tissue site 105. A Those skilled in the art will know that DC motor 114 will not operate or rotate the pump until the force of the DC motor 114 is sufficient to overcome the inertia or load of pump 112, which in this case may be a diaphragm pump.

[0017] More specifically, referring to FIG. 2, a graph 301 is shown illustrating the voltage for the pump motor 114 required to start the pump 112, where the X-axis represents the pump pressure (PP) that is the load on the DC pump motor, and the Y-axis represents the applied voltage (V A) represents the DC motor voltage (V) at which the DC motor 114 is loaded with a pressure of 100 mmHg, as shown by dashed lines 302 and 303. For example, as shown by dashed lines 302 and 303, when the DC motor 114 is loaded with a pressure of 100 mmHg, the controller 170 may need to apply a voltage of at least 2.3 V to the DC motor 114 to rotate the pump 112. Applying a voltage of less than 2.3 V to the DC motor 114 would result in the motor having insufficient power to rotate the pump, i.e., the loaded motor would remain stopped or "stalled," and as a result, the motor would not be able to rotate the pump. Thus, the value of 2.3 V is often referred to in the industry as the "stall voltage" and is calculated for a DC motor loaded with a pressure of 100 mmHg. Correspondingly, when loaded with a greater pressure of 125 mmHg, the controller 170 may need to apply a greater voltage of at least 2.45 V to the DC motor to rotate the pump (304). Applying less than the 2.45V stall voltage to the DC motor is not enough to make the DC motor turn the pump at a stall pressure of 125mmHg. The variation in stall voltage is proportional to the variation in pressure load on the motor; the greater the pressure load on the motor, the greater the stall voltage required to overcome the pressure load.

[0018] The specific stall voltage for a particular DC motor used to drive a diaphragm pump can typically be determined by one skilled in the art from available specifications for the DC motor. The diaphragm pump and DC motor may be an integrated device, such as a Thomas Model No. 30130002 series 4.5V diaphragm pump, for which such information is readily available. (Thomas; thomas.de@gardnerdenver.com) Referring again to FIG. 2, where graph 301 shows the stall voltage of a pump motor, the Y-axis represents the calculated stall voltage for this Thomas motor based on the specifications currently available on the Thomas website referenced above. The examples provided in the above paragraphs include voltages and pressures that are merely illustrative. Graph 301 simply illustrates that one skilled in the art can calculate various stall voltages for a DC motor based on the motor's typically available specifications. Those working with small diaphragm pumps driven by DC motors, such as Thomas DC motors, often refer to stall voltage as "stall power," i.e., the product of stall voltage and the rated current of a particular DC motor.

[0019] Data from pump specifications is typically limited to the relationship of maximum flow rate to vacuum pressure at maximum pump voltage (e.g., 4.5V for the Thomas pump specified above). Positive pressure is expressed in mbar (mmHg of positive pressure = 0.7500616827042 * mbar), and vacuum pressure is specified as a percentage vacuum. For example, if 100% maximum vacuum is specified as 760 mmHg, then 40% maximum vacuum is 304 mmHg vacuum (=0.4 * 760 mmHg). In this example, 304 mmHg vacuum is the theoretical maximum vacuum that can be achieved if the pump is operated at 4.5V and the DC motor is allowed to run until it stalls. Graph 301 in Figure 2 was generated based on the motor specifications and the observed stall voltage that was required to drive this pump.

[0020] The formula for calculating the stall voltage for this particular pump is: Stall Voltage = 1.638 V + (0.006515 V / mmHg * X mmHg), where X is the current vacuum pressure. Therefore, at 50 mmHg vacuum, the stall voltage is 1.96 V (1.638 + (0.006515 * 50)), and at 125 mmHg vacuum, the stall voltage is 2.45 V (1.638 + (0.006515)) as shown by dashed lines 304, 305. * 125), and at 175 mmHg vacuum, the stall voltage is 2.78 V (1.638 + (0.006515)), as shown by dashed lines 306, 307. * 175). Again, the higher the vacuum pressure, the higher the applied voltage required to start the pump. Otherwise, the pump will stall and not move until the required stall voltage is applied. Once the pump stalls, the DC motor will simply overheat, which can damage the DC motor and reduce battery life.

[0021] Referring back to FIG. 1 , reduced pressure source 110 may deliver reduced pressure to tissue site 105 through dressing 115. Dressing 115 may include a tissue interface, such as, for example, manifold 120, which may be positioned adjacent to or in contact with tissue site 105. Manifold 120 may be a biocompatible porous material capable of being placed in contact with tissue site 105 and distributing reduced pressure to tissue site 105. Manifold 120 may be made from foam, gauze, felt mat, or any other material suitable for a particular biological application. Manifold 120 may include multiple channels or pathways to facilitate the distribution of reduced pressure or fluid to or from tissue site 105.

[0022] In one embodiment, the manifold 120 is a porous foam containing a plurality of interconnected cells or pores that function as fluid channels. The porous foam may be a polyurethane, open-cell, reticulated foam, such as GranuFoam, manufactured by Kinetic Concepts, Inc. (San Antonio, Texas). When open-cell foam is used, the porosity may vary but is preferably approximately 400-600 microns. The fluid channels allow fluid communication throughout the portion of the manifold 120 that has open cells. The cells and fluid channels may be uniform in shape and size or may contain patterned or random variations in shape and size. Variations in the shape and size of the manifold's cells result in differences in the fluid channels, and such characteristics can be used to modify the fluid flow characteristics through the manifold 120. The manifold 120 may further include portions containing "closed cells." These closed-cell portions of the manifold 120 contain a plurality of cells, the majority of which are not fluidly connected to adjacent cells. Closed cell portions may be selectively positioned within the manifold 120 to prevent fluid transmission through the outer periphery of the manifold 120 .

[0023] Manifold 120 may also be constructed from a bioabsorbable material that does not need to be removed from the patient's body after use of reduced pressure treatment system 100. Suitable bioabsorbable materials may include, but are not limited to, polymer blends of polylactic acid (PLA) and polyglycolic acid (PGA). Polymer blends may also include, but are not limited to, polycarbonate, polyfumarate, and capralactone. Manifold 120 may further function as a scaffold for new cell growth, or a scaffold material may be used in conjunction with manifold 120 to promote cell growth. A scaffold is a substance or structure used to enhance or promote cell growth or tissue formation, such as a three-dimensional porous structure that provides a template for cell growth. Illustrative examples of scaffold materials include calcium phosphate, collagen, PLA / PGA, coral hydroxyapatite, carbonate, or engineered allograft materials. In one embodiment, the scaffold material has high void friction (i.e., high air content).

[0024] The dressing 115 may also include a sealing member 125, also referred to as a drape or cover. The manifold 120 may be secured to the tissue site 105 using the sealing member 125. The sealing member 125 may be a cover used to secure the manifold 120 to the tissue site 105. The sealing member 125 may be impermeable or semi-permeable, but in one embodiment, the sealing member 125 can maintain reduced pressure at the tissue site 105 after installation over the manifold 120. The sealing member 125 may be a flexible drape or film made from a silicone-based compound, acrylic, hydrogel or hydrogel foam material, or any other biocompatible material that has the impermeable or permeable properties desired for the tissue site 105. The sealing member 125 may be formed from a hydrophobic material to prevent moisture absorption by the sealing member 125. In one embodiment, seal member 125 is configured to provide a sealed connection with manifold 120 and the tissue surrounding tissue site 105. The sealed connection may be provided by an adhesive (not shown) disposed along the periphery of seal member 125 or disposed on any portion of seal member 125 to secure seal member 125 to manifold 120 or to the intact epidermis, i.e., surrounding tissue, around the tissue site. The adhesive may be pre-placed on seal member 125 or may be sprayed or otherwise applied to seal member 125 immediately prior to installation.

[0025] In general, components of treatment system 100 may be directly or indirectly coupled. Components can be fluidly connected to one another to provide a pathway for transferring fluid (i.e., liquid and / or gas) between the components. In some embodiments, for example, components may be fluidly coupled through tubing. As used herein, "tubing" broadly refers to a tube, pipe, hose, conduit, or other structure having one or more lumens adapted to carry fluid between two ends. Typically, tubing is an elongated, cylindrical structure with some flexibility, although geometry and stiffness can vary. In some embodiments, components may additionally or alternatively be coupled by physical proximity, by being integrated into a single structure, or by being formed from the same piece of material. In some contexts, coupling may also include mechanical, thermal, electrical, or chemical (e.g., chemical) bonding.

[0026] The reduced pressure generated by reduced pressure source 110 may be applied to tissue site 105 through source tube 130 and delivery tube 135. Source tube 130 and delivery tube 135 may be any tube through which a gas, liquid, gel, or other fluid may flow. For example, exudate from tissue site 105 may flow through delivery tube 135. In FIG. 1 , source tube 130 couples reduced pressure source 110 to canister 140, and delivery tube 135 couples canister 140 to dressing 115. However, in another embodiment, reduced pressure source 110 may be directly coupled to dressing 115 using delivery tube 135.

[0027] The source tube 130 and the delivery tube 135 may be made of any material. The source tube 130 and the delivery tube 135 may be either flexible or non-flexible. Additionally, the source tube 130 and the delivery tube 135 may include one or more passageways or lumens through which fluid may flow. For example, the delivery tube 135 may include two lumens. In this example, one lumen may be used for the passage of exudate from the tissue site 105 to the canister 140. The other lumen may be used to deliver a fluid, such as air, an antibacterial agent, an antiviral agent, a cell growth promoter, a lavage fluid, or other chemically active agent, to the tissue site 105. The source from which these fluids originate is not shown in FIG. 1 . Further details regarding the inclusion of multi-lumen tubes in the reduced pressure treatment system 100 are provided below.

[0028] In one embodiment, delivery tube 135 is coupled to manifold 120 via connecting member 145. Connecting member 145 allows fluid to pass from manifold 120 to delivery tube 135, and vice versa. For example, exudate collected from tissue site 105 using manifold 120 may enter delivery tube 135 via connecting member 145. In another embodiment, reduced pressure treatment system 100 does not include connecting member 145. In this embodiment, delivery tube 135 may be inserted directly into seal member 125 or manifold 120, such that the end of delivery tube 135 is adjacent to or contacts manifold 120.

[0029] Liquids, such as exudate from the tissue site 105, may flow through the delivery tube 135 to the canister 140. The canister 140 may be any device or cavity capable of containing fluids, such as gases and liquids, as well as fluids including solids. For example, the canister 140 may contain exudate from the tissue site 105. The source tube 130 and the delivery tube 135 may be directly connected to the canister 140 or may be coupled to the canister 140 via a connector, such as connector 150, as indicated by arrow 151. The canister 140 may be a flexible or rigid canister, bag, or pouch that is fluidly connected to the manifold 120 by the delivery tube 135. The canister 140 may be a separate canister or may be operatively associated with the reduced pressure source 110 to collect exudate and fluids.

[0030] Reduced pressure treatment system 100 may further include a first pressure sensor or wound pressure sensor 155 electrically coupled to controller 170. Wound pressure sensor 155 detects the actual reduced pressure at or near tissue site 105, i.e., tissue site pressure or wound pressure (WP). The reference to the word "wound" as part of the term wound pressure (WP) is exemplary only and does not limit the term or description herein as applied to measuring pressure at other types of tissue sites, such as an incision or subcutaneous cavity. In one non-limiting example, wound pressure sensor 155 is a silicon piezoresistive gauge pressure sensor. Wound pressure sensor 155 may be configured to detect wound pressure (WP) via control tubing 160 directly fluidly coupled to connecting member 145 or indirectly, via the portion of control tubing 160 that passes through canister 140. Control tubing 160 may include one or more pathways or lumens through which fluid may flow.

[0031] Reduced pressure treatment system 100 may further include a second pressure sensor or pump pressure sensor 156 electrically coupled to controller 170. Pump pressure sensor 156 may be configured to detect reduced pressure at or downstream of canister 140, i.e., pump pressure (PP), via control tubes 157 and 158, respectively. In other words, pump pressure sensor 156 may be coupled to canister 144 directly via control tube 157 or via source tube 130 via control tube 158 to detect pump pressure (PP). In one exemplary embodiment, pump pressure sensor 156 may be a silicon piezoresistive gauge pressure sensor. In yet another exemplary embodiment, pump pressure (PP) may be determined by controller 170 analyzing the DC voltage of pump motor 114, as shown with reference to FIG. 2 .

[0032] Pressure sensors 155 and 156 may be located at alternative locations on or within reduced pressure treatment system 100. Referring back to FIG. 1 , wound pressure sensor 155 is shown remote from tissue site 105. In this example, reduced pressure at tissue site 105 may be detected directly from remotely located wound pressure sensor 155 via control tubing 160, or indirectly from canister 140 via a portion of control tubing 160 coupled to wound pressure sensor 155. Also in this example, pump pressure sensor 156 may be directly or indirectly coupled to other remotely located components of reduced pressure treatment system 100, such as reduced pressure source 110, canister 140, or any other illustrated component of reduced pressure treatment system 100. In another example, wound pressure sensor 155 may not require the use of control tubing 160 to detect pressure at tissue site 105. In one non-limiting example, the wound pressure sensor 155 is directly coupled to the manifold 120 or is disposed between the seal member 125 and the manifold 120 .

[0033] Reduced pressure treatment system 100 may also include a regulator 165 electrically coupled to controller 170. Regulator 165 may be a valve having an input and an output, where the input may be fluidly coupled to the external environment, as indicated by arrow 166, and the output may be fluidly coupled to the treatment environment of reduced pressure treatment system 100. For example, the output of regulator 165 may be fluidly coupled to canister 140, as indicated by arrow 167. In another embodiment, the output of regulator 165 may be fluidly coupled to control tubing 160 (not shown). Regulator 165 may be any valve capable of releasing reduced pressure within the treatment environment of reduced pressure treatment system 100, thereby increasing the flow rate of fluid therein. Non-limiting examples of regulator 165 include a pneumatic solenoid valve, a proportional valve, or a mechanical valve. In one example, regulator 165 may be manually controlled by a caregiver. In another example, regulator 165 may be controlled by controller 170.

[0034] During operation, manifold 120 may be positioned within, over, on, or otherwise adjacent to a tissue site. Seal member 125 may be positioned over manifold 120 and sealed to tissue near tissue site 105. For example, seal member 125 may be sealed to the intact epidermis around the tissue site, i.e., peri-wound tissue. Dressing 115 may thus provide a sealed treatment environment adjacent to the tissue site that is substantially isolated from the external environment, and reduced pressure source 110 may reduce pressure in the sealed treatment environment. Reduced pressure applied across the tissue site via manifold 120 in the sealed treatment environment may induce macroscopic and microscopic strains in the tissue site and remove exudate and other fluids from the tissue site, which may be collected in canister 140 for appropriate disposal.

[0035] The fluid dynamics of using a reduced pressure source to reduce pressure at another component or location, such as within an enclosed treatment environment, can be mathematically complex. However, the basic principles of fluid dynamics applicable to negative pressure treatment are generally well known to those skilled in the art, and the process of reducing pressure may be illustratively described herein as "delivering," "distributing," or "generating" negative pressure, for example.

[0036] Generally, exudates and other fluids flow toward lower pressures along a fluid pathway. Thus, the term "downstream" typically refers to a location closer to the reduced pressure source within the fluid pathway, and conversely, the term "upstream" refers to a location further away from the negative pressure source. Similarly, it may be convenient to describe certain features in terms of the fluid's "inlet" or "outlet" in such a frame of reference. For purposes of describing the various features and components of reduced pressure treatment systems herein, this orientation is generally assumed. However, depending on the application, the fluid pathway may also be reversed (e.g., by using a positive pressure source instead of a reduced pressure source), and this descriptive convention should not be construed as a restrictive convention.

[0037] The term "tissue site" in this context refers broadly to a wound or defect located on or within tissue, including, but not limited to, bone tissue, adipose tissue, muscle tissue, nerve tissue, dermal tissue, vascular tissue, connective tissue, cartilage, tendon, or ligament. Wounds can include, for example, chronic, acute, traumatic, subacute, and dehiscence wounds, partial thickness burns, ulcers (such as diabetic ulcers, pressure ulcers, or venous insufficiency ulcers), skin flaps, and transplanted tissue. The term "tissue site" can also refer not necessarily to any area of ​​tissue where there is a wound or defect, but instead to an area where it may be desirable to add or promote the growth of additional tissue. For example, negative pressure can be used in a particular tissue area to grow additional tissue that can be harvested and transplanted to another tissue location.

[0038] "Negative or reduced pressure" generally refers to a pressure that is less than the local ambient pressure, such as ambient pressure, in a local environment outside the enclosed treatment environment provided by the dressing 102. In many cases, the local ambient pressure may also be the atmospheric pressure at which the tissue site is located. Alternatively, the pressure may be less than the hydrostatic pressure associated with the tissue at the tissue site. Unless otherwise indicated, pressure values ​​described herein are gauge pressures. Similarly, references to an increase in negative pressure typically refer to a decrease in absolute pressure, while a decrease in negative pressure typically refers to an increase in absolute pressure.

[0039] A negative pressure source, such as reduced pressure source 110, can be a reservoir of reduced pressure air or a manual or powered device capable of reducing pressure within an enclosed volume, such as a vacuum pump, suction pump, wall suction port available in many healthcare facilities, or a micropump. The negative pressure source may be housed within or used in conjunction with other components, such as a processing unit, alarm indicator, memory, database, software, display device, or user interface to further facilitate negative pressure treatment. For example, reduced pressure source 110 and controller 106 may be housed within a treatment control unit. The magnitude and nature of the negative pressure applied to the tissue site can vary depending on the treatment requirements, but the pressure is generally between -5 mmHg (-667 Pa) and -500 mmHg (-66.7 kPa), commonly referred to as a low vacuum, also known as a rough vacuum. A typical treatment range is between -75 mmHg (-9.9 kPa) and -300 mmHg (-39.9 kPa).

[0040] As described above, the applied voltage (V A ) can be used to control the pump pressure (PP) to ultimately achieve a desired or target pressure at the tissue site 105. Correspondingly, instead of using a pump pressure sensor 156, the applied voltage (V A) provides an indication of the pump pressure (PP) and may be monitored by the controller 170, which then determines the time rate of change of the applied voltage (V) which necessarily corresponds to the time rate of change of the pump pressure (PP). A ) as a time rate of change. Controller 170 may also use this calculation to determine the flow rate of air between reduced pressure source 110 and tissue site 105, i.e., flow rate (FR). In another embodiment, reduced pressure treatment system 100 may further include a sensing device (not shown) that directly measures flow rate (FR), such as, for example, a flow meter or a differential processor for calculating the time rate of change of the difference between wound pressure (WP) and pump pressure (PP). Alternatively, flow rate (FR) may be determined by calculating a pressure differential based on measurements obtained from two pressure sensors, such as pressure sensors 155 and 156 described above.

[0041] The flow rate (FR) may be measured, for example, as cubic centimeters per minute (cc / min) of air between the reduced pressure source 110 and the tissue site 105. The flow rate (FR) provides some indication of the likelihood that the dressing 115 or other component of the negative pressure system 100 is leaking (i.e., system leaking), reducing the pressure at the tissue site 105 below the desired pressure targeted for treatment. For example, a high flow rate (FR) may indicate that the dressing 115 or other component of the system 100 is considered to have a "high system leak" or a "high leak condition" that needs to be corrected, for example, by increasing the pump pressure (PP) or adjusting the seal around the tissue site. On the other hand, a lower flow rate (FR) may indicate that the dressing 115 or other component of the system 100 is considered to have a "low system leak" or a "low leak condition" that requires a lower pump pressure (PP), which requires less battery power to drive the DC motor 114 at a lower speed to maintain the desired wound pressure (WP).

[0042] Controller 170 may be an integrated or separate component of reduced pressure treatment system 100. Controller 170 may be any device capable of processing data, such as data from wound pressure sensor 155 and / or pump pressure sensor 156. Controller 170 may also control the operation of one or more components of reduced pressure treatment system 100, such as reduced pressure source 110, motor 114, regulator 165, pressure sensors 155 and 156, and indicator 172. Controller 170 may control and receive data from other components (not shown) of reduced pressure source 110, including pump 112 and motor 114. In one embodiment, controller 170 controls the wound pressure (WP) from wound pressure sensor 155, the pump pressure (PP) from pump pressure sensor 156, and the applied voltage (V) to motor 114, as described above. A ) and processes data such as flow rate (FR) from monitoring of the tissue site 105. Controller 170 may also control the operation of one or more components of reduced pressure treatment system 100 to manage wound pressure at tissue site 105. In one embodiment, controller 170 may include an input for receiving a desired target wound pressure (TWP) set by a clinical caregiver or other user, and may be a program that processes data related to the setting and input of the target wound pressure (TWP) to be applied to tissue site 105.

[0043] In one example embodiment, the target wound pressure (TWP) may be a fixed pressure value determined by a user / caregiver as a desired reduced pressure target for treatment at the tissue site 105 and then provided as an input to the controller 170. The user may be a nurse, or a physician, or other authorized clinical medical personnel who prescribes the desired negative pressure to be applied to the tissue site 105. The desired tissue site pressure will vary depending on the tissue site, but will generally be selected based on the type of tissue forming the tissue site, the type of injury or wound (if any), the patient's medical condition, and the attending physician's preferences. After selecting the desired target wound pressure (TWP), the reduced pressure source 110 is controlled to achieve the target wound pressure (TWP) applied to the tissue site 105.

[0044] 3 , the target wound pressure (TWP) can be set by the user in a continuous pressure mode, as shown by solid line 401 and dotted line 402, where reduced pressure is applied to the tissue site 105 until the user turns off the reduced pressure source 110. The target wound pressure (TWP) can also be set by the user in an intermittent pressure mode, as shown by solid lines 401, 403, and 405, where the wound pressure (WP) cycles between the target wound pressure (TWP) and atmospheric pressure. For example, the target wound pressure (TWP) may be set by the user to a value of 125 mmHg for a specified period of time (e.g., 5 minutes), followed by turning treatment off by venting the tissue site 105 to atmosphere for a specified period of time (e.g., 2 minutes), as shown by solid line 403, and then repeating the cycle by turning treatment back on, as shown by solid line 405, resulting in a square wave pattern continuously forming between the target wound pressure (TWP) level and no pressure.

[0045] It should be understood that the increase in wound pressure (WP) from ambient pressure at tissue site 105 to target wound pressure (TWP) is not instantaneous, but rather is limited depending on the type of treatment device and dressing. For example, reduced pressure source 110 and dressing 115 may have an initial rise time, as indicated by dashed line 407, that may vary depending on the type of dressing and treatment device being used. For example, the initial rise time for one treatment system may be in the range of approximately 20-30 mmHg / sec, or more specifically equal to approximately 25 mmHg / sec, while for another treatment system, it may be in the range of approximately 5-10 mmHg / sec. When operating in an intermittent mode, the recurring rise time of line 405 may be substantially equal to the initial rise time of line 407.

[0046] The target pressure may also be a variable target pressure (VTP) controlled or determined by the controller 170 that varies in a dynamic pressure mode. For example, the variable target pressure (VTP) may vary between a maximum pressure value and a minimum pressure value that may be set as a user input as a desired reduced pressure range for treatment at the tissue site 105. The variable target pressure (VTP) may also be processed and controlled by the controller 170 that varies the target wound pressure (TWP) according to a predetermined waveform, such as a sinusoidal waveform, sawtooth waveform, or triangular waveform, that may be set as a user input as a predetermined or time-varying reduced pressure desired for treatment at the tissue site 105. For example, the variable target pressure (VTP) may be a reduced pressure that provides effective treatment by applying reduced pressure to the tissue site 105 in the form of a triangular waveform that varies between 50 and 125 mmHg and has a rise time set at a rate of +25 mmHg / min and a fall time set at a rate of -25 mmHg / min. In another embodiment of reduced pressure treatment system 100, the variable target pressure (VTP) may be a reduced pressure that varies between 25 and 125 mmHg and applies reduced pressure to tissue site 105 in the form of a triangular waveform with a rise time set at a rate of +30 mmHg / min and a fall time set at -30 mmHg / min. Again, the type of system and tissue site will determine the type of reduced pressure treatment used.

[0047] After selecting the target wound pressure (TWP), the reduced pressure source 104 is operated to achieve the desired pressure at the wound site 105 by controlling the pump pressure (PP). In many cases, the reduced pressure source 110 should be operated at a higher pump pressure (PP) than the pressure at the target wound pressure (TWP) due to pressure loss between the reduced pressure source 110 and the tissue site 105. Additionally, the head pressure of exudate and other fluids in the conduits can result in a drop in vacuum pressure at the tissue site 105. The height of the canister 140 above the tissue site 105 can determine the amount of head pressure imposed on the tissue site 105 by the fluid in the conduits. For exudates and fluids with densities similar to water, the head pressure imposed by one foot of fluid is approximately 25 mmHg. Some fluids drawn from the tissue site 105 may be heavier or more viscous than water and therefore have a more significant effect on pressure loss at the tissue site 105.

[0048] As an example of potential losses caused by the weight of fluid within a conduit, referring to FIG. 4, the target wound pressure (TWP) prescribed for a particular tissue site may be −125 mmHg, and the wound pressure (WP) varies while reduced pressure is applied to the tissue site 105. (It should be understood that the steady sinusoidal variation in wound pressure (WP) shown in FIG. 4 is illustrative only and does not represent the actual variation in wound pressure (WP) under normal operating conditions, such as the variation shown in FIGS. 5A and 5B.) If the canister 140 is positioned two feet above the tissue site 105, and if the delivery tube 135 between the canister 140 and the tissue site 105 is completely filled with fluid, the head pressure imposed by the fluid can create a pressure differential (δP) of approximately 50 mmHg. This particular example occurs when the tissue site is located in a patient's lower extremity, such as the foot, and the canister 140 is attached near or above the patient's head (e.g., to an IV pole when the patient is in a wheelchair). Therefore, if the head pressure of the fluid in the delivery tube 135 is approximately 50 mmHg, the pump 112 needs to supply a pump pressure (PP) that rises to a maximum pump pressure value (PPmax) of approximately 185 mmHg and falls to a minimum pump pressure value (PPmin) of approximately 165 mmHg (a target pump pressure median (TPP) of approximately 175 mmHg) to produce a target wound pressure (TWP) at the tissue site 105 of approximately 125 mmHg.

[0049] The controller 170 may also be programmed and controlled by a user to maintain the target wound pressure (TWP) within an acceptable range of pressure. For example, if the target wound pressure (TWP) is set at 125 mmHg as the desired treatment pressure for the tissue site 105, the user may desire that the wound pressure (WP) not vary by more than ±10 mmHg from the desired target wound pressure (TWP), so that the wound pressure (WP) is controlled between a minimum wound pressure value (WPmin) of 115 mmHg and a maximum wound pressure value (WPmax) of 135 mmHg, i.e., with a differential wound pressure range (δWP) of approximately 20 mmHg. Thus, in this example, assuming the head pressure is about 50 mmHg as described above, the pump pressure (PP) must also vary within ±10 mmHg of the target pump pressure (TPP) so that the pump pressure (PP) ranges from a minimum pump pressure value (PPmin) of about 165 mmHg to a maximum pump pressure value (PPmax) of about 185 mmHg, i.e., within a pump pressure differential (δTTP) of about 20 mmHg. Controlling the pump pressure (PP) to stay within this range indirectly maintains the wound pressure (WP) within a range from a minimum wound pressure value (WPmin) of about 115 mmHg to a maximum wound pressure value (WPmax) of about 135 mmHg.

[0050] In contrast to the example shown in FIG. 4, referring to FIG. 5A as an example of wound pressure (WP) fluctuations under normal operating conditions, the pressure difference (δP) between the pump pressure (PP) and the wound pressure (WP) is the result of a fairly high flow rate (FR) of approximately 300 cc / min (high leak condition) at the dressing 115 and other components in the system. In this example, the wound pressure (WP) is controlled to cycle between approximately 135 mmHg and 115 mmHg, as described above, by providing a pump pressure (PP) that results in a target wound pressure (TWP) of approximately 125 mmHg at the tissue site 105 by rising to a maximum pump pressure value (PPmax) of approximately 155 mmHg and falling to a minimum pump pressure value (PPmin) of approximately 120 mmHg. Thus, the pressure difference (δP) is approximately 15 mmHg, which is much smaller than the pressure difference (δP) of approximately 50 mmHg resulting from the upper pressure in the example associated with FIG. 4 above. 5B illustrates yet another example in which the pressure differential (δP) between the pump pressure (PP) and the wound pressure (WP) is the result of a lower flow rate (FR) of approximately 50 cc / min (low leak condition) through the dressing 115 and other components in the system. In this example, by providing a pump pressure (PP) that rises to a maximum pump pressure value (PPmax) of 140 mmHg and falls to a minimum pump pressure value (PPmin) of 115 mmHg, the wound pressure (WP) is again controlled to cycle between approximately 135 mmHg and 115 mmHg, resulting in a target wound pressure (TWP) of approximately 125 mmHg at the tissue site 105. Thus, the pressure differential (δP) is approximately 5 mmHg, which is even smaller than the pressure differential in the previous example.

[0051] The controller 170 may also include a bang-bang controller (not shown), also referred to as an on-off controller or a hysteresis controller. A bang-bang controller is a feedback controller that switches abruptly between two states, e.g., on and off. Essentially, a bang-bang controller may apply an all-or-nothing type of control. A bang-bang controller may be used to generate the pressure fluctuations generally described above in connection with Figures 5A and 5B. Continuing with that general description, the bang-bang controller may operate in one mode as follows: For example, if the wound pressure (WP) drops too far to a minimum wound pressure value (WPmin), the applied voltage (V A ) is the stall voltage, i.e., the start (t on ) at the bang bang on voltage (V ON ), the vacuum pump 112 is turned on, increasing the pump pressure (PP) to a maximum pump pressure (PPmax). The increase in pump pressure (PP) is proportional to the applied voltage (V A Although there may be a slight delay in the application of the bang-bang voltage (V), the increasing pump pressure (PP) eventually also increases the wound pressure (WP), as shown at time t1. ON ) continues to be applied until either the pump pressure (PP) reaches a maximum pump pressure value (PPmax) or the wound pressure (WP) reaches a maximum wound pressure value (WPmax), whichever comes first. When either one of these maximum values ​​is reached or exceeded, the vacuum pump 112 switches off (t off ), so that pump pressure (PP) is not applied and residual pressure within reduced pressure treatment system 100 is allowed to decrease as a result of leaks within the system. Reduced pressure pump 112 remains off until either the wound pressure (WP) again falls below the minimum wound pressure value (WPmin) or the pump pressure (PP) falls below the minimum pump pressure value (PPmin), whichever occurs first.

[0052] The bang-bang controller switches between these two states, turning on the vacuum pump 112 if the wound pressure or pump pressure drops too low in the down mode and turning off the vacuum pump 112 if the wound pressure or pump pressure rises too high in the up mode. More specifically, referring to FIG. 5B , the bang-bang controller allows the wound pressure (WP) to oscillate near a target wound pressure (TWP) of 125 mmHg, encompassed between two limits programmed by the user into the controller 170, e.g., a minimum wound pressure value (WPmin) of 115 mmHg and a maximum wound pressure value (WPmax) of 135 mmHg. As long as the wound pressure (WP) does not exceed either of these limits, the wound pressure (WP) will not be pulled back within a wound pressure range (δWP) of 20 mmHg. Because the bang-bang controller does not need to compensate for increased leakage in a low-leakage environment, the bang-bang controller maintains the wound pressure (WP) substantially within this range.

[0053] The controller 170 may also include a PID controller (not shown) that provides a control loop feedback mechanism that calculates an error value as the difference between a measured process variable and a desired set point or target, in this case the wound pressure (WP) at the wound site 105 and the corresponding target wound pressure (TWP), respectively. PID controllers are well known to those skilled in the art as providing proportional, historical, and time rate of change information to maintain the wound pressure (WP) near the target pressure (TP). The PID summation adjusts the process, in this case the reduced pressure treatment process, by adjusting the power or voltage supplied to a control element, such as a DC motor, i.e., the applied voltage (V), which as noted above is directly related to the pump pressure (PP). A ) is used to adjust the applied voltage (V) by adjusting the pulse width modulation to achieve the desired pump pressure (PP) required to compensate for leakage of the dressing 115 and / or the pressure head described above. A) may be varied. The response of a PID controller depends on the responsiveness of the controller to errors, the extent to which the controller overshoots the set point, e.g., the target pressure (TP), and the degree of system oscillation, e.g., the degree to which the wound pressure (WP) oscillates within the tolerances described above. The preferred embodiment of a PID controller is a digital controller, but the PID controller may also be an analog controller or a simple RC circuit. Analog or digital PID controllers may also be implemented in software as part of a program logic controller.

[0054] After the wound pressure sensor 155 measures the wound pressure (WP), the PID controller determines the applied voltage (V) required to adjust the wound pressure (WP) back to the target pressure (TP). A ), i.e., pump pressure compensation (δPP). The pump pressure compensation (δPP) is the additional pressure needed to maintain the wound pressure (WP) at the desired target pressure (TP), e.g., 125 mmHg, and may be calculated every few seconds. As a result, the PID control adjusts the applied voltage (V) to the DC motor 114. A ) is varied to achieve a pump pressure (PP) between a minimum pump pressure value (PPmin) and a maximum pump pressure value (PPmax), thereby maintaining the wound pressure (WP) near the target wound pressure (TP).

[0055] Referring more specifically to FIG. 6 as an example of maintaining the wound pressure (WP) under normal operating conditions of the PID controller, in contrast to the example shown in FIG. 4, the pressure difference (δP) between the pump pressure (PP) and the wound pressure (WP) is the result of different leak rates (LR), as shown by three examples including a first pump pressure (PP1), a second pump pressure (PP2), and a third pump pressure (PP3). In the first example, the first pump pressure (PP1) has a relatively large pressure difference (δP1) of approximately 15-16 mmHg resulting from a fairly high flow rate (FR) of approximately 350 cc / min. The first pressure (PP1) is varied by the PID controller between a maximum pump pressure value (PPmax) and a minimum pump pressure value (PPmin) to maintain the wound pressure (WP) at a target wound pressure (TWP) of 125 mmHg. In other words, the PID controller varies the first pump pressure (PP1) between 140 mmHg and 141 mmHg to maintain the wound pressure (WP) at the target wound pressure (TWP) of 125 mmHg. In a second example, the second pump pressure (PP2) also has a relatively large pressure difference (δP2) of approximately 11-12 mmHg resulting from a fairly high flow rate (FR) of approximately 250 cc / min, and is varied by the PID controller between a maximum pump pressure value (PPmax) and a minimum pump pressure value (PPmin) to maintain the wound pressure (WP) at the target wound pressure (TWP) of 125 mmHg. In other words, the PID controller varies the second pump pressure (PP2) between 136 mmHg and 137 mmHg to maintain the wound pressure (WP) at the target wound pressure (TWP) of 125 mmHg. Essentially, the difference between these two examples is that a higher flow rate (FR) requires a larger pressure differential (δP) to maintain the wound pressure (WP) at the same target pressure (TP). The third example shows a similar difference, with the third pump pressure (PP3) also having a much smaller pressure differential (δP3) of approximately 4-5 mmHg resulting from an even lower flow rate (FR) of approximately 100 cc / min, and being varied by the PID controller between 129 mmHg and 130 mmHg to maintain the wound pressure (WP) at the target wound pressure (TWP) of 125 mmHg.

[0056] Unlike the bang-bang controller, the PID controller does not switch the vacuum pump 112 on and off, but rather continuously controls the application of pump pressure (PP) between a maximum pressure value (PPmax) and a minimum pressure value (PPmin). This maintains the wound pressure (WP) at a relatively constant level, e.g., a target wound pressure (TWP) of 125 mmHg as shown by the dashed line, rather than being allowed to vary between a maximum pressure value (WPmax) and a minimum pressure value (WPmin) as shown in the case of the bang-bang controller. Thus, the extent to which the pump pressure (PP) decreases towards the minimum pump pressure value (PPmin) causes the PID controller to adjust the applied voltage (V) supplied to the DC motor 114. A) Correspondingly, the greater the wound pressure (WP) varies from the target pressure (TP), the greater the PID controller will increase the applied voltage (V A ) to further respond. A The action taken to increase or decrease the wound pressure (WP) delivered by the reduced pressure system deviates from the target wound pressure (TP). The PID controller operates continuously to keep the wound pressure (WP) as close as possible to the target wound pressure (TP), especially for high leak rates (LR). As a result, the PID controller maintains the wound pressure (WP) closer to the target wound pressure (TP) on average, while the bang-bang controller allows the wound pressure (WP) to oscillate between the two limits as described above, causing the reduced pressure treatment system 100 to operate more smoothly than the bang-bang controller, as shown when comparing the wound pressure (WP) fluctuations in Figures 6 and 5, respectively.

[0057] When the flow rate (FR) is sufficiently small to indicate a low leak condition, e.g., when the pump pressure (PP) or wound pressure (WP) is decreasing at a very slow rate toward their respective minimum pressure values, i.e., (PPmin) or (WPmin), the bang-bang controller can provide a sufficiently smooth wound pressure (WP) during treatment while conserving battery power and reducing noise by intermittently turning off the vacuum pump 112 during the same treatment period. For example, the DC motor 114 and pump 112 are turned off for a significant percentage of the 1 minute shown in FIG. 5B, but operate continuously when the PID controller is operating as shown in FIG. 6. Thus, it is desirable to keep the bang-bang controller running as long as possible during a treatment session for low leak conditions, such as when the flow rate (FR) is below a fixed target flow rate (TFR), which indicates a low leak condition, and switch to the PID controller when the flow rate (FR) is above the fixed target flow rate (TFR), which indicates a high leak condition. As a result, another exemplary embodiment of the controller 170 includes both a PID controller and a bang-bang controller, i.e., a hybrid controller, with additional processing that switches between them depending on the degree of leakage in the reduced pressure treatment system 100, regardless of the location or type of leak.

[0058] Thus, the controller 170 may be programmed to use a bang-bang controller in conjunction with a PID controller operating as described above to enable or disable the PID controller depending on specific switching conditions related to the amount of system leak generated by the dressing 115 or other components of the reduced pressure treatment system 100 that affect the flow rate (FR). Using such a hybrid controller is preferable to utilizing a PID controller alone. The PID controller operates continuously during the continuous control mode described above (or an enabled portion of the intermittent control mode described above), which more closely maintains the wound pressure (WP) at the target wound pressure (TP), but may generate noise continuously and more rapidly drain the battery powering the motor 114. The hybrid controller may also activate a bang-bang controller, which turns the DC motor 114 on and off to conserve battery power and reduce the noise generated by the pump 112 during a treatment procedure. The controller 170 may further include inputs for a user / caregiver to set one or more target flow rates (TFR).

[0059] A user / caregiver may set a target flow rate (TFR) as a switching condition to determine whether the dressing 115 or other components are in a high-leakage state or a low-leakage state. If the flow rate (FR) is greater than the fixed target flow rate (TFR), i.e., a high-leakage state, the bang-bang controller is disabled and a PID controller takes over to keep the wound pressure (WP) as close as possible to the target wound pressure (TP). However, if the flow rate (FR) is equal to or less than the fixed target flow rate (TFR), i.e., a low-leakage state, the bang-bang controller is enabled to keep the wound pressure (WP) within the wound pressure differential (δWP) range while conserving battery power and reducing noise from the pump 112. For example, the fixed target flow rate (TFR) may be 65 cc / min. As mentioned above, when the dressing 115 is in a low-leakage state, it is desirable to keep the bang-bang controller running as long as possible during treatment. For example, the controller 170 may operate as a bang-bang controller when the flow rate (FR) is below a fixed target flow rate (TFR), and revert to a PID controller when the flow rate (FR) exceeds the fixed target flow rate (TFR) as a result of additional leakage occurring within the dressing 115 as the patient moves around, ultimately resulting in a high leakage condition.

[0060] In another embodiment, the bang-bang controller may have dual target flow rate (TFR) capability, and the controller 170 further includes an input for a user to set two target flow rates (TFR) as switching conditions for determining whether the dressing 115 or other component is in a high-leakage state or a low-leakage state: a rising target flow rate (TFRA) when the bang-bang controller is enabled at an increasing flow rate (FR) and a falling target flow rate (TFRD) when the PID controller is enabled at a decreasing flow rate (FR). In one embodiment, both the rising target flow rate (TFRA) and the falling target flow rate (TFRD) are greater than the fixed target flow rate (TFR), such that the controller 170 switches more quickly from the PID controller to the bang-bang controller and more slowly from the bang-bang controller to the PID controller. For example, the rising target flow rate (TFRA) and the falling target flow rate (TFRD) may both be set to approximately 80 cc / min, higher than the fixed target flow rate (TFR) of 65 cc / min in the previous example. In yet another embodiment, the ascending target flow rate (TFRA) may also be greater than the descending target flow rate (TFRD), causing the controller 170 to switch more quickly from the PID controller to the bang-bang controller, and more slowly from the bang-bang controller to the PID controller. In this case, the controller 170 prioritizes the benefits gained from using a bang-bang controller as opposed to the drawbacks associated with continuous operation of a PID controller. For example, the ascending target flow rate (TFRA) may be 75 cc / min and the descending target flow rate (TFRD) may be approximately 85 cc / min. If the PID controller is currently enabled in a high leakage condition where the flow rate (FR) is decreasing, the descending target flow rate (TFRA) may be increased so that the controller 170 switches from the PID controller to the bang-bang controller more quickly to enable it. D) is set to 85 cc / min instead of 65 cc / min. Alternatively, if the bang-bang controller is enabled in a low leak condition where the flow rate (FR) is increasing, the rising target flow rate (TFRA) is set to 75 cc / min instead of 65 cc / min so that the controller 170 will switch over to the bang-bang controller more slowly and disable it.

[0061] In one embodiment, the controller 170 may provide an output signal to the indicator 172 to emit a visual and / or audible signal in response to the wound pressure (WP) at the tissue site 105, as measured by the wound pressure sensor 155, not responding to an increase in the pump pressure (PP). For example, the indicator may be a light emitting diode (LED) that provides a visual signal. In this embodiment, the indicator 172 illuminates in response to the wound pressure (WP) at the tissue site 105 not responding to an increasing pump pressure. In another embodiment, the indicator 180 is a sound emitting device, such as a speaker. In this embodiment, the indicator 172 emits a sound in response to the wound pressure (WP) at the tissue site 105 not responding to an increasing pump pressure. The controller 170 may provide other output signals that indicate whether the negative pressure therapy system is in a low leak state or a high leak state.

[0062] While most reduced pressure treatment systems have some system leakage, improvements in system dressings and other components have significantly reduced system leakage, even below the low system leakages noted above, such as the target flow rate (TFR). For example, seal members, such as seal member 125, and adhesives for attaching seal members to surrounding tissue have been improved to make them more airtight and significantly reduce system leakage from the external environment to the treatment environment. In some cases, the wound pressure (WP) may already be equal to the target wound pressure (TWP), while the flow rate (FR) at the tissue site 105 may range from less than 50 cc / min to zero. A flow rate (FR) below the target flow rate (TFR) is often too low to heal the wound. A minimum level of fluid flow at the tissue site is required to promote proper healing. For example, a minimum level of fluid flow is required to remove exudate from the tissue site. If the Flow Rate (FR) falls below the Minimum Flow Rate (MinFR) required for proper healing of the wound (which in some embodiments is less than the Target Flow Rate (TFR)), it may be desirable to measure the Flow Rate (FR) associated with a system leak and increase the Flow Rate (FR) above the system flow rate to an acceptable level (i.e., "induced leakage") in order to heal the wound at the tissue site 105 in "induced leakage mode." In some embodiments, the Minimum Flow Rate (MinFR) may be dependent on or equal to the Target Flow Rate (TFR). In still other embodiments, the Minimum Flow Rate (MinFR) is more dependent on the specific type of dressing and / or the type of wound that needs to be healed.

[0063] In some embodiments, the controller 170 may be configured to determine a flow rate (FR) associated with a system leak and compare it to a minimum flow rate (MinFR) desired for a particular type of dressing and / or wound type. The controller 170 may be further configured to increase leakage by opening the regulator 165 if the flow rate (FR) is below the minimum flow rate (MinFR). When the regulator 165 opens to induce leakage from the external environment to the treatment environment and increase the flow rate (FR) of fluid within the treatment system and ultimately at the tissue site 105, the controller 170 may be further configured to adjust the flow rate (FR) as the flow rate (FR) increases toward the minimum flow rate (MinFR) and then maintain the flow rate (FR) above the minimum flow rate (MinFR). In some embodiments, the controller 170 may be further configured to include an algorithm for directing leakage into the treatment environment in a controlled manner. For example, the algorithm may include a linear or nonlinear ramp function to increase the flow rate (FR) to the minimum flow rate (MinFR). The algorithm may further comprise a sinusoidal function that varies amplitude and / or frequency to maintain the flow rate (FR) above a minimum flow rate (MinFR). The algorithm may further include a digital function that varies the flow rate (FR) by varying amplitude, frequency, and / or duty cycle.

[0064] As discussed above, system 100 may have system leaks arising from the dressings and / or other components of the system as well as the wound itself. Therefore, different reduced pressure treatment systems may have different system leak values ​​depending on these factors. For example, one system may have a low system leak of 50 cc / min, while another system may have a low leak condition of 100 cc / min. In some embodiments, the low leak condition may be set as a minimum flow rate (MinFR), which may also vary, for example, between 50 and 100 cc / min. Controller 170 may be configured to open regulator 165 to induce leakage into treatment system 100 when the flow rate (FR) of fluid within the treatment system falls below the minimum flow rate (MinFR).

[0065] In one example, a dressing has a minimum flow rate (MinFR) requirement of 100 cc / min, but the controller measures a flow rate (FR) of 50 cc / min, requiring the fluid to drain into a canister two feet above the dressing. Gravity and thicker fluid due to exudate can cause the fluid flow of such a small leak to remain stationary within the dressing. However, the controller detects that the flow rate (FR) is lower than the 100 cc / min minimum flow rate (MinFR) requirement and opens the regulator to control the leak, inducing airflow into the treatment environment of the dressing. The algorithm may further include digital functions that vary the flow rate (FR) by varying the amplitude, frequency, and / or duty cycle. For example, a minimum flow rate (MinFR) requirement of 100 cc / min equates to a 1.0 mmHg / 10 sec drop, and a flow rate (FR) of 50 cc / min equates to a 0.5 mmHg / 10 sec drop. If the algorithm includes, for example, a digital function, the frequency can be increased to a duty cycle of 20% / cycle, i.e., 1 cycle / 5 seconds or 12 cycles / minute with a 1 second width / cycle. This will decrease the wound pressure (WP) at the tissue site (alternatively, increase pressure decay) and increase the flow rate (FR) to at least 100 cc / min, thereby overcoming gravity and allowing the canister to move 2 feet above the dressing. The bang-bang controller, in conjunction with the PID controller, may then operate as described above to adjust the wound pressure (WP) as needed.

[0066] 7, an exemplary embodiment of a method or process for controlling wound pressure (WP) is implemented on a controller, such as, for example, controller 170 as described above, or alternatively, on another exemplary embodiment of controller 170. Controller 170 and other components may implement this process as described above according to a treatment loop 700, illustrated as a flowchart in FIG. 7. Treatment loop 700 includes a treatment algorithm 703 for selecting an appropriate controller, i.e., a PID controller or a bang-bang controller, to control the delivery of reduced pressure to the tissue site while simultaneously conserving power and reducing noise from pump 112 and motor 114. Controller 170 first checks at 705 whether negative pressure treatment system 100 is turned on, and if not, the applied voltage (VA) is set to 0 V at 707 and applied to motor 114 as the new motor voltage (VM) at 709, preventing motor 114 from operating. When the negative pressure therapy system 100 is turned on, the controller 170 checks at 711 to determine whether enough time has elapsed to activate the therapy algorithm 703, i.e., the duty cycle therapy time (tDC). The duty cycle of the therapy algorithm 703 may be, for example, approximately 50 ms. Thus, if it has been less than 50 ms since the therapy algorithm 703 was last calculated, the motor voltage (VM) remains set to the previously applied voltage (VA) at 709. The duty cycle of the therapy loop 700 itself may be, for example, 10 ms without activating the therapy algorithm 703. However, if it has been more than 50 ms, the controller 170 recalculates the therapy algorithm 703 and proceeds to check at 713 the current wound pressure (WP) and / or pump pressure (PP) against their corresponding maximum and minimum wound pressure and pump pressure values, i.e., (WPmax) and (WPmin), and (PPmax) and (PPmin), respectively, as described above.

[0067] The treatment algorithm 703 begins at 713 by determining whether the bang-bang controller is active. If the PID controller is active and the bang-bang controller is not active, then at 715 the local pump pressure (PPL) is set to the current pump pressure (PPC). As described above, the PID control adjusts the applied voltage (VA) to the DC motor 114 to achieve a pump pressure (PP) between a minimum pump pressure value (PPmin) and a maximum pump pressure value (PPmax) to maintain the wound pressure (WP) near the target wound pressure (TP). Referring back to FIG. 6 as an example, the PID controller continues to control the pump pressure (PP) during a high leak condition by varying the first pump pressure (PP1) between 140 mmHg and 141 mmHg to maintain the wound pressure (WP) at the target wound pressure (TWP) of 125 mmHg. The controller 170 determines at 717 the value of the applied voltage (VA) that corresponds to the current pump pressure (PPC) and applies that voltage as the motor voltage (VM) at 709. However, if the bang-bang controller is operating or active as shown in Figure 5B, the therapy algorithm 703 determines at 719 whether the bang-bang controller is increasing or decreasing.

[0068] If the wound pressure (WP) drops too low in the decrease mode, for example, below the minimum wound pressure value (WPmin) as described above, the vacuum pump 112 is turned on at an applied voltage (VA) greater than the stall voltage, i.e., the bang-bang on voltage (VON), to increase the pump pressure (PP) to the maximum pump pressure (PPmax) in the increase mode. The bang-bang on voltage (VON) continues to be applied until the pump pressure (PP) reaches the maximum pump pressure value (PPmax), for example, as shown at 501 and 503, or the wound pressure (WP) reaches the maximum wound pressure value (WPmax), for example, as shown at 502 and 504, whichever occurs first. When the wound pressure (WP) is in the increase mode, the treatment algorithm 703 sets the local target wound pressure (TPL) at 723 to the target wound pressure (TP) plus a hysteresis value (H). The hysteresis value (H) is the maximum amount that the pressure should increase above the target wound pressure (TP) when the wound pressure (WP) is in increase mode before the controller 170 turns off the pump 112 to protect the tissue site 105 from excessive, potentially damaging, reduced pressure. The hysteresis value (H) sets an upper limit above the target wound pressure (TP), which is the maximum pressure value (WPmax). For example, if the hysteresis value (H) is 10 mmHg, the maximum wound pressure value (WPmax) is set to 135 mmHg, as shown in FIG. 5B. The wound pressure (WP) typically follows the rising pump pressure (PP) as shown by the wound pressure peaks at 502 and 504 and the pump pressure peaks at 501 and 503, so that when the pump pressure (PP) reaches a maximum pump pressure value (PPmax), e.g., about 140 mmHg at 501 at 503, the wound pressure (WP) typically is less than the maximum wound pressure value (WPmax), e.g., about 132 mmHg at 505 and 506. As a result, the controller 170 allows the bang-bang controller to continue regulating the application of reduced pressure, but turns off the pump 112 during the ramp-down mode of the reduced pressure cycle.

[0069] Accordingly, if the wound pressure (WP) rises too high in the increase mode, for example, above the maximum wound pressure value (WPmax) or the maximum pump pressure value (PPmax) as described above, the reduced pressure pump 112 is turned off so that no pump pressure (PP) is applied, allowing residual pressure within the reduced pressure treatment system 100 to decrease in the decrease mode as a result of a leak in the system. The reduced pressure pump 112 remains off until the wound pressure (WP) again falls below the minimum wound pressure value (WPmin), for example, as shown at 508, or the pump pressure (PP) falls below the minimum pump pressure value (PPmin), for example, as shown at 507, whichever occurs first. When the wound pressure (WP) is in the decrease mode as described above, the treatment algorithm 703 sets the local target wound pressure (TPL) at 721 to the target wound pressure (TP) minus the hysteresis value (H). The hysteresis value (H) is the minimum amount that the wound pressure (WP) must decrease below the target wound pressure (TP) when the wound pressure (WP) is in a decrease mode before the controller 170 determines that the flow rate (FR) has increased to a rate large enough to require the PID controller to maintain the wound pressure (WP) closer to the target wound pressure (TP) as described above. Thus, the hysteresis value (H) also sets a lower limit below the target wound pressure (TP), which is the minimum pressure value (WPmin). For example, if the hysteresis value (H) is 10 mmHg, the minimum wound pressure value (WPmin) is set to 115 mmHg, as shown in FIG. 5B. As the pump pressure (PP) normally follows the decreasing wound pressure (WP) as shown between the pump and wound pressure peaks at 501 and 502 respectively and the pump and wound pressure minimums at 507 and 508 respectively, the bang bang controller turns the pump 112 back on at 507, after which the wound pressure (WP) begins to increase again in the ramp up mode. As a result, the controller 170 allows the bang bang controller to continue to regulate the application of reduced pressure, and does so by turning on the pump 112 in the ramp up mode of the reduced pressure cycle.The bang bang controller uses a minimum wound pressure value (WPmin) of 115 mmHg and a maximum wound pressure value (WPmax) of 135 mmHg, or a hysteresis value (H) to allow the wound pressure (WP) to effectively oscillate around a target wound pressure (TWP) of 125 mmHg such that it falls between two limits which can be separately programmed into the controller 170. In either case, the bang bang controller maintains the wound pressure (WP) within a wound pressure range (δWP), for example a wound pressure range (δWP) of 20 mmHg.

[0070] After the treatment algorithm 703 sets the motor voltage (VM) equal to the applied voltage (VA) at 709 to re-enter the treatment loop 700, the treatment loop 700 then reads the current flow rate (FR) measured by the controller 170 at 725 and determines at 727 whether the current flow rate (FR) is less than the target flow rate (TFR). If the flow rate (FR) is less than the target flow rate (TFR), indicating a low leak condition as described above, the bang-bang controller remains on or is enabled, as shown at 729. However, if the flow rate (FR) is equal to or greater than the target flow rate (TFR), indicating a high leak condition as described above, the bang-bang controller remains off or is disabled, as shown at 731. Finally, the treatment loop 700 checks at 733 whether the negative pressure wound therapy system 100 has been turned off, and if not, continues the treatment loop as shown at 735. If the negative pressure treatment system 100 is turned off, the treatment loop ends at 737.

[0071] 8, an exemplary embodiment of a method or process for controlling the flow rate (FR) is implemented on a controller, such as, for example, on controller 170 as described above, or alternatively, on another exemplary embodiment of controller 170. Controller 170 and other components can implement this process as described above according to a flow rate (FR) loop 800, shown as a flowchart in FIG. 8. Flow rate (FR) loop 800 is periodically executed to ensure that the flow rate (FR) has not decreased below a minimum flow rate (MinFR). Flow rate (FR) loop 800 begins by reading the flow rate (FR) at 802 and then comparing the flow rate (FR) to the minimum flow rate (MinFR) at 804. If the flow rate (FR) is equal to or greater than the minimum flow rate (MinFR) at 804, negative pressure therapy continues at 806, and in some embodiments, may return to the treatment loop at 700. If the flow rate (FR) is less than the minimum flow rate (MinFR) at 804, the flow rate (FR) loop 800 generates a low leak signal (not shown) and proceeds to trigger a leak in response to the low leak signal at 808, as described in more detail above. The flow rate (FR) loop 800 again reads the flow rate (FR) (not shown) and compares the new flow rate (TF) to the minimum flow rate (MinFR) at 810. If the new flow rate (FR) is still less than the minimum flow rate (MinFR) at 810, the flow rate (FR) loop 800 continues to trigger a leak at 808, as described in more detail above. This cycle continues until the new flow rate (FR) is greater than, rather than less than, the minimum flow rate (MinFR) at 810. Negative pressure therapy continues onto the treatment loop 700 at 806 in preferred embodiments, although treatment loop 700 should in some embodiments only begin after the system has reached equilibrium within the desired wound pressure (WP) range.

[0072] The systems, devices, and methods described herein can provide significant advantages. For example, a PID control algorithm constantly adjusts the negative pressure source to maintain pressure within a specified tolerance, which can be inefficient in low-leak applications and consume more power than a simple hysteresis control algorithm. Conversely, a hysteresis algorithm can perform well in low-leak applications and use relatively little power, but in high-leak applications, it can cause the negative pressure source to frequently turn on and off, which can be noisy and increase power consumption. Hybrid control, as described herein, can combine the advantages of PID and hysteresis control algorithms to minimize power consumption and noise. If the negative pressure therapy application is low-leak, for example, the hybrid control algorithm can select a hysteresis control algorithm to minimize power consumption. If the application changes or generates a higher leak, the hybrid control algorithm can switch to a PID control algorithm to minimize noise.

[0073] While illustrated in several exemplary embodiments, those skilled in the art will recognize that the systems, apparatus, and methods described herein are susceptible to various changes and modifications. Further, the description of various alternatives using terms such as "or" does not require mutual exclusivity unless clearly required by context, and the indefinite article "a" or "an" does not limit the subject to a single case unless clearly required by context.

[0074] The appended claims recite the novelty and inventive step of the above-described subject matter, but the claims may also encompass additional subject matter not specifically described in detail. For example, certain features, elements, or aspects may be omitted from the claims if they are not necessary to distinguish the novel and inventive feature from that already known to those skilled in the art. Features, elements, and aspects described herein may also be combined or replaced by alternative features serving the same, equivalent, or similar purpose without departing from the scope of the invention as defined by the appended claims.

Claims

1. 1. A system for promoting wound healing at a tissue site, comprising: a dressing configured to be placed at the tissue site and adapted to be covered by a drape to form a therapeutic environment isolated from an external environment for maintaining wound pressure (WP); a negative pressure source including a pump adapted to generate a pump pressure (PP), the pump further adapted to be fluidly coupled to the porous pad to apply negative pressure to the tissue site; a flow sensor having an input fluidly coupled between the pump and the porous pad and an output for providing a flow signal representative of a fluid flow rate (FR) indicative of leakage between the pump and the porous pad; a controller having an input coupled to the output of the flow sensor and an output, the controller configured to: (i) determine a flow rate (FR) based on the flow rate signal; (ii) compare the flow rate (FR) to a minimum flow rate (MinFR); and (iii) generate a low leak signal at the output of the controller when the flow rate (FR) is less than the minimum flow rate (MinFR); a regulator coupled to the output of the controller and adapted to increase the flow rate (FR) by increasing the leakage into the treatment environment in response to generation of a low leak signal; A system comprising:

2. 2. The system of claim 1, wherein the flow sensor comprises a first pressure sensor having a first input for sensing the pump pressure (PP) and a first output for providing a signal indicative of the pump pressure (PP), and a second pressure sensor having a second input for sensing the wound pressure (WP) and a second output for providing a signal indicative of the wound pressure (WP), and the controller is electrically coupled to the first output of the first pump and the second output of the second pump and is further configured to determine the flow rate (FR) based on a difference between the pump pressure (PP) and the wound pressure (WP).

3. The system of claim 1 , wherein the regulator is configured to increase the leakage by venting gas from the external environment to the treatment environment.

4. The system of claim 3 , wherein the regulator is a control valve.

5. The system of claim 3 , wherein the regulator is further configured to increase the leakage from 0 to a leakage threshold.

6. The system of claim 5 , wherein the regulator is further configured to increase the leakage by venting gas at a constant rate.

7. The system of claim 5 , wherein the regulator is further configured to increase the leakage by venting gas at a variable rate.

8. The system of claim 7 , wherein the regulator is further configured to vary an amplitude of the variable speed.

9. The system of claim 7 , wherein the regulator is further configured to vary a frequency of the variable speed.

10. The system of claim 7 , wherein the regulator is further configured to vary a duty cycle of the variable speed.

11. The system of claim 1 , wherein the minimum flow rate (MinFR) is less than about 50 cc / min.

12. The system of claim 1 , wherein the minimum flow rate (MinFR) is less than about 100 cc / min.

13. The system of claim 1 , wherein the minimum flow rate (MinFR) comprises two or more target flow rates.

14. The system of claim 13 , wherein the controller is further configured to adjust the flow rate (FR) based on a first target flow rate and a second target flow rate.

15. 10. The system of claim 1, wherein the minimum flow rates (MinFR) include a first target flow rate of less than about 50 cc / min and a second target flow rate of less than about 100 cc / min.

16. The system of claim 1 , wherein the pump is coupled to a motor for driving the pump in response to application of power from a power source.

17. 17. The system of claim 16, wherein the controller is further configured to increase negative pressure at the tissue site by increasing power applied to the motor when the wound pressure (WP) is less than a minimum wound pressure (WPMin).

18. 17. The system of claim 16, wherein the controller is further configured to decrease the negative pressure at the tissue site by decreasing the power applied to the motor when the wound pressure (WP) is greater than a maximum wound pressure (WpMAX).

19. 17. The system of claim 16, wherein the controller is further configured to compare the pump pressure (PP) to a target pump pressure (TPP) and, in response to the comparison, vary the power applied to the motor to maintain the pump pressure (PP) near the target pump pressure (TPP).

20. The system of claim 16 , wherein the controller is further configured to determine the flow rate (FR) based on a time rate of change of the power applied to the motor.

21. 17. The system of claim 16, wherein the controller is further configured to determine the flow rate (FR) by determining a time rate of change of a voltage applied to the motor.

22. 17. The system of claim 16, wherein the controller is further configured to determine the flow rate (FR) by determining a time rate of change of current drawn by the motor.

23. 1. A system for promoting wound healing at a tissue site, comprising: a dressing configured to be placed at the tissue site and adapted to be covered by a drape to form a treatment environment isolated from an external environment for receiving negative pressure; a flow sensor having an input fluidly coupled to the porous pad and an output for providing a flow signal representative of a flow rate (FR) of a fluid proximate the porous pad; a controller coupled to the output of the flow sensor and configured to (i) determine a flow rate (FR) based on the flow rate signal, (ii) compare the flow rate (FR) to a minimum flow rate (MinFR), and (iii) generate a low leak signal when the flow rate (FR) is less than the minimum flow rate (MinFR); a regulator coupled to the controller and adapted to increase the flow rate (FR) by increasing leakage into the treatment environment in response to generation of a low leak signal; A system comprising:

24. 24. The system of claim 23, wherein the flow sensor comprises a first pressure sensor for sensing a first pressure and a second pressure sensor for sensing a second pressure, and the controller is electrically coupled to the first pressure sensor and the second pressure sensor and is further configured to determine the flow rate (FR) based on a difference between the first pressure and the second pressure.

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