Patent Publication Number: US-2019184070-A1

Title: System, Method, And Apparatus For Regulating Pressure

Description:
RELATED APPLICATION 
     The present invention is a divisional of U.S. patent application Ser. No. 14/024,066, filed Sep. 11, 2013, which claims the benefit, under 35 USC § 119(e), of the filing of U.S. Provisional Patent Application Ser. No. 61/701,394, entitled “SYSTEM, METHOD, AND APPARATUS FOR REGULATING PRESSURE,” filed 14 Sep. 2012, which is incorporated herein by reference for all purposes 
    
    
     TECHNICAL FIELD 
     The subject matter described herein relates generally to regulating pressure. In more particular embodiments, the subject matter relates to regulating pressure for reduced-pressure therapy. 
     BACKGROUND 
     Clinical studies and practice have shown that reducing pressure in proximity to a tissue site can augment and accelerate growth of new tissue at the tissue site. The applications of this phenomenon are numerous, but it has proven particularly advantageous for treating wounds. Regardless of the etiology of a wound, whether trauma, surgery, or another cause, proper care of the wound is important to the outcome. Treatment of wounds with reduced pressure is commonly referred to as “reduced-pressure therapy,” but may also be known by other names, including “negative pressure wound therapy” and “vacuum therapy,” for example. Reduced-pressure therapy may provide a number of benefits, including migration of epithelial and subcutaneous tissues, improved blood flow, and micro-deformation of tissue at a wound site. Together, these benefits can increase development of granulation tissue and reduce healing times. 
     While the clinical benefits of reduced-pressure therapy are widely known, the cost and complexity of reduced-pressure therapy can be a limiting factor in its application, and the development and operation of reduced-pressure systems, components, and processes continues to present significant challenges to manufacturers, healthcare providers, and patients. 
     SUMMARY 
     Illustrative embodiments of systems, methods, and apparatuses for regulating pressure are described below. One such illustrative embodiment may be described as a reduced-pressure treatment system, which may include a dressing, a supply chamber, a control chamber, a charging chamber. The supply chamber can be fluidly coupled to the dressing through a supply lumen, and the control chamber can be fluidly coupled to the dressing through a feedback lumen. The charging chamber can be fluidly coupled to the supply chamber through a port. A regulator valve within the control chamber controls fluid communication through the port based on a differential between a control pressure in the control chamber and a therapy pressure. 
     Another illustrative embodiment relates to a method for regulating pressure, such as a therapeutic pressure. One such method may include placing a manifold in a sealed environment proximate to a tissue site, fluidly coupling the manifold to a supply chamber through a supply lumen, and fluidly coupling the manifold to a control chamber through a control lumen. The supply chamber may also be fluidly coupled to a charging chamber, and a charging pressure in the charging chamber can be reduced below a therapy pressure. Fluid communication between the supply chamber and the charging chamber can be regulated based on a differential between a control pressure in the control chamber and the therapy pressure. A regulated supply pressure from the supply chamber can be delivered to the manifold. 
     Yet another illustrative embodiment relates to an apparatus for regulating pressure. In one form, such an apparatus may include a supply chamber, a control chamber, and a charging chamber. The supply chamber may have a supply port adapted for coupling to a supply lumen, and the control chamber may have a control port adapted for coupling to a feedback lumen. The charging chamber can be fluidly coupled to the supply chamber through a charging port. A regulator valve within the control chamber can operate to control fluid communication through the charging port based on a differential between pressure in the control chamber and a target pressure. 
     Another illustrative embodiment of an apparatus for regulating pressure may include a lower housing having an end wall and a side wall. A first piston opposite the end wall of the lower housing may be engaged to the side wall of the lower housing to define a charging chamber within the lower housing. A partition in the first piston can separate a lower bowl from an upper bowl. A charging spring may be engaged to the first piston and the end wall of the lower housing. A charging port through the first piston can provide fluid communication between the charging chamber and a supply chamber defined by the partition and the lower bowl. An upper housing may have a floor and a side wall, wherein the side wall of the upper housing can be coupled to the side wall of the lower housing. A control chamber may be generally defined by the upper bowl and the floor of the upper housing. A second piston opposite the upper bowl may be engaged to the side wall of the lower housing, wherein the second piston can divide the control chamber into an ambient pressure region and a control pressure region. A valve body may extend through an aperture in the partition into the supply chamber, the valve body having a first end coupled to the second piston and a second end disposed adjacent to the charging port in the supply chamber. A regulator spring can engage the valve body between the charging port and the second piston. A multi-channel port can be exposed externally through the upper housing, and the multi-channel port can provide a supply port fluidly coupled to the supply chamber and a control port fluidly coupled to the control pressure region of the control chamber. The multi-channel port can be coupled with a multi-lumen tube. 
     Other features and advantages will become apparent with reference to the drawings and detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of an example embodiment of a reduced-pressure therapy system that can regulate therapeutic pressure in accordance with this specification; 
         FIGS. 2A-2B  are schematic cross-sections of an example embodiment of a regulator in the reduced-pressure therapy system; 
         FIGS. 3A-3B  are a cross-section of an example embodiment of a piston-driven pump in the reduced-pressure therapy system; 
         FIG. 4A  is a perspective view of an example embodiment of the reduced-pressure therapy system; 
         FIG. 4B  is a partial cross-sectional view of the example embodiment of the reduced-pressure therapy system in  FIG. 4A  taken along line  4 - 4 ; 
         FIG. 5  is a perspective view of a vacuum pump that may be associated with some embodiments of the reduced-pressure therapy system; 
         FIG. 6  is a front view of the vacuum pump illustrated in  FIG. 5 ; 
         FIG. 7  is an exploded side perspective view of the vacuum pump of  FIG. 5 ; 
         FIG. 8  is an exploded rear perspective view of the vacuum pump in  FIG. 5 ; 
         FIG. 9  is a cross-sectional side view of the vacuum pump of  FIG. 6  taken at  9 - 9 ; 
         FIG. 10  is a top-rear perspective view of a piston of the vacuum pump of  FIG. 5 ; 
         FIG. 11  is a bottom-rear perspective view of the piston of  FIG. 10 ; 
         FIG. 12  is a top-rear perspective view of a seal of the vacuum pump of  FIG. 5 ; 
         FIG. 13  is a bottom-rear perspective view of the seal of  FIG. 12 ; 
         FIG. 14  is a top-rear perspective view of a second barrel of the vacuum pump of  FIG. 5 ; 
         FIG. 15  is a bottom-rear perspective view of the second barrel of  FIG. 14 ; 
         FIG. 16  is a cross-sectional side view of the vacuum pump of  FIG. 5 ; 
         FIG. 17  is an enlarged cross-sectional view of the vacuum pump of  FIG. 16 ; and 
         FIG. 18  is an enlarged cross-sectional view of the vacuum pump of  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION 
     New and useful systems, methods, and apparatuses associated with regulating pressure are set forth in the appended claims. Objectives, advantages, and a preferred mode of making and using the systems, methods, and apparatuses may be understood best by reference to the following detailed description in conjunction with the accompanying drawings. The description provides information that enables a person skilled in the art to make and use the claimed subject matter, but may omit certain details already well-known in the art. Moreover, descriptions of various alternatives using terms such as “or” do not necessarily require mutual exclusivity unless clearly required by the context. The claimed subject matter may also encompass alternative embodiments, variations, and equivalents not specifically described in detail. The following detailed description should therefore be taken as illustrative and not limiting. 
     The example embodiments may also be described herein in the context of a reduced-pressure therapy applications, but many of the features and advantages are readily applicable to other environments and industries. Spatial relationships between various elements or to the spatial orientation of various elements may be described as depicted in the attached drawings. In general, such relationships or orientations assume a frame of reference consistent with or relative to a patient in a position to receive reduced-pressure therapy. However, as should be recognized by those skilled in the art, this frame of reference is merely a descriptive expedient rather than a strict prescription. 
       FIG. 1  is a simplified functional block diagram of an example embodiment of a reduced-pressure therapy system  100  that can regulate therapeutic pressure in accordance with this specification. As illustrated, reduced-pressure therapy system  100  may include a dressing  102  fluidly coupled to a reduced-pressure source  104 . A regulator or controller, such as regulator  106 , may also be fluidly coupled to dressing  102  and reduced-pressure source  104 . Dressing  102  generally includes a drape, such as drape  108 , and a manifold, such as pressure distribution manifold  110 . Reduced-pressure therapy system  100  may also include a fluid container, such as container  112 , coupled to dressing  102  and reduced-pressure source  104 . 
     In general, components of reduced-pressure therapy system  100  may be coupled directly or indirectly. For example, reduced-pressure source  104  may be directly coupled to regulator  106  and indirectly coupled to dressing  102  through regulator  106 . Components may be fluidly coupled to each other to provide a path for transferring fluids (i.e., liquid and/or gas) between the components. In some embodiments, components may be fluidly coupled with a tube, for example. A “tube,” as used herein, broadly refers to a tube, pipe, hose, conduit, or other structure with one or more lumina adapted to convey fluids between two ends. Typically, a tube is an elongated, cylindrical structure with some flexibility, but the geometry and rigidity may vary. In some embodiments, components may additionally or alternatively be coupled by virtue of physical proximity, being integral to a single structure, or being formed from the same piece of material. Coupling may also include mechanical, thermal, electrical, or chemical coupling (such as a chemical bond) in some contexts. 
     In operation, pressure distribution manifold  110  may be placed within, over, on, or otherwise proximate to a tissue site. Drape  108  may be placed over pressure distribution manifold  110  and sealed to tissue proximate to the tissue site. The tissue proximate to the tissue site is often undamaged epidermis peripheral to the tissue site. Thus, dressing  102  can provide a sealed therapeutic environment proximate to a tissue site, substantially isolated from the external environment, and reduced-pressure source  104  can reduce the pressure in the sealed therapeutic environment. Reduced pressure applied uniformly through pressure distribution manifold  110  in the sealed therapeutic environment can induce macrostrain and microstrain in the tissue site, as well as remove exudates and other fluids from the tissue site, which can be collected in container  112  and disposed of properly. 
     The fluid mechanics of using a reduced-pressure source to reduce pressure in another component or location, such as within a sealed therapeutic environment, can be mathematically complex. However, the basic principles of fluid mechanics applicable to reduced-pressure therapy are generally well-known to those skilled in the art, and the process of reducing pressure may be described illustratively herein as “delivering,” “distributing,” or “generating” reduced pressure, for example. 
     In general, exudates and other fluids flow toward lower pressure along a fluid path. This orientation is generally presumed for purposes of describing various features and components of reduced-pressure therapy systems herein. Thus, the term “downstream” typically implies something in a fluid path relatively closer to a reduced-pressure source, and conversely, the term “upstream” implies something relatively further away from a reduced-pressure source. Similarly, it may be convenient to describe certain features in terms of fluid “inlet” or “outlet” in such a frame of reference. However, the fluid path may also be reversed in some applications, such as by substituting a positive-pressure source, and this descriptive convention should not be construed as a limiting convention. 
     The term “tissue site” in this context broadly refers to a wound or defect located on or within tissue, including but not limited to, bone tissue, adipose tissue, muscle tissue, neural tissue, dermal tissue, vascular tissue, connective tissue, cartilage, tendons, or ligaments. A wound may include chronic, acute, traumatic, subacute, and dehisced wounds, partial-thickness burns, ulcers (such as diabetic, pressure, or venous insufficiency ulcers), flaps, and grafts, for example. The term “tissue site” may also refer to areas of tissue that are not necessarily wounded or defective, but are instead areas in which it may be desired to add or promote the growth of additional tissue. For example, reduced pressure may be used in certain tissue areas to grow additional tissue that may be harvested and transplanted to another tissue location. 
     “Reduced pressure” generally refers to a pressure less than a local ambient pressure, such as the ambient pressure in a local environment external to a sealed therapeutic environment provided by dressing  102 . In many cases, the local ambient pressure may also be the atmospheric pressure in a patient&#39;s vicinity. Alternatively, the pressure may be less than a hydrostatic pressure associated with tissue at the tissue site. Unless otherwise indicated, values of pressure stated herein are gauge pressures. Similarly, references to increases in reduced pressure typically refer to a decrease in absolute pressure, while decreases in reduced pressure typically refer to an increase in absolute pressure. 
     A reduced-pressure source, such as reduced-pressure source  104 , may be a reservoir of air at a reduced pressure, or may be a manual or electrically-powered device that can reduced 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, for example. The reduced-pressure source may be housed within or used in conjunction with other components, such as sensors, processing units, alarm indicators, memory, databases, software, display devices, or user interfaces that further facilitate reduced-pressure therapy. While the amount and nature of reduced pressure applied to a tissue site may vary according to therapeutic requirements, the pressure typically ranges between −5 mm Hg (−667 Pa) and −500 mm Hg (−66.7 kPa). Common therapeutic ranges are between −75 mm Hg (−9.9 kPa) and −300 mm Hg (−39.9 kPa). 
     Pressure distribution manifold  110  can generally be adapted to contact a tissue site. Pressure distribution manifold  110  may be partially or fully in contact with the tissue site. If the tissue site is a wound, for example, pressure distribution manifold  110  may partially or completely fill the wound, or may be placed over the wound. Pressure distribution manifold  110  may take many forms, and may be many sizes, shapes, or thicknesses depending on a variety of factors, such as the type of treatment being implemented or the nature and size of a tissue site. For example, the size and shape of pressure distribution manifold  110  may be adapted to the contours of deep and irregular shaped tissue sites. 
     More generally, a manifold is a substance or structure adapted to distribute reduced pressure to or remove fluids from a tissue site, or both. In some embodiments, though, a manifold may also facilitate delivering fluids to a tissue site, if the fluid path is reversed or a secondary fluid path is provided, for example. A manifold may include flow channels or pathways that distribute fluids provided to and removed from a tissue site around the manifold. In one illustrative embodiment, the flow channels or pathways may be interconnected to improve distribution of fluids provided to or removed from a tissue site. For example, cellular foam, open-cell foam, porous tissue collections, and other porous material such as gauze or felted mat generally include structural elements arranged to form flow channels. Liquids, gels, and other foams may also include or be cured to include flow channels. 
     In one illustrative embodiment, pressure distribution manifold  110  may be a porous foam material having interconnected cells or pores adapted to uniformly (or quasi-uniformly) distribute reduced pressure to a tissue site. The foam material may be either hydrophobic or hydrophilic. In one non-limiting example, pressure distribution manifold  110  may be an open-cell, reticulated polyurethane foam such as GranuFoam® dressing available from Kinetic Concepts, Inc. of San Antonio, Tex. 
     In some embodiments, such as embodiments in which pressure distribution manifold  110  may be made from a hydrophilic material, pressure distribution manifold  110  may also wick fluid away from a tissue site while continuing to distribute reduced pressure to the tissue site. The wicking properties of pressure distribution manifold  110  may draw fluid away from a tissue site by capillary flow or other wicking mechanisms. An example of a hydrophilic foam is a polyvinyl alcohol, open-cell foam such as V.A.C. WhiteFoam® dressing available from Kinetic Concepts, Inc. of San Antonio, Tex. Other hydrophilic foams may include those made from polyether. Other foams that may exhibit hydrophilic characteristics include hydrophobic foams that have been treated or coated to provide hydrophilicity. 
     Pressure distribution manifold  110  may further promote granulation at a tissue site if pressure within a sealed therapeutic environment is reduced. For example, any or all of the surfaces of pressure distribution manifold  110  may have an uneven, coarse, or jagged profile that can induce microstrains and stresses at a tissue site if reduced pressure is applied through pressure distribution manifold  110 . 
     In one example embodiment, pressure distribution manifold  110  may be constructed from bioresorbable materials. Suitable bioresorbable materials may include, without limitation, a polymeric blend of polylactic acid (PLA) and polyglycolic acid (PGA). The polymeric blend may also include without limitation polycarbonates, polyfumarates, and caprolactones. Pressure distribution manifold  110  may further serve as a scaffold for new cell-growth, or a scaffold material may be used in conjunction with pressure distribution manifold  110  to promote cell-growth. In general, a scaffold material may be a substance or structure used to enhance or promote the growth of cells or 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 hydroxy apatites, carbonates, or processed allograft materials. 
     Drape  108  is an example of a sealing member. A sealing member may be constructed from a material that can provide a fluid seal between two environments or components, such as between a therapeutic environment and a local external environment. The sealing member may be, for example, an impermeable or semi-permeable, elastomeric material that can provide a seal adequate to maintain a reduced pressure at a tissue site for a given reduced-pressure source. For semi-permeable materials, the permeability generally should be low enough that a desired reduced pressure may be maintained. An attachment device may be used to attach a sealing member to an attachment surface, such as undamaged epidermis, a gasket, or another sealing member. The attachment device may take many forms. For example, an attachment device may be a medically-acceptable, pressure-sensitive adhesive that extends about a periphery, a portion, or an entire sealing member. Other example embodiments of an attachment device may include a double-sided tape, paste, hydrocolloid, hydrogel, silicone gel, organogel, or an acrylic adhesive. 
     Container  112  is representative of a container, canister, pouch, or other storage component that can be used to manage exudates and other fluids withdrawn from a tissue site. In many environments, a rigid container may be preferred or required for collecting, storing, and disposing of fluids. In other environments, fluids may be properly disposed of without rigid container storage, and a re-usable container could reduce waste and costs associated with reduced-pressure therapy. 
     In general, reduced-pressure therapy can be beneficial for wounds of all severity, but the cost and complexity of reduced-pressure therapy systems often limit the application of reduced-pressure therapy to large, highly-exudating wounds present on patients undergoing acute or chronic care, as well as other severe wounds that are not readily susceptible to healing without application of reduced pressure. For example, the complexity of conventional reduced-pressure therapy systems can limit the ability of a person with little or no specialized knowledge from administering reduced-pressure therapy. The size of many reduced-pressure therapy systems may also impair mobility. Many reduced-pressure therapy systems also require careful cleaning after each treatment, and may require electrical components or other powered devices to supply the reduced pressure for treatment. Although some reduced-pressure therapy systems deploy a purely mechanical method for reducing pressure, such systems have been unable to provide adequate control of the level of reduced pressure. 
     Reduced-pressure therapy system  100  may overcome these shortcomings and others by providing mechanical regulation of therapeutic pressure. In one example embodiment, reduced-pressure therapy system  100  may include a manually-actuated hand pump for reducing pressure. A valve can regulate pressure down to a mechanically pre-determined target pressure and supply this pressure to a sealed therapeutic environment proximate a tissue site via a supply lumen, and a feedback lumen can be fluidly connected to the valve within the pump. Pressure transmitted by the feedback lumen can control the action of the valve, which controls the pressure delivered to the tissue site. Thus, such an embodiment of reduced-pressure therapy system  100  can accurately control the pressure within the sealed therapeutic environment, including offsetting blockage that may occur in a dressing or storage system by further reducing the supplied pressure. 
       FIGS. 2A-2B  are simplified schematic cross-sections of an example embodiment of an apparatus for regulating pressure, such as regulator  106 . In this example embodiment, regulator  106  can include a housing  200  having a charging chamber  202 , a supply chamber  204 , and a control chamber  206 . Charging chamber  202  may be fluidly coupled to supply chamber  206  through a conduit, passage, or port, such as charging port  205 . A port  208  can provide fluid communication between control chamber  206  and a source of ambient pressure. Charging chamber  202  may also include a port, such as port  210 , which can be fluidly coupled to a source of reduced pressure, such as reduced-pressure source  104 . The charging chamber  202  may be adapted to receive reduced pressure from a device that can be manually-actuated, or alternatively that can be powered by electrical or other means. 
     A supply port  212  may fluidly connect supply chamber  204  to a dressing, such as dressing  102  in  FIG. 1 , and a control port  214  may fluidly couple control chamber  206  to the dressing. For example, in one embodiment, a first lumen such as supply lumen  216   a  may fluidly connect supply port  212  and supply chamber  204  to a dressing, and a second lumen such as feedback lumen  216   b  may fluidly couple control port  214  and control chamber  206  to the dressing. In some embodiments, the first lumen and the second lumen may be disposed within a single multi-lumen tube, such as tube  218 . In other embodiments, more than one tube may be used to couple a dressing to supply port  212  and control port  214 . 
     A regulator valve  220  can be operably associated with charging port  205  to regulate fluid communication between the charging chamber  202  and supply chamber  204 . In some embodiments, regulator valve  220  may include a piston, a valve body, and an elastic member. A piston can be a flexible or movable barrier, for example, illustrated in  FIGS. 2A-2B  as piston  222 . A valve body can be, for example, a generally rigid structure having a first end coupled to, adjoining, abutting, or otherwise engaging the piston, and movable with the piston. A second end of the valve body can be generally sized and shaped to engage and/or seal charging port  205 . The valve body in  FIGS. 2A-2B  is illustrated as stem  224 . As illustrated, stem  224  may extend through a partition into supply chamber  204 . An elastic member, represented in  FIGS. 2A-2B  as regulator spring  226 , can be a spring, rubber, or other elastic structure, for example, generally disposed between piston  222  and charging port  205 . In  FIGS. 2A-2B , for example, regulator spring  226  can be disposed within control chamber  206 , but may be disposed in supply chamber  204  in other embodiments. Regulator spring  226  in this embodiment can be a coil spring and coaxial with stem  224 , for example, which biases piston  222  against ambient pressure  228  in control chamber  206 . 
     In some embodiments, housing  200  may be formed from two components. For example, housing  200  may be formed from a lower housing  200   a  and a upper housing  200   b , as illustrated in  FIGS. 2A-2B . Lower housing  200   a  and upper housing  200   b  in this example each include an end wall, a side wall adjoining the end wall, and an open end opposite the end wall. Either lower housing  200   a  or upper housing  200   b  may have an outside dimension less than an inside dimension of the other so that one may be inserted into the other to form a structure that provides a substantially closed interior. In some embodiments, lower housing  200   a  and upper housing  200   b  may be engaged to allow relative movement between them. In more particular embodiments, lower housing  200   a  and upper housing  200   b  may each have cylindrical side walls and rounded end walls. 
     Charging chamber  202  may be generally defined by adjoining walls of housing  200 , such as an end wall of housing  200 , a side wall or walls of housing  200 , and a partition within housing  200 , such as chamber wall  207   a . Supply chamber  204  may also be generally defined by adjoining walls within housing  200 . For example, supply chamber  204  in  FIGS. 2A-2B  can be generally defined by chamber wall  207   a , a side wall or walls of housing  200 , and another partition, such as chamber wall  207   b . Control chamber  206  may be similarly described, for example, as a chamber defined by chamber wall  207   b , the side wall or walls of housing  200 , and another end wall of housing  200 . Thus, in this example embodiment, charging chamber  202  and supply chamber  204  may have a common wall (i.e., chamber wall  207   a ); supply chamber  204  and control chamber  206  may have a common wall (i.e., chamber wall  207   b ); charging chamber  202  and supply chamber  204  can be fluidly isolated from each other except through charging port  205 ; charging chamber  202  and supply chamber  204  can be fluidly isolated from the ambient environment; and control chamber  206  can be fluidly isolated from charging chamber  202  and supply chamber  204 . 
     Regulator valve  220  in this example can be disposed partially within control chamber  206  and partially within supply chamber  204 , with circumferential edges of piston  222  abutting or engaging the side wall or walls of control chamber  206 . The interface between piston  222  and the walls of control chamber  206  may also provide a fluid seal, dividing control chamber  206  into a region of ambient pressure  228  and a region of control pressure  230 . However, regulator valve  220  may also reciprocate within control chamber  206  while maintaining the fluid seal. For example, regulator valve  220  may additionally include flexible o-rings disposed between piston  222  and the side wall of control chamber  206 , and the o-rings may be lubricated so that regulator valve  220  can reciprocate within control chamber  206 . 
     In operation, pressure in supply chamber  204  can be distributed to a remote chamber, environment, or other location through supply port  212 . For example, pressure in supply chamber  204  may be distributed to a controlled environment, such as a sealed therapeutic environment associated with reduced-pressure therapy system  100 . Control pressure  230  in control chamber  206  can be equalized with the pressure in the remote location through control port  214 . In reduced-pressure therapy applications, control pressure  230  should be less than ambient pressure  228 , resulting in a pressure differential across regulator valve  220 . To simplify further description, the force on regulator valve  220  resulting from the pressure differential on opposing sides of piston  222  may be referred to as a “differential force.” Regulator spring  226  also generally exerts a force on regulator valve  220 . In expected operating ranges, the force of regulator spring  226  is directly proportional to the spring constant of regulator spring  226  and to a displacement X (i.e., displacement from a state of equilibrium) of the ends of regulator spring  226 . Thus, if control pressure  230  is less than ambient pressure  228 , the differential force on piston  222  tends to compress regulator spring  226  and, consequently, the force of regulator spring  226  opposes the differential force. The differential force and the force of regulator spring  226  can be combined to determine a net force acting on regulator valve  220 . The net force can cause regulator valve  220  to move reciprocally within control chamber  206 , such as along a central axis  231  aligned with charging port  205 . 
     Regulator spring  226  may be selected, adjusted, modified, tuned, or otherwise calibrated so that control pressure  230  must drop below a threshold value (such as a target pressure) before the net force can move regulator valve  220  into a position that closes charging port  205 . In some embodiments, for example, piston  222  may rotate within housing  200  to adjust the compression of regulator spring  226 . In the embodiment illustrated in  FIGS. 2A-2B , piston  222  includes a boss  232  that can be rigidly mated with a sleeve  234  of upper housing  200   b , and stem  224  may be threaded or have a threaded portion engaged to boss  232 . Stem  224  may be locked radially with housing  200  with a keyed feature. In such embodiments, piston  222  and upper housing  234  are generally locked radially and compression of regulator spring  226  may be adjusted by rotating upper housing  200   b , which can cause piston  222  to rotate relative to stem  224 . The change in compression of regulator spring  226  results in a change to the force of regulator spring  226  acting on regulator valve  220 , and thus a change in the threshold value of control pressure  230  needed to actuate regulator valve  220 . In many applications, this threshold value of control pressure  230  should generally correlate to a target pressure prescribed for reduced-pressure therapy, and may be referred to herein as the “therapy pressure” or “therapeutic pressure.” Thus, in some embodiments, the therapy pressure may be adjusted by rotating upper housing  200   b . In yet more particular embodiments, upper housing  200   b  may be calibrated to indicate various levels of therapy pressure. 
     Thus, charging chamber  202  may be charged and the pressure in the therapeutic environment may be controlled based on a differential between the therapy pressure and control pressure  230 , by balancing the force of regulator spring  226  and the differential force (i.e., control pressure  230  on one side of piston  222  against ambient pressure  228  on an opposing side of piston  222 ). For reduced-pressure therapy applications, charging chamber  202  may be charged to a pressure lower than the therapy pressure. In one embodiment, for example, the desired therapy pressure may be about −125 mm Hg and pressure in charging chamber  202  may be reduced to a pressure of about −150 mm Hg. 
     If regulator valve  220  is calibrated to a particular therapy pressure and control pressure  230  is higher than the therapy pressure, the force of regulator spring  226  should exceed the differential force and the net force should actuate regulator valve  220 , moving regulator valve  220  into an open position (see  FIG. 2B ) in which stem  224  disengages from (i.e., opens) charging port  205 . Pressure between charging chamber  202  and supply chamber  204  can equalize through open charging port  205 . As the pressure in charging chamber  202  and supply chamber  204  continues to equalize, the pressure in supply chamber  204  continues to decrease. Unless there is a complete blockage in the fluid path between supply chamber  204  and the therapeutic environment, pressure in the therapeutic environment also decreases and equalizes with the pressure in supply chamber  204  through supply lumen  216   a . And unless there is a complete obstruction in the fluid path between the therapeutic environment and control chamber  206 , control pressure  230  also decreases and equalizes with the pressure in the therapeutic environment through feedback lumen  216   b . As control pressure  230  decreases and approaches the therapy pressure, the differential force increases until it exceeds the force of regulator spring  226 , causing stem  224  to engage (i.e., close) charging port  205 , which can substantially reduce or prevent fluid communication between charging chamber  202  and supply chamber  204  through charging port  205 , as illustrated in  FIG. 2A . Charging port  205  generally remains open, though, until control pressure  230  is less than or substantially equal to the therapy pressure. Advantageously, regulator valve  220  can keep charging port  205  open to compensate for pressure drops and partial blockages, particularly in the fluid path between supply chamber  204  and a controlled environment, because pressure in the controlled environment can be directly measured by feedback lumen  216   b.    
     Referring to  FIGS. 3A-3B , a cross-section of an example embodiment of a piston-driven pump  300  is illustrated. Piston-driven pump  300  may, for example, produce reduced pressure for a chamber such as charging chamber  202 . Piston-driven pump  300  generally includes a piston  302 , a piston spring  304 , and a housing  306 . Piston  302  can be disposed within a cavity of housing  306 , such as a cylinder  308 . A sealed portion of cylinder  308 , such as vacuum chamber  310 , may be disposed between piston  302  and an opposing end of cylinder  308 . As illustrated, a seal  312  may be disposed within cylinder  308  to fluidly seal vacuum chamber  310  from the remainder of cylinder  308 . A port  314  in housing  306  may allow fluid to flow out of vacuum chamber  310 . For example, port  314  may be fluidly coupled to port  210  to allow fluid to flow between vacuum chamber  310  and charging chamber  202 . In some embodiments, port  314  and port  210  may be the same port. 
     A check valve may be used to allow unidirectional flow out of vacuum chamber  310 . For example, an o-ring may seal piston  302  against the side wall of cylinder  308  and a ball check valve in piston  302  may allow fluid to flow out of vacuum chamber  310  through a port in piston  302 . In other embodiments, such as the embodiment illustrated in  FIGS. 3A-3B , a flexible seal  312  may be disposed within cylinder  308  to fluidly seal vacuum chamber  310 . Pressure on a compression stroke creates a pressure differential that can cause seal  312  to flex and allow fluid to flow out of vacuum chamber  310  along the wall of cylinder  308 . Seal  312  flexes back to a sealing position on an expansion stroke, or when pressure is released on a compression stroke. 
     Piston  302  can reciprocate within cylinder  308  between a compressed position (as illustrated in  FIG. 3A ) and an expanded position (as illustrated in  FIG. 3B ). An elastic member such as piston spring  304  can be operably associated with piston  302  to bias piston  302  toward the expanded position. For example, a first end of piston spring  304  may abut or otherwise engage a first end of cylinder  308 , and a second end of piston spring  304  may abut or otherwise engage piston spring  304 , either directly or indirectly through seal  312  (as illustrated in  FIGS. 3A-3B ). 
     In operation, port  314  may be fluidly coupled to a charging chamber, such as charging chamber  202 . To reduce pressure in the charging chamber, piston  302  can be moved to the compressed position, which decreases the volume of vacuum chamber  310 . Seal  312  allows fluid within vacuum chamber  310  to exit during the compression stroke. After moving piston  302  to the compressed position, piston spring  304  exerts a force on seal  312  that attempts to return piston  302  to the expanded position, which increases the volume of vacuum chamber  310 . As the volume of vacuum chamber  310  increases, seal  312  prevents fluid from entering vacuum chamber  310 , which reduces the pressure in vacuum chamber  310 . The pressure between vacuum chamber  310  and the charging chamber can be equalized through port  314 , which results in a pressure reduction in the charging chamber. After piston  302  has moved to an expanded position, piston  302  may be moved again to a compressed position to recharge the charging chamber. 
     Piston-driven pump  300  may be manually-actuated, or may be actuated by an electrical, hydraulic, or pneumatic actuator, for example. For all of the charging chambers described herein, pressure may be reduced by manual or electrically powered means. In some embodiments, for example, charging chamber  202  may initially be charged or re-charged to a selected reduced pressure by a reduced pressure pump or a vacuum pump driven by an electric motor. In another illustrative embodiment, a wall suction unit (such as are commonly available in hospitals and other medical facilities) may be used to reduce pressure in charging chamber  202  to a selected pressure. 
       FIG. 4A  is a perspective view of an illustrative embodiment of reduced-pressure therapy system  100 . In this example embodiment, the reduced-pressure source is a vacuum pump  402  that may be manually operated. Dressing  102  may be positioned at a tissue site  404 , and includes drape  108  adapted for sealing around tissue site  404 . Dressing  102  may be fluidly coupled to vacuum pump  402  through a tube  406 , which may be a multi-lumen tube. Tube  406  may fluidly communicate with dressing  102  through an adapter  408 , as illustrated, or through one or more apertures in dressing  102 . 
       FIG. 4B  is a partial cross-sectional view of the example embodiment of reduced-pressure therapy system  100  in  FIG. 4A  taken along line  4 - 4 , which illustrates additional details that may be associated with certain embodiments. In such embodiments, dressing  102  may include pressure distribution manifold  110  and a sealant  410 . In operation, pressure distribution manifold  110  may be positioned within, over, on, or otherwise proximate to tissue site  404 , sealant  410  may be applied to drape  108  or to epidermis surrounding tissue site  404 , and drape  108  may be placed over pressure distribution manifold  110 . Sealant  410  may be activated or engaged to provide a sealing layer between drape  108  and epidermis surrounding tissue site  404  (preferably undamaged epidermis). Thus, drape  108  encloses pressure distribution manifold  110  and tissue site  404  in a sealed therapeutic environment in which pressure may be controlled. 
       FIG. 5  is a perspective view of vacuum pump  402  illustrating additional details that may be associated with some embodiments.  FIG. 6  is a front view of the embodiment of vacuum pump  402  illustrated in  FIG. 5 . In these illustrative embodiments, vacuum pump  402  generally includes a first barrel  515  and a second barrel  519 . While first barrel  515  and second barrel  519  are illustrated as having substantially cylindrical shapes, the barrels could be other shapes that permit operation of the device. First barrel  515  may be an outer barrel having an interior dimension greater than an exterior dimension of second barrel  519 , which may be an inner barrel. 
     Referring to  FIGS. 5-9 , first barrel  515  may include a closed end, an adjoining side wall, and an open end opposite the closed end. A cavity, such as cylinder  523  may be defined generally by the side wall. Cylinder  523  may slidingly receive second barrel  519  through the open end of first barrel  515 , and second barrel  519  can be movable between an extended position and a compressed position. Vacuum pump  402  may additionally include a barrel ring  529  and two pistons, referred to as piston  531  and seal  535 . Barrel ring  529  may be positioned at the open end of first barrel  515  to circumscribe second barrel  519 . Barrel ring  529  can eliminate large gaps between first barrel  515  and second barrel  519  at an open end of first barrel  515 . Piston  531  and seal  535  may be slidingly received within cylinder  523  of first barrel  515 . Both piston  531  and seal  535  can be positioned in cylinder  523  between second barrel  519  and a closed end of first barrel  515 , seal  535  being positioned between second barrel  519  and piston  531 . 
     Referring more specifically to  FIG. 9 , first barrel  515  may include a protrusion  539  extending from the closed end of first barrel  515  toward the open end of first barrel  515 . An elastic member, such as charging spring  543 , can be positioned within first barrel  515 . Protrusion  539  can receive one end of charging spring  543 , which can reduce lateral movement of charging spring  543  within cylinder  523 . An opposite end of charging spring  543  can be received against piston  531 . Charging spring  543  can bias piston  531 , seal  535 , and second barrel  519  toward the expanded position. 
     Referring again to  FIGS. 7-9 , but also to  FIGS. 10 and 11 , piston  531  in this example embodiment generally includes an outer wall  547  and an inner wall  551  joined by an outer floor  553 . An annulus  555  may be disposed between outer wall  547  and inner wall  551 , and a plurality of radial supports  559  can be positioned between outer wall  547  and inner wall  551  in annulus  555 . Radial supports  559  can provide additional rigidity to piston  531 , while reducing the weight of piston  531  relative to a single-wall piston that includes no annulus. However, a single-wall piston, a double-wall piston, or other variations may be suitable for various applications. 
     A plurality of guides  563  can be disposed on piston  531 , and in one embodiment, one of guides  563  may be disposed on each radial support  559 . Guides  563  can align piston  531  relative to seal  535  and second barrel  519 . Guides  563  can further serve to secure piston  531  to second barrel  519  by means of a friction fit. 
     In the illustrated embodiment, piston  531  further includes a lower bowl  567  defined by inner wall  551 , a partition  569 , and an inner floor  571 . Piston  531  may also include an upper bowl  568 , generally defined by inner wall  551  and partition  569 , wherein lower bowl  567  and upper bowl  568  are disposed on opposing sides of partition  569 . In one embodiment, inner floor  571  may be two-tiered or multi-tiered, but inner floor  571  may instead be single-tiered and/or substantially planar. Inner floor  571  may also be positioned such that a recess  573  is defined beneath inner floor  571  to receive an end of charging spring  543  (see  FIGS. 9 and 11 ). A charging port  575  may pass through inner floor  571 . A valve seat  579  may be positioned in lower bowl  567  near charging port  575  such that fluid communication through charging port  575  may be selectively controlled by selective engagement of valve seat  579  with a valve body. 
     A well  583  may also be positioned in annulus  555  of piston  531 , and a channel  587  can fluidly connect well  583  and lower bowl  567 . Channel  587  can allow fluid communication between well  583  and lower bowl  567 . 
     Referring still to  FIGS. 7-9 , but also to  FIGS. 12 and 13 , seal  535  may include a central portion  591  circumscribed by a skirt portion  595 . A plurality of guidance apertures  599  can be disposed in central portion  591  to receive guides  563  of piston  531  when vacuum pump  402  is assembled. A multi-channel aperture, such as communication aperture  601 , may be similarly disposed in central portion  591 , and in one embodiment, communication aperture  601  can be located at a distance from a center of seal  535  equal to the distance of guidance apertures  599  from the center. Communication aperture  601  can permit fluid communication through central portion  591  of seal  535 . 
     Skirt portion  595  of seal  535  extends axially from an edge of central portion  591 . As illustrated in  FIG. 9 , skirt portion  595  can engage an inner surface  605  of first barrel  515  to permit unidirectional fluid communication past seal  535 . In other words, skirt portion  595  of seal  535  can allow fluid to flow past skirt portion  595  if the fluid flow is directed from the side of seal  535  on which piston  531  is disposed toward the opposite side of seal  535 . Skirt portion  595 , however, substantially prevents fluid flow in the opposite direction. While the skirt portion  595  of seal  535  effectively controls fluid communication past skirt portion  595 , a valve member such as, for example, a check valve or other valve could instead be used to perform this function. 
     As illustrated in more detail in  FIGS. 9 and 13 , a valve body  603  may be coupled to, abut, or otherwise engage central portion  591  of seal  535 . Although valve bodies of many types, shapes and sizes may be used, valve body  603  in this illustrative embodiment can be generally conical with an apex  609  adapted to sealingly engage valve seat  579  of piston  531 . While valve body  603  is illustrated as being an integral part of seal  535  in this example, valve body  603  may alternatively be a separate component from seal  535  that is provided to engage valve seat  579 . 
     In one embodiment, both seal  535  and valve body  603  can be made from an elastomeric material, such as a medical grade silicone, for example. While many different materials may be used to construct, form, or otherwise create seal  535  and valve body  603 , a flexible material can improve the sealing properties of skirt portion  595  with inner surface  605  and valve body  603  with valve seat  579 . 
     Referring more specifically to  FIG. 9 , a regulator spring  607  can be disposed between seal  535  and charging port  575  to bias valve body  603  away from charging port  575 . For example, one end of regulator spring  607  may be positioned concentrically around valve seat  579  within lower bowl  567  of piston  531 , while another end of regulator spring  607  may engage a shoulder of valve body  603 . Regulator spring  607  generally biases regulator valve  604  toward an open position, in which valve body  603  may be disengaged from port  575  and valve seat  579  to permit fluid communication through charging port  575 . In one example embodiment, only central portion  591  of seal  535  moves due to the flexibility of the seal  535  (see  FIG. 18 ). In another embodiment, regulator spring  607  may move seal  535  in its entirety. 
     Referring again to  FIGS. 7-9 , but also to  FIGS. 14 and 15 , an example embodiment of second barrel  519  includes a first housing portion  611  and a second housing portion  615 . First housing portion  611  can include an outer shell  619  having an aperture  623 , which may be disposed near an open end of first housing portion  611 , for example. A floor  627  may be integrally formed with or otherwise connected to outer shell  619  on an end of first housing portion  611  opposite the open end. An aperture  631  may be centrally disposed in floor  627 . A boss  633  can be integrated with or connected to first housing portion  611 . Boss  633  may include supply port  527 , which can be physically aligned with aperture  623  to allow a tube to be fluidly connected to supply port  527  through aperture  623 . One embodiment of boss  633  is a ninety degree fluid fitting that can couple supply port  527  to a fluid channel  635  positioned within first housing portion  611 , and can couple a control port  528  to a fluid channel  636 . Fluid channel  635  and fluid channel  636  may be, for example, rigid conduits formed from the same or similar material as that of outer shell  619 , or in alternative embodiments, fluid channel  635  and fluid channel  636  may be lumina in a flexible, multi-lumen conduit. 
     Referring more specifically to  FIG. 15 , a plurality of guidance apertures  637  can be disposed in floor  627  of first housing portion  611 . A multi-channel aperture, such as communication aperture  638 , may also be disposed in first housing portion  611 , such as to allow fluid communication through floor  627 . Guidance apertures  637  can receive guides  563  of piston  531 , for example, to align communication aperture  638  with communication aperture  601 . In one illustrative embodiment, a first channel of communication aperture  638  may also be aligned with fluid channel  635  and a second channel may be aligned with fluid channel  636 , for example. A friction fit between guides  563  and guidance apertures  637  can also assist in securing the relative positions of piston  531  and second barrel  519 . It should be readily apparent, however, that piston  531  and second barrel  519  may be secured by alternative means. 
     Second housing portion  615  may include an end cap  639  integrally or otherwise connected to a guide  643 . Together, end cap  639  and guide  643  may slidingly engage outer shell  619  of first housing portion  611  to create a substantially closed second barrel  519  (with the exception of various apertures and passages). While second barrel  519  may be constructed from fewer components, the existence of first housing portion  611  and second housing portion  615  can allow easier access within second barrel  519  and easier assembly of vacuum pump  402 . 
     In certain example embodiments, a shaft  647  may extend from end cap  639  and can include an engagement end  649  opposite end cap  639 . When second barrel  519  is assembled, shaft  647  may be substantially coaxial with a longitudinal axis of second barrel  519  and extend through aperture  631  in floor  627  of first housing portion  611 . An elastic member such as spring  651  may be positioned within second barrel  519  such that one end of spring  651  bears upon floor  627  of first housing portion  611  and another end of spring  651  bears upon shaft  647  or another portion of second housing portion  615 . Spring  651  can bias shaft  647  and other portions of second housing portion  615  toward a disengaged position (see position of shaft  647  in  FIG. 9 ) in which engagement end  649  of shaft  647  does not bear upon seal  535  or valve body  603 . A sliding relationship and engagement between first housing portion  611  and second housing portion  615  allows a force to be exerted on second housing portion  615  (against the biasing force of spring  651 ) to move second housing portion  615  to an engaged position. In the engaged position, engagement end  649  of shaft  647  can bear upon seal  535  above valve body  603  (see  FIG. 16 ), which forces valve body  603  against valve seat  579 , thereby substantially reducing or preventing fluid communication through charging port  575 . 
     When vacuum pump  402  is assembled as illustrated in  FIG. 9 , for example, a charging chamber  655  can be generally defined by a sealed portion of cylinder  523  between piston  531  and the closed end of first barrel  515 . A supply chamber  659  may be generally defined beneath partition  569 , within lower bowl  567  of piston  531 . A control chamber  661  can be generally defined between upper bowl  568  of piston  531  and floor  627  of first housing  611 . Seal  535  can be disposed at least partially within control chamber  661  to divide control chamber  661  into a region of control pressure  662  and a region of ambient pressure  663 . A port such as charging port  575  can allow fluid communication between charging chamber  655  and supply chamber  659  depending on the position of valve body  603 . Supply chamber  659  can fluidly communicate with well  583  of piston  531  through fluid channel  587 , and control chamber  661  may fluidly communicate with fluid channel  636  through channel  589 . Well  583  can be aligned with communication aperture  601  of seal  535  and communication aperture  638  of first housing portion  611 , which can allow fluid communication between well  583 , fluid channel  635 , and supply port  527  of second barrel  519 . 
     While charging port  575  is illustrated as being disposed within piston  531  in this example, charging port  575  could instead be routed through the wall of first barrel  515 . Charging port  575  could be any conduit or passage suitable for allowing fluid communication between the chambers. 
     In operation, vacuum pump  402  can be used with other components of a reduced pressure treatment system similar to those of reduced pressure treatment system  100 . Supply port  527  of vacuum pump  402  can be adapted to be connected to a delivery tube or other conduit, for example, which may be fluidly connected to a tissue site. Although a fluid container could be integrated into vacuum pump  402 , in some embodiments, vacuum pump  402  may not be intended to collect wound exudates or other fluids within an internal chamber. In certain embodiments, vacuum pump  402  may either be used with low-exudating wounds, or an alternative collection system such as an external canister or absorptive dressing may be used to collect fluids. 
     Referring to  FIGS. 9 and 16 , an expanded position (see  FIG. 9 ) and a compressed position (see  FIG. 16 ) of vacuum pump  402  are illustrated. In an initial state, vacuum pump  402  may be in an expanded position and not “charged” with reduced pressure. To charge vacuum pump  402 , second barrel  519  can be manually compressed into first barrel  515  such that vacuum pump  402  is placed in the compressed position. As second barrel  519  compresses within first barrel  515  and moves toward the closed end of first barrel  515 , the force being exerted on second barrel  519  can be generally transmitted to seal  535  and piston  531 . The movement of second barrel  519 , seal  535 , and piston  531  into the compressed position decreases the volume of charging chamber  655 . As the volume of charging chamber  655  decreases, pressure in charging chamber  655  increases and seal  535  flexes to permit air and other gases within charging chamber  655  to exit past skirt portion  695 . 
     If the compressive force exerted upon second barrel  519  is removed, the biasing force exerted by charging spring  543  on piston  531  moves piston  531 , seal  535 , and second barrel  519  toward an expanded position. As this movement occurs, the volume of charging chamber  655  increases. Since skirt portion  595  of seal  535  allows only unidirectional flow, air and other gases are not permitted to enter charging chamber  655  past skirt portion  595 . A reduction in pressure (i.e., a generation of reduced pressure) occurs within charging chamber  655  as the volume increases. The pressure reduction within charging chamber  655  is generally dependent on the size of charging chamber  655 , range of motion of piston  531 , properties of charging spring  543 , and the integrity of seal  535 . Thus, the pressure limits of charging chamber  655  may be controlled by adjusting these parameters. In some embodiments, for example, a range of motion of piston  531  may be calibrated at so that a complete stroke (i.e., compression and expansion) reduces pressure in charging chamber  655  below a prescribed therapy pressure. For example, if the prescribed therapy pressure is −125 mmHg, a range may be selected to reduce the pressure in charging chamber  655  to −150 mmHg. 
     In the example embodiment of vacuum pump  402 , regulator valve  604  includes seal  535 , valve body  603 , and regulator spring  607 . The operation of regulator valve  604  can be controlled by two forces acting primarily on seal  535 . One of the forces is the result of a pressure differential between control pressure  662  and ambient pressure  663 . The force resulting from the pressure differential may again be referred to as a “differential force.” Regulator spring  607  also generally exerts another force on regulator valve  604 . In expected operating ranges, the force of regulator spring  607  is directly proportional to the spring constant of regulator spring  607  and displacement of the ends of regulator spring  607  from a state of equilibrium. The force exerted by regulator spring  607  is generally in direct opposition to the direction of displacement. Thus, the differential force tends to compress regulator spring  607  if control pressure  662  is less than ambient pressure  663 , and the force of regulator spring  607  in a compressed position opposes the differential force. The differential force and the force of regulator spring  607  can be combined to determine a net force acting on regulator valve  604 . 
     Regulator valve  604  can leverage the differential force and the force of regulator spring  607  to regulate a therapy pressure that can be delivered to supply port  527  and a dressing applied to a tissue site. In some embodiments, regulator spring  607  may be tuned based on a prescribed therapy. For example, a spring constant may be selected based on a prescribed therapy pressure, or the compression of regulator spring  607  may be adjusted based on the prescribed therapy pressure. In one illustrative embodiments for example, first barrel  515  and second barrel  519  may be threaded so that second barrel  519  can be rotated to change the compression of regulator spring  607 . Since changing the compression of regulator spring  607  changes the force of regulator spring  607  acting on valve body  603 , the pressure differential required to actuate regulator valve  607  can also be changed. 
     Thus, if regulator spring  607  is calibrated to a particular therapy pressure and control pressure  662  in control chamber  661  is higher than the therapy pressure, the force of regulator spring  607  should exceed the differential force and move regulator valve  604  into an open position (see  FIG. 18 ) in which valve body  603  disengages valve seat  579 . If valve body  603  disengages valve seat  579 , pressure between charging chamber  655  and supply chamber  659  can equalize through charging port  575 . As the pressure in charging chamber  655  and supply chamber  659  continues to equalize, the pressure in supply chamber  659  continues to decrease. The pressure in the dressing also decreases as the pressure in supply chamber  659  and the pressure in the dressing equalize through supply port  527 , unless there is a complete blockage in the fluid path between supply chamber  659  and the dressing. Likewise, control pressure  662  also decreases as control pressure  662  equalizes with the pressure in the dressing through control port  528 , unless there is a complete blockage in the fluid path between the dressing and control chamber  661 , which causes the differential force to increase. Thus, if control pressure  662  is reduced below the therapy pressure, the differential force should exceed the force of regulator spring  607  and move regulator valve  604  into the closed position (see  FIG. 17 ) so that valve body  603  engages valve seat  579  and closes charging port  575 . 
     When vacuum pump  402  is initially connected to a delivery tube and tissue site for treatment, it may be necessary to compress second barrel  519  within first barrel  515  more than once. As each compression stroke is completed, air and other gases may be pulled from the delivery tube and the tissue site until the pressure within the tube and at the tissue site begins to approach the desired therapy pressure. 
     If second barrel  519  is compressed within first barrel  515 , second housing portion  615  can move relative to first housing portion  611  so that shaft  647  exerts a force on valve body  603  that holds valve body  603  in the closed position to prevent positively pressurized gas (such as gas from charging chamber  655 ) from entering supply chamber  659 . Since shaft  647  remains engaged during the entire charging stroke of vacuum pump  402 , the air within charging chamber  655  can be vented past seal  535  and not into supply chamber  659 . 
     While in some embodiments of vacuum pump  402 , first barrel  515 , second barrel  519 , piston  531 , seal  535 , and other components may be cylindrical, the size and/or shape of the components may be varied. Additionally, the relative positions of valve seat  579  and valve body  603  may be reversed such that valve body  603  is positioned below valve seat  579 . 
     It should be apparent from the foregoing that systems, methods, and apparatuses having significant advantages has been described. While shown in only a few forms, the systems, methods, and apparatuses illustrated are susceptible to various changes, modifications, and uses encompassed within the claims that follow.