Apparatus for controlling leakage of reduced pressure therapy system

Abstract

In one example embodiment, a system for stimulating healing of tissue at a wound site is disclosed. The system includes a dressing including a porous pad and a sterile drape covering the pad at the wound site to maintain negative pressure at the wound site; a negative pressure source including a pump and an electric motor to generate a pump pressure to apply to the wound site; and a first pressure sensor to sense 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 to induce a leak into a therapeutic environment of the system to maintain a minimum level of fluid flow within the dressing.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 062,218, filed August 6, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The invention as set forth in the appended claims relates generally to tissue treatment systems, and more specifically, but not in a limiting way, to controlling leakage in negative pressure wound therapy systems. Background Technology

[0004] Clinical research and practice have shown that reducing pressure near tissue sites can enhance and accelerate the growth of new tissue at those sites. This phenomenon has numerous applications, but it has proven particularly beneficial for wound management. Regardless of the cause of the wound—whether trauma, surgery, or other reasons—proper wound care is crucial for outcome. Treating wounds or other tissues with pressure reduction is commonly referred to as "negative pressure therapy," but it is also known by other names, including "negative pressure wound therapy," "decompression therapy," "vacuum therapy," and "vacuum-assisted closure." Negative pressure therapy offers numerous benefits, including the migration of epithelial and subcutaneous tissues, improved blood flow, and micro-deformation of tissues at the wound site. These benefits can work together to increase granulation tissue development and reduce healing time.

[0005] While the clinical benefits of negative pressure therapy are well-known, its cost and complexity may be limiting factors in its application, and the development and operation of negative pressure systems, components, and processes continue to pose significant challenges to manufacturers, healthcare providers, and patients. Summary of the Invention

[0006] The appended claims set forth novel and useful systems, apparatus, and methods for maintaining negative pressure under both low-leakage and high-leakage conditions in negative pressure therapeutic environments. Exemplary embodiments are also provided to enable those skilled in the art to make and use the claimed subject matter.

[0007] Reduced pressure provided to a tissue site, such as, for example, an incision or a wound, by a reduced pressure treatment system can need to be properly controlled to improve the effectiveness of the reduced pressure treatment. A reduced pressure treatment system can include a pump for providing reduced pressure, a wound dressing disposed adjacent to a wound, and a sterile drape covering both to provide a sealed environment to provide reduced pressure therapy from the pump to the sealed environment. However, leaks can occur in the dressing and other components of the reduced pressure treatment system, such as, for example, between the sterile drape and the tissue site, i.e., system leaks. System leaks can be high system leaks or low system leaks, depending on the flow rate of the fluid within the system, particularly adjacent to the dressing. In a low system leak system, it can be necessary to monitor and control the flow rate to ensure that the flow rate does not fall below a minimum value, i.e., a 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 can be configured to increase the flow rate by inducing air to leak from the external environment into the treatment environment of the dressing to maintain effective treatment of the wound.

[0008] In one example embodiment, a system for promoting healing of a wound at a tissue site includes a dressing including a porous pad configured to be positioned at the tissue site and adapted to be covered by a sterile drape to form a treatment environment sealed from an external environment to maintain wound pressure (WP) at the tissue site. The system can also include a reduced 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 reduced pressure to the tissue site. The system can also include a flow rate sensor having an input fluidly coupled between the pump and the porous pad and an output to provide a flow rate signal indicative of a flow rate (FR) of a fluid indicative of a leak between the pump and the porous pad. The system can also include a controller having an input coupled to the output of the flow rate sensor and the controller having an output. In some embodiments, the controller can be configured to determine a flow rate (FR) based on the flow rate signal and then compare the measured flow rate (FR) to a minimum flow rate (MinFR). The controller can be further configured to generate a low leak signal at the output of the controller when the flow rate (FR) is less than the minimum flow rate (MinFR). The system can also include a regulator coupled to the output of the controller, the regulator can be adapted to increase the flow rate (FR) by increasing the leak between the pump and the porous pad in response to the occurrence of a low leak signal.

[0009] In some example embodiments, the flow rate sensor can include 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). The controller can be electrically coupled to the first output of the first pump and the second output of the second pump and can be further configured to determine the flow rate (FR) based on a difference between the pump pressure (PP) and the wound pressure (WP).

[0010] In one example embodiment, a method for stimulating healing of a wound at a tissue site can include positioning a porous pad at the tissue site and covering the porous pad with a sterile drape to form a treatment environment isolated from an external environment to maintain a wound pressure (WP) at the tissue site. The method can also include applying negative pressure to the porous pad using a pump to generate a pump pressure (PP) to apply negative pressure to the tissue site. The method can also include determining an indicative leak flow rate (FR) of fluid between the pump and the porous pad and 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 also includes generating a low leak signal and increasing the flow rate (FR) by increasing a leak between the pump and the porous pad in response to the occurrence of the low leak signal.

[0011] The objectives, advantages and preferred modes of use of the claimed subject matter will be best understood by making reference to the following detailed description of illustrative embodiments when considered in connection with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is a functional block diagram of an embodiment of an example of a reduced pressure therapy system according to the present specification, including a controller coupled to a pump motor and a pump, the controller can provide mixed control of pressure provided to a tissue site and control of leaks of the reduced pressure therapy system;

[0013] Figure 2 is a graph illustrating a stall voltage characteristic of a pump motor that can be used in a reduced pressure therapy system of Figure 1

[0014] Figure 3 ​is a graph showing pressure control of a motor driven system according to an illustrative embodiment, where the x-axis represents time in minutes (min) and / or seconds (sec) and the y-axis represents pressure generated by a pump in millimeters of mercury (mmHg) that varies over time in a continuous control mode and an intermittent mode used in a reduced pressure treatment system of Figure 1 ;

[0015] Figure 4 is a graph showing pressure control of a motor driven system according to an illustrative embodiment, where the x-axis represents time in minutes (min) and / or seconds (sec) and the y-axis represents pressure generated by a pump in millimeters of mercury (mmHg) that is used with a PID controller and / or an on-off controller to compare a manipulated variable such as a tissue site or wound pressure (WP) at a tissue site with a controlled variable such as a pump pressure (PP);

[0016] Figure 5A and Figure 5B is a graph showing pressure control of an on-off controller according to an illustrative embodiment, where the x-axis represents time in seconds (sec) and the y-axis represents pressure generated by a pump in millimeters of mercury (mmHg) that varies over time in a continuous control mode, and where the pressure control of the on-off controller is subject to a greater discharge pressure generated by a reduced pressure treatment system of Figure 5B as shown in Figure 5A Figure 1 ;

[0017] Figure 6 is a graph showing pressure control of a PID controller according to an illustrative embodiment, where the x-axis represents time in seconds (sec) and the y-axis represents pressure generated by a pump in millimeters of mercury (mmHg) that varies over time in a continuous control mode, and where the horizontal time scale is substantially the same as the horizontal time scale shown in Figure 5B for comparing pressure control with pressure control of an on-off controller;

[0018] Figure 7 is a flowchart showing a process or treatment cycle for controlling reduced pressure at a tissue site according to an illustrative embodiment of the present disclosure, which can be stored on a controller of Figure 1 , which includes a treatment algorithm for selecting an appropriate pump pressure control to control reduced pressure at a tissue site; and

[0019] Figure 8 ​is a flowchart illustrating a process for controlling a gas leak or flow rate cycle from an external environment of a reduced pressure treatment system into a treatment environment in accordance with an illustrative embodiment. DETAILED DESCRIPTION

[0020] The following description of example embodiments provides information applicable to the subject matter set forth in the appended claims that enable a person of ordinary skill in the art to make and use the subject matter. Certain details may be omitted, however, for the sake of clarity.

[0021] Example embodiments may also be described herein with reference to spatial relationships or spatial orientations between various elements or with respect to the spatial orientation of various elements depicted in the drawings. Generally, such relationships or orientations are assumed to coincide with a reference frame of a patient in a location to be treated or relative to the patient. However, as will be recognized by those skilled in the art, this reference frame is merely a descriptive convenience and not a strict requirement.

[0022] Figure 1 is a simplified functional block diagram of an example embodiment of a reduced pressure treatment system 100 that can provide negative pressure therapy in accordance with the present specification. More specifically, the treatment system 100 can be used to control which pump pressure control to utilize to provide an appropriate amount of reduced pressure to a tissue site 105. The tissue site 105 can be any human, animal, or other organism's body tissue, including bone tissue, adipose tissue, muscle tissue, dermal tissue, vascular tissue, connective tissue, cartilage, tendon, ligament, or any other tissue. While the tissue site 105 can include a wound, diseased, or defective tissue, 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 exudate and other fluids from the tissue site 105, as well as to promote growth of additional tissue. In the case where the tissue site 105 is a wound site, growth of granulation tissue and removal of exudate and bacteria promotes healing of the wound. Application of reduced pressure to tissue that is not wounded or defective, including healthy tissue, can be used to promote growth of tissue that can be harvested and transplanted to another tissue location.

[0023] The reduced pressure applied to the tissue site 105 can be provided by a reduced pressure source 110. The reduced pressure source 110 can be any type of manually, mechanically, or electrically operated pump. Non-limiting examples of reduced pressure sources 110 include devices driven by stored energy and capable of generating reduced pressure. Examples of these stored energy reduced pressure sources include, but are not limited to, pumps driven by piezoelectric energy, spring energy, solar energy, kinetic energy, energy stored in a capacitor, combustion, and energy generated by a Stirling or similar cycle. Additional other devices and processes that can be used or included in the reduced pressure source 110 include syringes, lead screws, ratchets, clockwork drive devices, pendulum drive devices, hand generators, osmotic processes, thermal heating processes, and processes in which vacuum pressure is generated by condensation. In another embodiment, the reduced pressure source 110 can include a pump 112 that provides negative or reduced pressure to the tissue site 105, i.e., pump pressure (PP), which can be driven by a motor 114 that is electrically coupled to a controller 170, which is also a component of the reduced pressure treatment system 100, also referred to as a system controller. The motor 114 can be a DC motor powered by a DC power source such as, for example, a battery (not shown). Preferably, the pump 112 uses a small amount of electrical power and is capable of operating for an extended period of time on a single charge of a battery, such as, for example, a diaphragm pump.

[0024] In one exemplary embodiment, the reduced pressure source 110 includes a DC motor 114 powered by a battery (i.e., an external power source). The external power can be varied by changing the current or voltage applied to the motor, i.e., the "applied voltage" (V A ), to control the speed of the motor. The applied voltage (V 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. The reduced pressure source 110 also includes a pump 112 that provides reduced pressure or vacuum to the tissue site 105. Thus, the pump 112 represents a load on the DC motor 114 such that when the therapy requires an increase in reduced pressure at the tissue site 105, the applied voltage (VA) to the DC motor 114 is increased to achieve the target reduced pressure at the tissue site 105. It is known to those skilled in the art that the DC motor 114 will not run or turn the pump until the applied voltage (VA) is sufficient to overcome the inertia or load of the pump 112, which in this case can be a diaphragm pump. A

[0025] Referring more particularly to Figure 2 , a graph 301 is shown that illustrates the voltage to the pump motor 114 necessary to start the pump 112, where the X-axis represents the pump pressure (PP) load of the DC pump motor and the Y-axis represents the applied voltage (VA). The graph 301 illustrates that the pump motor 114 requires a minimum applied voltage (V A ​). For example, when loaded at a pressure of 100 mmHg as indicated by dashed lines 302, 303, controller 170 can need to apply at least 2.3 V to DC motor 114 before the DC motor turns pump 112. Applying any voltage less than 2.3 V to DC motor 114 will produce insufficient power for the motor to turn the pump, i.e., the loaded motor will remain stopped or “stalled” such that the motor cannot turn the pump. Thus, the 2.3 V value is commonly referred to in the industry as the “stall voltage” that will be calculated for a DC motor under a load at 100 mmHg pressure, i.e., the “stall pressure.” Correspondingly, controller 170 can need to apply a larger voltage of at least 2.45 V 304 to the DC motor before the DC motor turns the pump when loaded at a larger pressure of 125 mmHg. Applying any voltage less than the stall voltage of 2.45 V to the DC motor will be insufficient for the DC motor to turn the pump at the stall pressure of 125 mmHg. The change in stall voltage is proportional to the change in pressure load on the motor, i.e., the greater the pressure load on the motor, the greater the stall voltage needed to overcome the pressure load.

[0026] The particular stall voltage for a particular DC motor used to drive a diaphragm pump can be determined by one of skill in the art from the specifications available for the DC motor. The diaphragm pump and DC motor can be an integrated device such as, for example, a Thomas model 30130002 series 4.5 V diaphragm pump, for which this information is readily available. (Thomas; thomas.de@gardnerdenver.com) Referring again to Figure 2 , graph 301 shows the stall voltage for a pump motor, with the Y-axis representing the stall voltage calculated for the Thomas motor based on the specifications currently provided at the Thomas website mentioned above. The examples provided in the above paragraph include only exemplary voltages and pressures. Graph 301 simply shows that one of skill in the art can calculate various stall voltages for a DC motor based on the specifications commonly available for the DC motor. One working with a micro-diaphragm pump driven by a DC motor such as the Thomas DC motor commonly refers to the stall voltage as the “stall power,” i.e., the stall voltage multiplied by the rated current for the particular DC motor.

[0027] Data from pump specifications is typically limited to the maximum flow rate and the relationship of vacuum pressure at the maximum pump voltage (e.g., 4.5V for the Thomas pump described above). Positive pressure is specified in millibars (positive pressure in mmHg = 0.7500616827042 * millibars), and vacuum pressure is specified in percent of vacuum. For example, if 100% maximum vacuum is specified as 760 mmHg, then 40% maximum vacuum would equal a vacuum of 304 mmHg (= 0.4 * 760 mmHg). In this example, a vacuum pressure of 304 mmHg would be the theoretical maximum vacuum pressure available for the pump to run at 4.5V and be allowed to run until the DC motor stalls. Figure 2 The graph 301 in FIG. 3 is generated based on the motor specifications and the observed stall voltage required to drive the pump.

[0028] The formula for calculating the stall voltage for this particular pump is as follows: stall voltage = 1.638V + (0.006515V / mmHg * X mmHg), where X is the current vacuum pressure. Thus, at a vacuum of 50 mmHg, the stall voltage equals 1.96V (1.638 + (0.006515 * 50)); at a vacuum of 125 mmHg, the stall voltage equals 2.45V (1.638 + (0.006515 * 125)), as indicated by dashed lines 304, 305; and at a vacuum of 175 mmHg, the stall voltage equals 2.78V (1.638 + (0.006515 * 175)), as indicated by dashed lines 306, 307. Again, the higher the vacuum pressure, the higher the applied voltage required to start the pump. Otherwise, the pump stalls and will not move until the necessary stall voltage is applied. When the pump stalls, the DC motor simply overheats, which can damage the DC motor and reduce battery life.

[0029] Referring back to Figure 1 , the reduced pressure source 110 can provide reduced pressure to the tissue site 105 via a dressing 115. The dressing 115 can include a tissue interface, such as, for example, a manifold 120, which can be placed adjacent to or in contact with the tissue site 105. The manifold 120 can be a biocompatible porous material that can be placed in contact with the tissue site 105 and distribute reduced pressure to the tissue site 105. The manifold 120 can be made of a foam, gauze, felt pad, or other material suitable for a particular biological application. The manifold 120 can include a plurality of flow channels or pathways to facilitate distribution of reduced pressure or fluid to or from the tissue site 105.

[0030] In one embodiment, manifold 120 is a porous foam and includes a plurality of interconnected small pores or holes that act as flow channels. The porous foam can be a polyurethane open cell reticulated foam such as GranuFoam manufactured by Kinetic Concepts, Inc. of San Antonio, Texas. If an open cell foam is used, the porosity can vary but is preferably about 400 microns to 600 microns. The flow channels allow fluid communication throughout the portion of manifold 120 having open cells. The small pores and flow channels can be uniform in shape and size or can include a patterned or random variation in shape and size. Variation in the small pores of the manifold in shape and size results in variation in the flow channels, and this characteristic can be used to alter the flow characteristics of the fluid through manifold 120. Manifold 120 can also include portions having "closed cells." These closed cell portions of manifold 120 contain a plurality of holes that are not fluidly connected to adjacent holes. The closed cell portions can be selectively placed in manifold 120 to prevent fluid transfer through the perimeter surface of manifold 120.

[0031] Manifold 120 can also be constructed from a bioabsorbable material that does not have to be removed from the patient's body after use of reduced pressure treatment system 100. Suitable bioabsorbable materials can include, but are not limited to, a polymer blend of polylactic acid (PLA) and polyglycolic acid (PGA). The polymer blend can also include, but is not limited to, polycarbonates, polyfumarates, and caprolactones. Manifold 120 can also be used as a scaffold for new cell growth or a scaffold material can be used in conjunction with manifold 120 to promote cell growth. A scaffold is a substance or structure used to enhance or promote the growth of cells or the formation of tissue, 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 processed allograft materials. In one example, the scaffold material has a high void fraction (i.e., high air content).

[0032] The dressing 115 can also include a sealing member 125 (also referred to as a drape or cover). The manifold 120 can be secured to the tissue site 105 using the sealing member 125. The sealing member 125 can be a cover used to secure the manifold 120 at the tissue site 105. While the sealing member 125 can be impermeable or semi-permeable, in one example, the sealing member 125 is capable of maintaining reduced pressure at the tissue site 105 after the sealing member 125 is installed over the manifold 120. The sealing member 125 can be a flexible drape or film formed from a silicone-based compound, an acrylic, a hydrogel, or a hydrogel foam material, or any other biocompatible material that includes the desired impermeable or permeable properties for the tissue site 105. The sealing member 125 can be formed from a hydrophobic material to prevent the sealing member 125 from absorbing moisture. In one embodiment, the sealing member 125 is configured to provide a sealed connection with the tissue surrounding the manifold 120 and the tissue site 105. The sealed connection can be provided by an adhesive (not shown) positioned along the perimeter of the sealing member 125 or on any portion of the sealing member 125 to secure the sealing member 125 to the manifold 120 or to the unbroken epidermis at the tissue site perimeter (i.e., around the tissue). The adhesive can be pre-positioned on the sealing member 125 or can be sprayed or otherwise applied to the sealing member 125 just prior to installation of the sealing member 125.

[0033] Generally, components of the therapy system 100 can be coupled, either directly or indirectly. Components can be fluidly coupled to one another to provide a path for transferring fluids (i.e., liquids and / or gases) between the components. In some embodiments, for example, components can be fluidly coupled by a tube. As used herein, “tube” broadly refers to a tube, pipe, hose, conduit, or other structure having one or more lumens suitable for conveying a fluid between two ends. Typically, a tube is an elongated cylindrical structure having some flexibility, although the geometry and rigidity can vary. In some embodiments, components can additionally or alternatively be coupled by virtue of being in physical proximity, integral with a single structure, or formed from the same piece of material. In some cases, coupling can also include mechanical, thermal, electrical, or chemical coupling (such as a chemical bond).

[0034] Reduced pressure generated by the reduced-pressure source 110 can be applied to the tissue site 105 through a source tube 130 and a delivery tube 135. The source tube 130 and the delivery tube 135 can be any tube through which a gas, liquid, gel, or other fluid can flow. For example, exudate from the tissue site 105 can flow through the delivery tube 135. In Figure 1 In the depicted embodiment, the source tube 130 couples the reduced-pressure source 110 to a canister 140 and the delivery tube 135 couples the canister 140 to the dressing 115. However, in another embodiment, the reduced-pressure source 110 can be directly coupled to the dressing 115 using the delivery tube 135.

[0035] Source tube 130 and delivery tube 135 can be made of any material. Source tube 130 and delivery tube 135 can be flexible or non-flexible. Also, source tube 130 and delivery tube 135 can include one or more pathways or lumens through which fluid can flow. For example, delivery tube 135 can include two lumens. In this example, one lumen can be used to transfer exudate from tissue site 105 to canister 140. The other lumen can be used to deliver fluids such as air, antiseptics, antivirals, cell growth promoters, irrigation fluids, or other chemically active agents to tissue site 105. The fluid sources from which these fluids originate are not shown in FIG. 1. Additional details regarding the inclusion of multi-lumen tubes in reduced-pressure treatment systems 100 are provided below. Figure 1

[0036] In one embodiment, delivery tube 135 is coupled to manifold 120 via connection member 145. Connection member 145 permits fluid to be transferred from manifold 120 to delivery tube 135, and vice versa. For example, exudate collected from tissue site 105 using manifold 120 can enter delivery tube 135 via connection member 145. In another embodiment, reduced-pressure treatment system 100 does not include connection member 145. In this embodiment, delivery tube 135 can be inserted directly into sealing member 125 or manifold 120 such that an end of delivery tube 135 is adjacent to or in contact with manifold 120.

[0037] Fluid from tissue site 105, such as exudate, can flow through delivery tube 135 into canister 140. Canister 140 can be any device or cavity capable of containing fluids, such as gases and liquids, as well as fluids containing solids. For example, canister 140 can contain exudate from tissue site 105. Source tube 130 and delivery tube 135 can be directly connected to canister 140, or can be coupled to canister 140 via a connector, such as connector 150, as indicated by arrow 151. Canister 140 can be a flexible or rigid canister, bag, or bladder fluidly coupled to manifold 120 by delivery tube 135. Canister 140 can be a separate canister or can be operably combined with reduced-pressure source 110 to collect exudate and fluids.

[0038] ​The reduced pressure treatment system 100 can also include a first pressure sensor or wound pressure sensor 155 electrically coupled to the controller 170. The wound pressure sensor 155 detects the actual reduced pressure at or near the tissue site 105, i.e., the 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 terms or descriptions herein to application to the measurement of pressure at other types of tissue sites, such as, for example, an incision or a subcutaneous cavity. In one non-limiting example, the wound pressure sensor 155 is a silicon piezoresistive pressure sensor. The wound pressure sensor 155 can be configured to detect the wound pressure (WP) via the control tube 160, which is directly fluidly coupled to the connection member 145 or indirectly fluidly coupled to the connection member by a plurality of segments of the control tube 160 that pass through the canister 140. The control tube 160 can include one or more paths or lumens through which fluid can flow.

[0039] The reduced pressure treatment system 100 can also include a second pressure sensor or pump pressure sensor 156 electrically coupled to the controller 170. The pump pressure sensor 156 can be configured to detect the reduced pressure at or downstream of the canister 140, i.e., the pump pressure (PP), through control tubes 157 and 158, respectively. In other words, the pump pressure sensor 156 can be directly fluidly coupled to the canister 144 through the control tube 158 or to the source tube 130 through the control tube 157 to detect the pump pressure (PP). In one exemplary embodiment, the pump pressure sensor 156 can be a silicon piezoresistive pressure sensor. In yet another exemplary embodiment, the pump pressure (PP) can be determined by the controller 170 analyzing the DC voltage of the pump motor 114, as described with reference to Figure 2 .

[0040] The pressure sensors 155 and 156 can be located at alternating positions on or within the reduced pressure treatment system 100. Referring back to Figure 1 , the wound pressure sensor 155 is shown as being remote from the tissue site 105. In this example, the reduced pressure at the tissue site 105 can be detected directly from the remotely located wound pressure sensor 155 through the control tube 155 or indirectly from the canister 140 through a segment of the control tube 160 coupled to the wound pressure sensor 160. Also, in this example, the pump pressure sensor 156 can be directly or indirectly coupled to other remotely located components of the reduced pressure treatment system 100, such as the reduced pressure source 110, the canister 140, or any other illustrated component of the reduced pressure treatment system 100. In another example, the wound pressure sensor 155 can not require the use of the control tube 160 to detect the pressure at the tissue site 105. In one non-limiting example, the wound pressure sensor 155 is directly coupled to the manifold 120 or placed between the sealing member 125 and the manifold 120.

[0041] The reduced-pressure treatment system 100 can also include a regulator 165 electrically coupled to the controller 170. The regulator 165 can be a valve having an input and an output, where the input can be fluidly coupled to an external environment, as indicated by arrow 166, and the output fluidly coupled to the treatment environment of the reduced-pressure treatment system 100. For example, the output of the regulator 165 can be fluidly coupled to the canister 140, as indicated by arrow 167. In another embodiment, the output of the regulator 165 can be fluidly coupled to the control tube 160 (not shown). The regulator 165 can be any valve capable of releasing reduced pressure in the treatment environment of the reduced-pressure treatment system 100 to increase the flow rate of fluid therein. Non-limiting examples of the regulator 165 include a pneumatic solenoid valve, a proportional valve, or a mechanical valve. In one example, the regulator 165 can be manually controlled by an attendant. In another example, the regulator 165 can be controlled by the controller 170.

[0042] In operation, the manifold 120 can be placed within, over, on, or otherwise proximate to a tissue site. The sealing member 125 can be placed over the manifold 120 and sealed to tissue near the tissue site 105. For example, the sealing member 125 can be sealed to unbroken epidermis around the perimeter of the tissue site (i.e., around the wound). Thus, the dressing 115 can provide a sealed treatment environment proximate to the tissue site that is substantially isolated from the external environment, and the reduced-pressure source 110 can reduce pressure in the sealed treatment environment. The reduced pressure applied to the tissue site through the manifold 120 in the sealed treatment environment can induce macrostrain and microstrain in the tissue site, and remove exudate and other fluids from the tissue site, which can be collected in the canister 140 and disposed of appropriately.

[0043] The fluid mechanics of using a reduced-pressure source to reduce pressure in another component or location, such as within a sealed treatment environment, can be mathematically complex. However, the basic principles of fluid mechanics applicable to negative-pressure therapy are generally well known to those skilled in the art, and the process of reducing pressure can be illustratively described herein as, for example, "delivering," "distributing," or "generating" negative pressure.

[0044] In general, exudate and other fluids flow along a fluid path toward lower pressure. Thus, the term "downstream" generally implies things in the fluid path relatively closer to a reduced-pressure source, and, conversely, the term "upstream" implies things relatively farther from the negative-pressure source. Similarly, certain features can be conveniently described in terms of fluid "inlets" or "outlets" in this frame of reference. This orientation is generally assumed for the purposes of describing various features and components of the reduced-pressure treatment systems herein. However, in some applications, the fluid path can also be reversed (such as by replacing a reduced-pressure source with a positive-pressure source), and this descriptive convention should not be understood as a limiting convention.

[0045] In this context, the term "tissue site" broadly refers to a wound or defect located on or in tissue, including but not limited to bone tissue, adipose tissue, muscle tissue, neural tissue, dermal tissue, vascular tissue, connective tissue, cartilage, tendon, or ligament. Wounds can include, for example, chronic wounds, acute wounds, traumatic wounds, subacute wounds, and dehisced wounds, partial-thickness burns, ulcers such as diabetic ulcers, pressure ulcers, or venous insufficiency ulcers, flaps, and grafts. The term "tissue site" can also refer to a region of any tissue that is not necessarily wounded or defective, but where additional tissue growth can be desired or promoted. For example, negative pressure can be used in certain tissue regions to cause additional tissue growth, which can then be harvested and transplanted to another tissue location.

[0046] "Negative pressure or reduced pressure" generally refers to a pressure that is less than the pressure of the local environment, such as the ambient pressure in the local environment outside of the sealed treatment environment provided by the dressing 102. In many cases, the local environment pressure can also be the atmospheric pressure at which the tissue site is located. Alternatively, the pressure can be less than the hydrostatic pressure associated with the tissue at the tissue site. Unless otherwise specified, pressure values described herein are gauge pressures. Similarly, references to increases in negative pressure generally refer to decreases in absolute pressure, while decreases in negative pressure generally refer to increases in absolute pressure.

[0047] For example, the negative pressure source, such as the reduced pressure source 110, can be a reservoir of air at reduced pressure, or can be a manual or electric device that can reduce the pressure in a sealed volume, such as a vacuum pump, a suction pump, a wall suction port available at many healthcare facilities, or a micro-pump. The negative pressure source can be housed within or used in conjunction with other components, such as a treatment control unit, an alarm indicator, a memory, a database, software, a display device, or a user interface that further facilitates negative pressure therapy. For example, the reduced pressure source 110 and the controller 106 can be housed within a treatment control unit. While the amount and nature of negative pressure applied to a tissue site can vary according to the requirements of the therapy, the pressure is typically a low vacuum (also commonly referred to as rough vacuum) of between -5 mm Hg (-667 Pa) and -500 mm Hg (-66.7 kPa). A common therapeutic range is between -75 mm Hg (-9.9 kPa) and -300 mm Hg (-39.9 kPa).

[0048] As indicated above, the applied voltage (V A ) to the DC motor 114 can be used to control the pump pressure (PP) and ultimately achieve a desired or target pressure at the tissue site 105. Correspondingly, the applied voltage (V A ) provides an indication of the pump pressure (PP) and can be monitored by the controller 170, which in turn can determine the applied voltage (V Athe time-varying rate of change of the pressure differential must correspond to the time-varying rate of change of the pump pressure (PP). The controller 170 can also use this calculation to determine the flow rate of air between the reduced-pressure source 110 and the tissue site 105, i.e., the flow rate (FR). In another embodiment, the reduced-pressure treatment system 100 can also include a sensing device (not shown) that directly measures the flow rate (FR), such as, for example, a flow meter or a differentiator processor that calculates the time-varying rate of change of the difference between the wound pressure (WP) and the pump pressure (PP). Additionally, the flow rate (FR) can be determined by calculating the pressure differential based on measurements taken from two pressure sensors, such as, for example, the pressure sensors 155 and 156 described above.

[0049] Between the reduced-pressure source 110 and the tissue site 105, the flow rate (FR) can be measured, for example, in cubic centimeters of air per minute (cc / min). The flow rate (FR) provides some indication of the degree to which the dressing 115 or other components of the negative-pressure system 100 can be leaking, i.e., system leakage, which reduces the pressure at the tissue site 105 below the desired pressure for therapy. For example, a high flow rate (FR) can indicate that the dressing 115 or other components of the system 100 are considered to have a "high system leak" or "high leak condition," which requires remediation, 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) can indicate that the dressing 115 or other components of the system 100 are considered to have a "low system leak" or "low leak condition," which requires a lower pump pressure (PP) and, thus, less battery power to drive the DC motor 114 at a lower rate to maintain the desired wound pressure (WP).

[0050] The controller 170 can be an integral or separate component of the reduced-pressure treatment system 100. The controller 170 can be any device capable of processing data, such as data from the wound pressure sensor 155 and / or the pump pressure sensor 156. The controller 170 can also control the operation of one or more components of the reduced-pressure treatment system 100, such as the reduced-pressure source 110, the motor 114, the regulator 165, the pressure sensors 155 and 156, and the indicator 172. The controller 170 can control and receive data from other components (not shown) of the reduced-pressure source 110, including the pump 112 and the motor 114. In one embodiment, the controller 170 receives and processes data, such as the wound pressure (WP) from the wound pressure sensor 155, the pump pressure (PP) from the pump pressure sensor 156, and the applied voltage (V Athe flow rate (FR) of the reduced pressure source 110, as described above. The controller 170 can also control the operation of one or more components of the reduced pressure treatment system 100 to manage the wound pressure (WP) at the tissue site 105. In one embodiment, the controller 170 can include an input for receiving a desired target wound pressure (TWP) set by a clinician or other user, and can be a program for processing data related to the setting and input of the target wound pressure (TWP) to be applied to the tissue site 105.

[0051] In one exemplary embodiment, the target wound pressure (TWP) can be a fixed pressure value determined by a user / caregiver as the desired reduced pressure target for treatment at the tissue site 105, and then provided as an input to the controller 170. The user can be a nurse or doctor or other approved clinician who dictates the desired reduced pressure to be applied to the tissue site 105. The desired tissue site pressure will vary from tissue site to tissue site, but will generally be selected based on the type of tissue comprising the tissue site, the type of injury or wound, if any, the medical condition of the patient, and the preference of the attending physician. After the desired target wound pressure (TWP) is selected, the reduced pressure source 110 is controlled to achieve the target wound pressure (TWP) for application to the tissue site 105.

[0052] With more particular reference to Figure 3 The target wound pressure (TWP) can be set by the user in a continuous mode as indicated by solid line 401 and dashed line 402, in which reduced pressure is applied to the tissue site 105 until the user deactivates the reduced pressure source 110. The target wound pressure (TWP) can also be set by the user in an intermittent mode as indicated by solid lines 401, 403, and 405, in which the wound pressure (WP) cycles between the target wound pressure (TWP) and atmospheric pressure. For example, the target wound pressure (TWP) can be set by the user to 125 mmHg for a specified period of time (e.g., 5 min), then the therapy is turned off by venting the tissue site 105 to atmospheric pressure for a specified period of time (e.g., 2 min) as indicated by line 403, after which the cycle is repeated by turning the therapy back on as indicated by line 405, creating a square wave pattern between the target wound pressure (TWP) level and no pressure.

[0053] It should be appreciated that the increase in wound pressure (WP) at the tissue site 105 from ambient pressure to the target wound pressure (TWP) is not instantaneous, but is limited, depending on the type of therapy equipment and dressing. For example, the reduced pressure source 110 and dressing 115 can have an initial rise time as indicated by dashed line 407, which can vary depending on the type of dressing and therapy equipment used. For example, one therapy system can have an initial rise time in the range of between about 20-30 mmHg / sec, or more specifically equal to about 25 mmHg / sec, and another therapy system can have an initial rise time in the range of between about 5-10 mmHg / sec. The repeat rise time (line 405) can be substantially equal to the initial rise time (line 407) when operating in an intermittent mode.

[0054] The target pressure can also be a variable target pressure (VTP) controlled or determined by the controller 170, which varies in a dynamic pressure mode. For example, the variable target pressure (VTP) can vary between a maximum pressure value and a minimum pressure value, which can be set as an input by a user input of a desired range of reduced pressure for therapy at the tissue site 105. The variable target pressure (VTP) can also be processed and controlled by the controller 170, which varies the target wound pressure (TWP) according to a predetermined waveform, such as for example a sinusoidal waveform or a sawtooth waveform or a triangular waveform, which can be set as an input by a user as a desired predetermined or time-varying reduced pressure for therapy at the tissue site 105. For example, the variable target pressure (VTP) can be a reduced pressure that provides effective therapy by applying reduced pressure to the tissue site 105 in the form of a triangular waveform varying between 50 mmHg to 125 mmHg, with a rise time set at +25 mmHg / min and a fall time set at -25 mmHg / min. In another embodiment of the reduced pressure therapy system 100, the variable target pressure (VTP) can be a reduced pressure that applies reduced pressure to the tissue site 105 in the form of a triangular waveform varying between 25 mmHg to 125 mmHg, 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 determines the type of reduced pressure therapy to be used.

[0055] After the target wound pressure (TWP) is selected, the reduced pressure source 104 is operated to achieve the desired pressure at the wound site 105 by controlling the pressure (PP). In many cases, the reduced pressure source 110 will operate at a pump pressure (PP) that is higher than the target wound pressure (TWP) due to pressure losses between the reduced pressure source 110 and the tissue site 105. In addition, the discharge pressure of exudate and other fluids within the conduit can cause the vacuum pressure at the tissue site 105 to decrease. The height of the canister 140 above the tissue site 105 can determine the amount of discharge pressure exerted on the tissue site 105 by the fluid in the conduit. For exudate and fluids with a density similar to water, the discharge pressure exerted by one foot of fluid is almost 25 mmHg. Some fluids withdrawn from the tissue site 105 can even be heavier or more viscous than water and, therefore, have a more significant effect on the pressure loss at the tissue site 105.

[0056] Referring to Figure 4 As one example of a potential loss due to the weight of fluid in the conduit, the target wound pressure (TWP) specified for a particular tissue site can be -125 mmHg, where the wound pressure (WP) varies as reduced pressure is applied to the tissue site 105. (It will be understood that Figure 4 The steady sinusoidal variation in wound pressure (WP) shown is illustrative only and does not represent the actual variation in wound pressure (WP) under normal operating conditions, such as, for example Figure 5A and Figure 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 discharge pressure exerted by the fluid can create a pressure differential (δP) of about 50 mmHg. This particular example arises when the tissue site is located on a lower extremity of the patient, such as a foot, and the canister 140 is mounted near or above the patient's head (e.g., on an IV pole when the patient is in a wheelchair). Thus, if the discharge pressure of the fluid in the delivery tube 135 is about 50 mmHg, the pump 112 needs to provide a pump pressure (PP) that rises to a maximum pump pressure value (PPmax) of about 185 mmHg and falls to a minimum pump pressure value (PPmin) of about 165 mmHg (a median target pump pressure (TPP) of about 175 mmHg) to produce a target wound pressure (TWP) of about 125 mmHg at the tissue site 105.

[0057] The controller 170 can also be programmed and controlled by a user to maintain a target wound pressure (TWP) within an acceptable pressure range. For example, if a target wound pressure (TWP) is set at 125 mmHg as a desired therapeutic pressure for the tissue site 105, a user can desire that the wound pressure (WP) not vary more than ±10 mmHg from the desired target wound pressure (TWP) such 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., a wound differential pressure range (δWP) of about 20 mmHg. Thus, assuming there is a drain pressure of about 50 mmHg as described above for this example, the pump pressure (PP) must also be able to vary ±10 mmHg from a target pump pressure (TPP) such that the pump pressure (PP) can vary within a range extending from a minimum pump pressure value (PPmin) of about 165 mmHg to a maximum pump pressure value (PPmax) of about 185 mmHg, i.e., a pump differential pressure (δTTP) of about 20 mmHg. Controlling the pump pressure (PP) to remain within this range indirectly maintains the wound pressure (WP) within a range extending from a minimum wound pressure value (WPmin) of about 115 mmHg to a maximum wound pressure value (WPmax) of about 135 mmHg.

[0058] Referring to Figure 5A , as an example of wound pressure (WP) variation under normal operating conditions, as opposed to the example shown in Figure 4 , the pressure differential (δP) between the pump pressure (PP) and the wound pressure (WP) is the result of a relatively high flow rate (FR) of about 300 cc / min in the dressing 115 and other components in the system (high leak condition). In this example, the wound pressure (WP) is controlled as described above to cycle between about 135 mmHg and 115 mmHg by providing a pump pressure (PP) that rises to a maximum pump pressure value (PPmax) of about 155 mmHg and falls to a minimum pump pressure value (PPmin) of about 120 mmHg to produce a target wound pressure (TWP) of about 125 mmHg at the tissue site 105. Thus, the pressure differential (δP) is about 15 mmHg, which is much less than the pressure differential (δP) of about 50 mmHg produced by the drain pressure in the example associated with Figure 4 Figure 5B ​Another example is shown in which the pressure differential (δΡ) between the pump pressure (PP) and the wound pressure (WP) is the result of a lower flow rate (FR) of about 50 cc / min in the dressing 115 and other components in the system (low leak condition). In this example, the wound pressure (WP) is again controlled to cycle between about 135 mmHg and 115 mmHg 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 to produce a target wound pressure (TWP) of about 125 mmHg at the tissue site 105. Thus, the pressure differential (δΡ) is about 5 mmHg, which is even less than the pressure differential in the previous example.

[0059] The controller 170 can also include a start-stop controller (not shown), also known as an on-off controller or hysteresis controller. A start-stop controller is a feedback controller that switches abruptly between two states, e.g., between on and off. Essentially, a start-stop controller can apply control in an all-or-nothing form. A start-stop controller can be used to generate the pressure variations described above in connection with Figure 5A and Figure 5B the general description. Continuing the general description, a start-stop controller can operate in one mode as follows. For example, when the wound pressure (WP) falls too low to the minimum wound pressure value (WPmin), the reduced-pressure pump 112 turns on at a start time (t on ) with an applied voltage (V A ) that is greater than the stall voltage, i.e., a start-stop on voltage (V ON ) to increase the pump pressure (PP) to the maximum pump pressure (PPmax). Although the increase in pump pressure (PP) can be slightly lagged behind the application of the applied voltage (V A ), the increased pump pressure (PP) eventually results in an increase in the wound pressure (WP), as shown at time ti. The start-stop on voltage (V ON ) continues to be applied until either the pump pressure (PP) reaches the maximum pump pressure value (PPmax) or the wound pressure (WP) reaches the maximum wound pressure value (WPmax), whichever occurs first. When either of these maximum values is reached or exceeded, the reduced-pressure pump 112 turns off at an off time (t off ) so that no pump pressure (PP) is applied, allowing the residual pressure in the reduced-pressure therapy system 100 to decrease due to leaks in the system. The reduced-pressure pump 112 remains off until either the wound pressure (WP) again falls below or equal to the minimum wound pressure value (WPmin) or the pump pressure (PP) falls below or equal to the minimum pump pressure value (PPmin), whichever occurs first.

[0060] The start-stop controller switches between these two states, where the off- pressure pump 112 is turned on when the wound pressure or pump pressure drops too low in the falling mode, and turned off when the wound pressure or pump pressure rises too high in the rising mode. Referring more particularly to Figure 5B , the start-stop controller allows the wound pressure (WP) to oscillate around a target wound pressure (TWP) of 125 mmHg, as contained 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). Unless the wound pressure (WP) exceeds either of these limits, the wound pressure (WP) is not pulled back within a 20 mmHg wound pressure range (δWP). The start-stop controller maintains the wound pressure (WP) substantially within this range because the start-stop controller does not need to over-compensate for leaks in a low-leak environment.

[0061] The controller 170 can 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 setpoint or target (in this case, the wound pressure (WP) at the wound site 105 and the corresponding target wound pressure (TWP), respectively). As is well known to those skilled in the art, the PID controller provides proportional information, historical information, and rate of change information to maintain the wound pressure (WP) close to the target pressure (TP). The PID sum is used to adjust the process, in this case the reduced pressure therapy process, by controlling elements such as the power or voltage supplied to the DC motor (i.e., the applied voltage (V A )) that is directly related to the pump pressure (PP) as described above. The applied voltage (V A ) can be varied by adjusting the pulse width modulation as described above to achieve the desired pump pressure (PP) necessary to compensate for leaks in the dressing 115 and / or the discharge pressure mentioned above. The response of the PID controller depends on the responsiveness of the controller to error, the degree to which the controller overshoots the setpoint (e.g., the target pressure (TP)), and the degree of system oscillation (e.g., the degree of oscillation of the wound pressure (WP) within the acceptable range described above). Although the preferred embodiment of the PID controller is a digital controller, the PID controller can also be an analog controller or a simple RC circuit. The analog or digital PID controller can be implemented in software as part of the program logic controller.

[0062] After the wound pressure (WP) is measured by the wound pressure sensor 155, the PID controller adjusts the applied voltage (V A) to adjust the pump pressure (PP), i.e. the pump pressure correction (δPP). The pump pressure correction (δPP) is the additional pressure needed to maintain the wound pressure (WP) at the desired target pressure (TP) (e.g. 125 mmHg) and can be calculated every few seconds. Thus, the PID control changes the applied voltage (V A ) to achieve a pump pressure (PP) between a minimum pump pressure value (PPmin) and a maximum pump pressure value (PPmax) that maintains the wound pressure (WP) close to the target wound pressure (TP).

[0063] More specifically referring to Figure 6 , as an example of maintaining the wound pressure (WP) in normal operating conditions of the PID controller, with Figure 4In contrast, the pressure differential (δΡ) between the pump pressure (PP) and the wound pressure (WP) for the illustrated examples is a result of different leak rates (LR), as illustrated 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 differential (δΡ1) of about 15 mmHg to 16 mmHg due to a relatively high flow rate (FR) of about 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 the second example, the second pump pressure (PP2) also has a relatively large pressure differential (δΡ2) of about 11 mmHg to 12 mmHg due to a relatively high flow rate (FR) of about 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 a 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 (δΡ) to maintain the wound pressure (WP) at the same target pressure (TP). The third example illustrates the same difference, where the third pump pressure (PP3) also has a much smaller pressure differential (δΡ3) of about 4 mmHg to 5 mmHg due to a lower flow rate (FR) of about 100 cc / min, and is varied by the PID controller between 129 mmHg and 130 mmHg to maintain the wound pressure (WP) at a target wound pressure (TWP) of 125 mmHg.

[0064] Unlike the start-stop controller, the PID controller does not turn the pump 112 on and off, but rather continuously controls the pump pressure (PP) applied between a maximum pressure value (PPmax) and a minimum pressure value (PPmin) to maintain the wound pressure (WP) at a relatively constant level, e.g., at a target wound pressure (TWP) of 125 mmHg as shown by the dashed line, rather than allowing the wound pressure to vary between a maximum pressure value (WPmax) and a minimum pressure value (WPmin) as shown by the start-stop controller. Thus, the greater the extent to which the pump pressure (PP) falls toward the minimum pump pressure value (PPmin), the more the PID controller increases the applied voltage (V A ) supplied to the DC motor 114. Correspondingly, the greater the difference between the wound pressure (WP) and the target pressure (TP), the more the PID controller responds by adjusting the applied voltage (V A ) supplied to the DC motor 114. The action taken to increase or decrease the applied voltage (V A ) is proportional to the extent to which the wound pressure (WP) provided 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). Thus, when comparing the wound pressure (WP) variations of Figure 6 and FIG. 5, respectively, as shown, the PID controller causes the reduced pressure treatment system 100 to operate more smoothly than the start-stop controller because the PID controller maintains the wound pressure (WP) closer to the target wound pressure (TP) on average, whereas the start-stop controller allows the wound pressure (WP) to oscillate between the two limits as described above.

[0065] When the flow rate (FR) is small enough to indicate a low leak condition, e.g., when the pump pressure (PP) or the wound pressure (WP) is decreasing at a very slow rate toward their respective minimum pressure values (i.e., (PPmin) or (WPmin)), the start-stop controller can provide a sufficiently smooth wound pressure (WP) during treatment while conserving battery power and reducing noise due to the intermittent shutdown of the reduced pressure pump 112 during the same treatment cycle. For example, the DC motor 114 and pump 112 are turned off for a substantial percentage of time during a one minute cycle as shown in Figure 5B , but when the PID controller is used as shown in Figure 6The start-stop controller is continuously running during the operations shown in the middle. Thus, for low leakage conditions, such as when the flow rate (FR) is less than or equal to a fixed target flow rate (TFR) representing a low leakage condition, it is desirable to keep the start-stop controller running during the therapy session as much as possible, but to switch to the PID controller when the flow rate (FR) is greater than a fixed target flow rate (TFR) representing a high leakage condition. Thus, another example embodiment of the controller 170 includes both a PID controller and a start-stop controller (i.e., a hybrid controller) and additional processing that switches between the two depending on the degree of leakage of the reduced pressure therapy system 100, regardless of the location of the leak or the leak.

[0066] Thus, the controller 170 can be programmed to use the start-stop controller in conjunction with the PID controller operating as described above to enable or disable the PID controller depending on a particular switching condition related to the amount of system leakage created by the dressing 115 or other components of the reduced pressure therapy system 100 affecting the flow rate (FR). Using such a hybrid controller would preferably utilize only the PID controller that is continuously running during the continuous control mode as described above (or the enabled portion of the intermittent control mode as described above) to more tightly maintain the wound pressure (WP) at the target wound pressure (TP), but can continuously generate noise and deplete the battery powering the drive motor 114 more quickly. The hybrid controller can interface the start-stop controller such that the DC motor 114 turns on and off to conserve battery power and reduce the noise generated by the pump 112 during the therapy process. The controller 170 can also include an input for the user / caregiver to set one or more target flow rates (TFR).

[0067] The user / caregiver can set the target flow rate (TFR) as the switching condition for determining whether the dressing 115 or other components are in a high or low leakage state. If the flow rate (FR) is greater than the fixed target flow rate (TFR), i.e., a high leakage condition, the start-stop controller is disabled so that the PID controller takes over to keep the wound pressure (WP) as close to the target wound pressure (TP) as possible. However, if the flow rate (FR) is less than or equal to the fixed target flow rate (TFR), i.e., a low leakage condition, the start-stop controller is enabled to keep the wound pressure (WP) within the wound pressure differential (δWP) range while conserving battery power and reducing the noise from the pump 112. For example, the fixed target flow rate (TFR) can be 65 cc / min. As indicated above, it is desirable to keep the start-stop controller running during therapy as much as possible when the dressing 115 is in a low leakage condition. For example, the controller 170 can interface the start-stop controller when the flow rate (FR) is less than or equal to the fixed target flow rate (TFR), but switches back to the PID controller when the flow rate (FR) is greater than the fixed target flow rate (TFR) due to additional leakage forming in the dressing 115 as the patient moves around eventually creating a high leakage condition.

[0068] In another embodiment, the start-stop controller can have dual target flow rate (TFR) capability, where the controller 170 further includes inputs for the user to set two target flow rates (TFR) as follows: an upper target flow rate (TFR A ) when the start-stop controller is enabled to increase the flow rate (FR) and a lower target flow rate (TFR D ) when the PID controller is enabled to decrease the flow rate (FR) as the switching condition for determining whether the dressing 115 or other component is in a high leak condition or a low leak condition. In one embodiment, both the upper target flow rate (TFR A ) and the lower target flow rate (TFR D ) are greater than the fixed target flow rate (TFR) such that the controller 170 switches more quickly from the PID controller to the start-stop controller and more slowly from the start-stop controller to the PID controller. For example, both the upper target flow rate (TFR A ) and the lower target flow rate (TFR D ) can be set to about 80 cc / min, which is higher than the fixed target flow rate TFR (65 cc / min) in the previous example. In yet another embodiment, the upper target flow rate (TFR A ) can also be greater than the lower target flow rate (TFR D ) such that the controller 170 switches even more quickly from the PID controller to the start-stop controller and even more slowly from the start-stop controller to the PID controller. In this case, the controller 170 supports the benefits derived from using the start-stop controller as opposed to continuous operation with the PID controller. For example, the upper target flow rate (TFR A ) can be 75 cc / min and the lower target flow rate (TFR D ) can be about 85 cc / min. If the PID controller is currently enabled in a high leak condition where the flow rate (FR) is decreasing, the lower target flow rate (TFR D ) will be set to 85 cc / min instead of 65 cc / min such that the controller 170 switches more quickly from the PID controller to enable the start-stop controller. Alternatively, if the start-stop controller is enabled in a low leak condition where the flow rate (FR) is increasing, the upper target flow rate (TFR A ) will be set to 75 cc / min instead of 65 cc / min such that the controller 170 switches more slowly to disable the start-stop controller.

[0069] In one embodiment, the controller 170 can provide an output signal to the indicator 172 to signal a visual and / or audible signal in response to wound pressure (WP) at the tissue site 105 being unresponsive to an increased pump pressure (PP). For example, the indicator can be a light emitting diode (LED) that provides a visual signal. In this embodiment, the indicator 172 illuminates in response to wound pressure (WP) at the tissue site 105 being unresponsive to an increased 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 wound pressure (WP) at the tissue site 105 being unresponsive to an increased pump pressure. The controller 170 can provide other output signals that indicate whether the negative pressure treatment system is in a low leak condition or a high leak condition.

[0070] While most reduced pressure treatment systems have some system leak, improvements in the dressing and other components of the system have greatly reduced system leak to even below low system leak, such as the target flow rate (TFR) mentioned above. For example, sealing members, such as the sealing member 125 and the adhesive used to attach the sealing member to the tissue around the tissue site, have been improved so that they are more airtight and significantly reduce system leak from the external environment into the treatment environment. In some cases, the wound pressure (WP) can have equaled the target wound pressure (TWP) while the flow rate (FR) at the tissue site 105 can range from less than 50 cc / min up to zero. A flow rate (FR) that is less than the target flow rate (TFR) is generally too low to heal a wound. Some minimum level of fluid flow is needed at the tissue site to promote adequate healing. For example, a minimum level of fluid flow is needed to remove exudate from the tissue site. If the flow rate (FR) drops below the minimum flow rate (MinFR) necessary for proper healing of the wound [which in some embodiments is less than the target flow rate (TFR)], it would be desirable to measure the flow rate (FR) associated with system leak and increase the flow rate (FR) to exceed and be acceptably higher than the system flow rate (i.e., "induce leak") to heal the wound at the tissue site 105 in an "induced leak mode." In some embodiments, the minimum flow rate (MinFR) can depend on or be equal to the target flow rate (TFR). In other embodiments, the minimum flow rate (MinFR) depends more on the particular type of dressing and / or the type of wound that needs to heal.

[0071] In some embodiments, the controller 170 can be configured to determine a flow rate (FR) associated with system leakage and compare it to a minimum flow rate (MinFR) expected for a particular type of dressing and / or wound type. The controller 170 can be further configured to increase the leakage by opening the regulator 165 if the flow rate (FR) is less than the minimum flow rate (MinFR). When the regulator 165 is opened to induce leakage from the external environment into the therapy environment to increase the flow rate (FR) of fluid within the therapy system and ultimately at the tissue site 105, the controller 170 can be further configured to adjust the flow rate (FR) as it 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 can be further configured to include an algorithm for inducing leakage into the therapy environment in a controlled manner. For example, the algorithm can include a linear or non-linear ramp function to increase the flow rate (FR) to the minimum flow rate (MinFR). The algorithm can also include a sinusoidal function that maintains the flow rate (FR) above the minimum flow rate (MinFR) by varying the amplitude and / or frequency. The algorithm can also include a digital function that varies the flow rate (FR) by varying the amplitude, frequency, and / or duty cycle.

[0072] As indicated above, the system 100 can have system leakage that originates from the dressing and / or other components of the system as well as the wound itself. Thus, depending on these factors, different reduced pressure therapy systems can have different system leakage values. For example, one system can have a low system leakage of 50 cc / min, while another system can have a low leakage condition of 100 cc / min. In some embodiments, the low leakage condition can be set to a minimum flow rate (MinFR) that can also vary, for example, from 50 cc / min to 100 cc / min. The controller 170 can be configured to open the regulator 165 to induce leakage into the therapy system 100 when the flow rate (FR) of fluid within the therapy system falls below the minimum flow rate (MinFR).

[0073] In one example, the dressing is a 100 cc / min minimum flow rate (MinFR) requirement, but the controller measures a flow rate (FR) of 50 cc / min, where the fluid needs to be evacuated into a canister that is two feet above the dressing. The action of gravity and the more viscous fluid due to exudate can cause such a small leak of fluid flow to remain stagnant in the dressing. However, the controller detects that the flow rate (FR) is below the minimum flow rate (MinFR) requirement of 100 cc / min, and opens and controls the regulator to induce airflow leakage into the therapeutic environment of the dressing. The algorithm can also include a digital function that changes the flow rate (FR) by changing the amplitude, frequency, and / or duty cycle. For example, the minimum flow rate (MinFR) requirement of 100 cc / min equals a drop of 1.0 mmHg / 10 seconds, and the flow rate (FR) of 50 cc / min equals a drop of 0.5 mmHg / 10 seconds. If the algorithm includes a digital function, for example, the frequency can be increased to one cycle / 5 seconds or 12 cycles / minute, with a duty cycle of 20% / cycle, i.e., a 1 second width / cycle. This would cause the wound pressure (WP) at the tissue site to decrease (alternatively, cause the pressure decay to increase) and cause the flow rate (FR) to increase to at least 100 cc / min, thereby overcoming the gravity and allowing the canister to move 2 feet above the dressing. The start-stop controller in combination with the PID controller can then operate as described above to adjust the wound pressure (WP) as needed.

[0074] Referring now to Figure 7 , the example embodiments of the method or process for controlling the wound pressure (WP) are implemented on a controller, such as, for example, the controller 170 as described above, or alternatively, on another example embodiment of the controller 170. The controller 170 and other components can be implemented according to the therapy cycle 700 as shown in the flowchart in FIG. 7A, and the therapy cycle 800 as shown in the flowchart in FIG. 8A, in accordance with the example embodiments described above. Figure 7 The therapy cycle 700 includes a therapy algorithm 703 for selecting the appropriate controller (i.e., the PID controller or the start-stop controller) to control the delivery of reduced pressure to the tissue site while conserving power and while reducing the noise from the pump 112 and the motor 114. The controller 170 first checks to see if the negative pressure therapy system 100 has been turned on at 705, such that if the negative pressure therapy system 100 is not turned on, the applied voltage (V A ) is set to 0 V at 707 and applied to the motor 114 as the new motor voltage (V M ) at 709, such that the motor 114 does not run. If the negative pressure therapy system 100 is turned on, the controller 170 checks at 711 to determine if enough time has passed to turn on the therapy algorithm 703, i.e., the duty cycle therapy time (t DC). The duty cycle of the therapy algorithm 703 can be, for example, about 50 ms. Thus, if less than 50 ms has occurred since the last calculation of the therapy algorithm 703, the motor voltage (V M ) is held at 709 at the previously applied voltage (V A ). The duty cycle of the therapy cycle 700 itself can be, for example, 10 ms without engaging the therapy algorithm 703. However, if more than 50 ms has occurred, the controller 170 recalculates the therapy algorithm 703 and continues at 713 to check the current wound pressure (WP) and / or pump pressure (PP) against the corresponding maximum and minimum wound pressure and pump pressure values (as described above, i.e., (WPmax) and (WPmin) and (PPmax) and (PPmin), respectively) and the current wound pressure and / or pump pressure.

[0075] The therapy algorithm 703 begins by determining at 713 whether the on-off controller is active. If the PID controller is engaged instead of the on-off controller, the local pump pressure (PPL) is set at 715 to the current pump pressure (PPC). As described above, the PID control adjusts the applied voltage (V A ) to the DC motor 114 to achieve a pump pressure (PP) between the minimum pump pressure value (PPmin) and the maximum pump pressure value (PPmax) to maintain the wound pressure (WP) near the target wound pressure (TP). Referring back to Figure 6 As an example, 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 and continues to control the pump pressure (PP) during a high leak condition. The controller 170 determines at 717 the value of the applied voltage (V A ) corresponding to the current pump pressure (PPC) and applies that voltage at 709 as the motor voltage (V M ). However, as shown at 717, if the on-off controller is engaged or activated, the therapy algorithm 703 determines at 719 whether the on-off controller is rising or falling. Figure 5B

[0076] When the wound pressure (WP) falls too low in the falling mode, for example, below the minimum wound pressure value (WPmin) as described above, the reduced pressure pump 112 is turned on with an applied voltage (V A ) greater than the stall voltage, i.e., the on-off on voltage (V ON ) to increase the pump pressure (PP) to the maximum pump pressure (PPmax) in the rising mode. The on-off on voltage (V ON ​), until either the pump pressure (PP) reaches a maximum pump pressure value (PPmax) as shown, for example, at 501 and 503, or the wound pressure (WP) reaches a maximum wound pressure value (WPmax) as shown, for example, at 502 and 504, whichever occurs first. When the wound pressure (WP) is in the rising mode, the therapy algorithm 703 sets the local target wound pressure (TPL) to the target wound pressure (TP) plus a hysteresis value (H) at 723. The hysteresis value (H) is the maximum amount by which the wound pressure (WP) should increase above the target wound pressure (TP) before the controller 170 shuts down the pump 112 to protect the tissue site 105 from excessive decompression that can cause injury, while in the rising mode. The hysteresis value (H) sets an upper limit above the target wound pressure (TP) that is the maximum pressure value (WPmax). For example, if the hysteresis value (H) is 10 mmHg, as shown at 701, the maximum wound pressure value (WPmax) is set to 135 mmHg. Since the wound pressure (WP) generally 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, when the pump pressure (PP) reaches the maximum pump pressure value (PPmax) (e.g., about 140 mmHg at 501 and 503), the wound pressure (WP) is generally less than the maximum wound pressure value (WPmax) (e.g., about 132 mmHg at 505 and 506). Thus, the controller 170 allows the start-stop controller to continue to modulate the application of reduced pressure, but shuts down the pump 112 in the falling mode of the reduced pressure cycle. Figure 5B

[0077] ​Correspondingly, when the wound pressure (WP) rises too high in the rising mode, e.g., 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 to allow the residual pressure in the reduced pressure therapy system 100 to decrease in the falling mode due to leaks in the system. The reduced pressure pump 112 remains turned off until the wound pressure (WP) is again less than or equal to the minimum wound pressure value (WPmin) as shown in the figure, e.g., at 508, or the pump pressure (PP) is less than or equal to the minimum pump pressure value (PPmin) as shown in the figure, e.g., at 507, whichever occurs first. When the wound pressure (WP) is in the falling mode as described above, the therapy algorithm 703 sets the local target wound pressure (TPL) to the target wound pressure (TP) minus the hysteresis value (H) at 721. The hysteresis value (H) is the minimum amount of pressure below the target wound pressure (TP) that the wound pressure (WP) should be decreased to before the controller 170 determines that the flow rate (FR) has increased enough in the falling mode 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) that is the minimum pressure value (WPmin). For example, with a hysteresis value (H) of 10 mmHg, the minimum wound pressure value (WPmin) is set to 115 mmHg as shown in Figure 5B the figure. Since the pump pressure (PP) generally follows the falling wound pressure (WP) as shown between the pump pressure peak and the wound pressure peak at 501 and 502, respectively, and the pump pressure minimum and the wound pressure minimum at 507 and 508, respectively, the start-stop controller turns the pump 112 back on at 507 after which the wound pressure (WP) begins to increase again in the rising mode. Thus, the controller 170 allows the start-stop controller to continue to regulate the application of reduced pressure and does so by turning the pump 112 on in the rising mode of the reduced pressure cycle. The start-stop controller allows the wound pressure (WP) to effectively oscillate around the target wound pressure (TWP) of 125 mmHg contained between the minimum wound pressure value (WPmin) of 115 mmHg and the maximum wound pressure value (WPmax) of 135 mmHg that can be used or the hysteresis value (H) programmed into the controller 170. In either case, the start-stop controller maintains the wound pressure (WP) within the wound pressure range (δWP), e.g., a wound pressure range (δWP) of 20 mmHg.

[0078] At 709, the therapy algorithm 703 sets the motor voltage (V M ) equal to the applied voltage (V A) to re-enter therapy cycle 700, therapy cycle 700 then reads the current flow rate (FR) measured by controller 170 at 725 and determines whether the current flow rate (FR) is less than the target flow rate (TFR) at 727. If the flow rate (FR) is less than the target flow rate (TFR) indicating a low leak condition as described above, the start-stop controller remains on or enabled as indicated at 729. However, if the flow rate (FR) is greater than or equal to the target flow rate (TFR) indicating a high leak condition as described above, the start-stop controller remains off or disabled as indicated at 731. Finally, therapy cycle 700 checks to see if the negative pressure wound therapy system 100 has been turned off at 733 and if not, continues the therapy cycle as indicated at 735. If the negative pressure therapy system 100 has been turned off, the therapy cycle ends at 737.

[0079] Referring now to Figure 8 , an exemplary embodiment of the method or process for controlling the flow rate (FR) 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 can be implemented according to the flow rate (FR) cycle 800 as shown in the flow chart in FIG. 8. Figure 8 The flow rate (FR) cycle 800 is executed periodically to ensure that the flow rate (FR) has not decreased below the minimum flow rate (MinFR). The flow rate (FR) cycle 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 at 804, the flow rate (FR) is not less than the minimum flow rate (MinFR), the negative pressure therapy continues at 806, which in some embodiments can return to therapy cycle 700 at 700. If at 804, the flow rate (FR) is less than the minimum flow rate (MinFR), the flow rate (FR) cycle 800 generates a low leak signal (not shown) and continues inducing a leak at 808 as described in greater detail above in response to the low leak signal. The flow rate (FR) cycle 800 again reads the flow rate (FR) (not shown) and compares the new flow rate (FR) to the minimum flow rate (MinFR) at 810. If at 810, the new flow rate (FR) is still less than the minimum flow rate (MinFR), the flow rate (FR) cycle 800 continues inducing a leak at 808 as described in greater detail above. This cycle continues until at 810, the new flow rate (FR) is no longer less than but greater than the minimum flow rate (MinFR). In preferred embodiments, the negative pressure therapy continues at 806 and proceeds to therapy cycle 700, but in some embodiments, therapy cycle 700 should only begin after the system has reached equilibrium within the desired wound pressure (WP) range.

[0080] The systems, devices, and methods described herein can provide significant advantages. For example, the PID control algorithm constantly adjusts the negative pressure source to maintain the pressure within a specified tolerance, which can be inefficient in low-leakage applications, drawing more power than a simple hysteresis control algorithm. Conversely, the hysteresis algorithm can work well in low-leakage applications and use relatively little power, but can cause the negative pressure source to close and open frequently in high-leakage applications, which can be noisy and increase power consumption. The hybrid control as described herein can combine the benefits of the PID and hysteresis control algorithms to minimize power consumption and noise. For example, if the negative pressure therapy application has low leakage, the hybrid control algorithm can select the hysteresis control algorithm to minimize power consumption. If the application changes or creates higher leakage, the hybrid control algorithm can switch to the PID control algorithm to minimize noise.

[0081] While shown in a few illustrative embodiments, a person having ordinary skill in the art will see that the systems, devices, and methods described herein are susceptible to various changes and modifications. Moreover, the use of the terms "or" and "and" described various alternatives in the use of complementary aspects, and the use of the term "and / or" does not require complementary aspects unless the context requires otherwise. Additionally, the use of the terms "one or more" and "at least one" will be understood to require one or more than one, unless the context requires otherwise.

[0082] The appended claims set forth the new and novel aspects of the subject matter described above, but the claims can also encompass additional subject matter not specifically recited. For example, certain features, elements or aspects can be omitted from the claims if they are not needed to distinguish over features known to one of ordinary skill in the art. The features, elements, and aspects described herein can also be combined or replaced by alternative features serving the same, equivalent, or similar purposes in order to realize the scope of the invention as defined by the claims without departing from the scope of the invention as set forth in the claims.

Claims

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

2. The system of claim 1, wherein the flow rate sensor comprises a first pressure sensor and a second pressure sensor, the first pressure sensor having a first input for sensing the pump pressure (PP) and a first output for providing a signal indicating the pump pressure (PP), the second pressure sensor having a second input for sensing the wound pressure (WP) and a second output for providing a signal indicating the wound pressure (WP), and wherein the controller is electrically connected 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 the 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 discharging gas from the external environment into the treatment environment.

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

5. The system of claim 3, wherein the leakage has a first leakage threshold and a second leakage threshold.

6. The system of claim 3, wherein the regulator is further configured to increase the leakage from zero to a first leakage threshold.

7. The system of claim 6, wherein the regulator is further configured to increase the leakage by discharging gas at a fixed rate.

8. The system of claim 6, wherein the regulator is further configured to increase the leakage by discharging gas at a variable rate.

9. The system of claim 8, wherein the regulator is further configured to change the amplitude of the variable rate.

10. The system of claim 8, wherein the regulator is further configured to change the frequency of the variable rate.

11. The system of claim 8, wherein the regulator is further configured to change the duty cycle of the variable rate.

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

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

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

15. The system of claim 14, 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.

16. The system of claim 1, wherein the minimum flow rate (MinFR) comprises a first target flow rate of less than 50 cc / min and a second target flow rate of less than 100 cc / min.

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

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

19. The system of claim 17, wherein the controller is further configured to reduce the negative pressure source at the wound site by reducing the power applied to the motor when the wound pressure (WP) is greater than the maximum wound pressure (WPMax).

20. The system of claim 17, wherein the controller is further configured to compare the pump pressure (PP) with a target pump pressure (TPP), and to change the electrical power applied to the motor in response to the comparison to maintain the pump pressure (PP) close to the target pump pressure (TPP).

21. The system of claim 17, wherein the controller is further configured to determine the flow rate (FR) based on the rate of change of the electrical power applied to the motor over time.

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

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

24. A system for promoting wound healing at a tissue site, the system comprising: The dressing includes a porous pad configured to be positioned at the tissue site and adapted to be covered by a sterile drape to form a therapeutic environment isolated from the external environment to receive negative pressure. A flow rate sensor having an input fluidly coupled to the porous pad and an output for providing a flow rate signal representing the flow rate (FR) of fluid approaching the porous pad. A controller, connected to the output of the flow rate sensor, and configured to (i) determine the flow rate (FR) based on the flow rate signal, (ii) compare the flow rate (FR) with a minimum flow rate (MinFR), and (iii) generate a low leakage signal when the flow rate (FR) is less than the minimum flow rate (MinFR); and A regulator, coupled to the controller, is adapted to increase the flow rate (FR) by increasing leakage into the treatment environment in response to the occurrence of the low leakage signal.

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

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