Abstract:
A method and apparatus for measuring a volume of a wound are described, the method comprising the steps of: applying a dressing over a wound, the volume of which is to be measured, the dressing including at least a sealing drape over the wound so as to create a sealed wound cavity; creating a vacuum in said wound cavity by vacuum pump means so as to produce a predetermined vacuum in the wound cavity; measuring a volume of air extracted from said wound cavity in producing said predetermined vacuum; and, calculating a volume of said wound.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is divisional of U.S. application Ser. No. 12/746,492, filed Jun. 4, 2010, and issued as U.S. Pat. No. 8,814,841, which is a U.S. National Phase of the PCT International Application No. PCT/GB2008/050917, filed Oct. 7, 2008, designating the U.S. and published on Jun. 11, 2009 as WO 2009/071924, which claims priority to Great Britain Patent Application No. 0723855.3 filed Dec. 6, 2007. The disclosures of all these prior applications are incorporated by reference in their entirety and should be considered a part of this specification. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to apparatus and a method for the measurement of wound volume to assess progress of wound healing particularly, though not exclusively, during topical negative pressure (TNP) therapy. 
     2. Background 
     There is much prior art available relating to the provision of apparatus and methods of use thereof for the application of TNP therapy to wounds together with other therapeutic processes intended to enhance the effects of the TNP therapy. Examples of such prior art include those listed and briefly described below. 
     TNP therapy assists in the closure and healing of wounds by reducing tissue oedema; encouraging blood flow and granulation of tissue; removing excess exudates and may reduce bacterial load and thus, infection to the wound. Furthermore, TNP therapy permits less outside disturbance of the wound and promotes more rapid healing. 
     In our co-pending International patent application, WO 2004/037334, apparatus, a wound dressing and a method for aspirating, irrigating and cleansing wounds are described. In very general terms, this invention describes the treatment of a wound by the application of topical negative pressure (TNP) therapy for aspirating the wound together with the further provision of additional fluid for irrigating and/or cleansing the wound, which fluid, comprising both wound exudates and irrigation fluid, is then drawn off by the aspiration means and circulated through means for separating the beneficial materials therein from deleterious materials. The materials which are beneficial to wound healing are recirculated through the wound dressing and those materials deleterious to wound healing are discarded to a waste collection bag or vessel. 
     In our co-pending International patent application, WO 2005/04670, apparatus, a wound dressing and a method for cleansing a wound using aspiration, irrigation and cleansing wounds are described. Again, in very general terms, the invention described in this document utilises similar apparatus to that in WO 2004/037334 with regard to the aspiration, irrigation and cleansing of the wound, however, it further includes the important additional step of providing heating means to control the temperature of that beneficial material being returned to the wound site/dressing so that it is at an optimum temperature, for example, to have the most efficacious therapeutic effect on the wound. 
     In our co-pending International patent application, WO 2005/105180, apparatus and a method for the aspiration, irrigation and/or cleansing of wounds are described. Again, in very general terms, this document describes similar apparatus to the two previously mentioned documents hereinabove but with the additional step of providing means for the supply and application of physiologically active agents to the wound site/dressing to promote wound healing. 
     The content of the above references is included herein by reference. 
     However, the above apparatus and methods are generally only applicable to a patient when hospitalised as the apparatus is complex, needing people having specialist knowledge in how to operate and maintain the apparatus, and also relatively heavy and bulky, not being adapted for easy mobility outside of a hospital environment by a patient, for example. 
     Some patients having relatively less severe wounds which do not require continuous hospitalisation, for example, but whom nevertheless would benefit from the prolonged application of TNP therapy, could be treated at home or at work subject to the availability of an easily portable and maintainable TNP therapy apparatus. 
     One particular area of wound therapy which is desirable is to monitor the volume of a wound during its treatment such as, for example, at dressing change time so as to be able to quantify the healing process. However, there are presently no simple and/or accurate tools with which to make this assessment of wound volume. 
     Methods which have been used have include measuring the length, width and depth of a wound and making some assessment from the dimensions but this is very inaccurate; Taking a tracing around the wound, calculating the area and taking depth measurements; filling the wound with a filler and measuring the wound volume based on a known density of the filler; and photographic methods. All of these prior methods involve making calculated guesses resulting in inaccurate volume figures or are time consuming or are rather impractical. 
     Desirably, wound volume should be measured when a dressing is changed at, for example, every few days so as to keep a continuous record of progress. 
     Whatever method was used it would result in the figures having to be stored in a file, for example, and generally not with the patient being treated. 
     SUMMARY 
     It is an aim of the present invention to at least partly mitigate the above-mentioned problems. 
     According to a first aspect of the present invention there is provided a method of measuring a volume of a wound, the method comprising the steps of: applying a dressing over a wound, the volume of which is to be measured, the dressing including at least a sealing drape over the wound so as to create a sealed wound cavity; creating a vacuum in said wound cavity by vacuum pump means so as to produce a predetermined vacuum in the wound cavity; measuring a volume of air extracted from said wound cavity in producing said predetermined vacuum; and, calculating a volume of said wound. 
     According to a second aspect of the present invention there is provided apparatus for measuring the volume of a wound, the apparatus comprising: a dressing covering and sealing the wound to form a cavity over the wound; an aspiration conduit leading from the wound cavity to a waste canister; a vacuum source; and, flow measuring sensor means. 
     In one embodiment of the present invention, the volume of air may be measured by signals received from flow sensor means in an apparatus control system. 
     Desirably, the wound volume measurement may be made at a time of dressing change and with an empty waste canister. 
     Desirably, the wound may be dressed in the same manner as closely as possible each time so as to minimise variables due to differences in the type and degree of packing of the wound, for example. For example, the wound may be packed with suitable filler material (if large enough and if appropriate) such as gauze, foam or any other type of filler appropriate to the wound and the sealing drape may be applied such that when the filler is compressed by the applied vacuum to the predetermined vacuum pressure then the sealing drape may be flush, for example, with the patient&#39;s sound flesh surrounding the wound. This is explanation is merely exemplary but is intended to emphasize the desirability of a consistent manner of wound dressing. 
     The method of the present may be applied by apparatus for the provision of TNP therapy to a patient in almost any environment. The apparatus is lightweight, may be mains or battery powered by a rechargeable battery pack contained within a device (henceforth, the term “device” is used to connote a unit which may contain all of the control, power supply, power supply recharging, electronic indicator means and means for initiating and sustaining aspiration functions to a wound and any further necessary functions of a similar nature). When outside the home, for example, the apparatus may provide for an extended period of operation on battery power and in the home, for example, the device may be connected to the mains by a charger unit whilst still being used and operated by the patient. 
     The overall apparatus of which the present invention is a part comprises: a dressing covering the wound and sealing at least an open end of an aspiration conduit to a cavity formed over the wound by the dressing; an aspiration tube comprising at least one lumen therethrough leading from the wound dressing to a waste material canister for collecting and holding wound exudates/waste material prior to disposal; and, a power, control and aspiration initiating and sustaining device associated with the waste canister. 
     The dressing covering the wound may be any type of dressing normally employed with TNP therapy and, in very general terms, may comprise, for example, a semi-permeable, flexible, self-adhesive drape material, as is known in the dressings art, to cover the wound and seal with surrounding sound tissue to create a sealed cavity or void over the wound. There may aptly be a porous barrier and support member in the cavity between the wound bed and the covering material to enable an even vacuum distribution to be achieved over the area of the wound. The porous barrier and support member being, for example, a gauze, a foam, an inflatable bag or known wound contact type material resistant to crushing under the levels of vacuum created and which permits transfer of wound exudates across the wound area to the aspiration conduit sealed to the flexible cover drape over the wound. 
     The aspiration conduit may be a plain flexible tube, for example, having a single lumen therethrough and made from a plastics material compatible with raw tissue, for example. However, the aspiration conduit may have a plurality of lumens therethrough to achieve specific objectives relating to the invention. A portion of the tube sited within the sealed cavity over the wound may have a structure to enable continued aspiration and evacuation of wound exudates without becoming constricted or blocked even at the higher levels of the negative pressure range envisaged. 
     It is envisaged that the negative pressure range for the apparatus embodying the present invention may be between about −50 mmHg and −200 mmHg (note that these pressures are relative to normal ambient atmospheric pressure thus, −200 mmHg would be about 560 mmHg in practical terms). Aptly, the pressure range may be between about −75 mmHg and −150 mmHg. Alternatively a pressure range of up to −75 mmHg, up to −80 mmHg or over −80 mmHg can be used. Also aptly a pressure range of below −75 mmHg could be used. Alternatively a pressure range of over −100 mmHg could be used or over −150 mmHg. 
     The aspiration conduit at its distal end remote from the dressing may be attached to the waste canister at an inlet port or connector. The device containing the means for initiating and sustaining aspiration of the wound/dressing may be situated between the dressing and waste canister, however, in a preferred embodiment of the apparatus embodying the present invention, the device may aspirate the wound/dressing via the canister thus, the waste canister may preferably be sited between the wound/dressing and device. 
     The aspiration conduit at the waste material canister end may preferably be bonded to the waste canister to prevent inadvertent detachment when being caught on an obstruction, for example. 
     The canister may be a plastics material moulding or a composite unit comprising a plurality of separate mouldings. The canister may aptly be translucent or transparent in order to visually determine the extent of filling with exudates. However, the canister and device may in some embodiments provide automatic warning of imminent canister full condition and may also provide means for cessation of aspiration when the canister reaches the full condition. 
     The canister may be provided with filters to prevent the exhaust of liquids and odours therefrom and also to prevent the expulsion of bacteria into the atmosphere. Such filters may comprise a plurality of filters in series. Examples of suitable filters may comprise hydrophobic filters of 0.2 μm pore size, for example, in respect of sealing the canister against bacteria expulsion and 1 μm against liquid expulsion. 
     Aptly, the filters may be sited at an upper portion of the waste canister in normal use, that is when the apparatus is being used or carried by a patient the filters are in an upper position and separated from the exudate liquid in the waste canister by gravity. Furthermore, such an orientation keeps the waste canister outlet or exhaust exit port remote from the exudate surface. 
     Aptly the waste canister may be filled with an absorbent gel such as ISOLYSEL (trade mark), for example, as an added safeguard against leakage of the canister when full and being changed and disposed of. Added advantages of a gel matrix within the exudate storing volume of the waste canister are that it prevents excessive movement, such as slopping, of the liquid, minimises bacterial growth and minimises odours. 
     The waste canister may also be provided with suitable means to prevent leakage thereof both when detached from the device unit and also when the aspiration conduit is detached from the wound site/dressing. 
     The canister may have suitable means to prevent emptying by a user (without tools or damage to the canister) such that a full or otherwise end-of-life canister may only be disposed of with waste fluid still contained. 
     The device and waste canister may have mutually complementary means for connecting a device unit to a waste canister whereby the aspiration means in the device unit automatically connects to an evacuation port on the waste canister such that there is a continuous aspiration path from the wound site/dressing to an exhaust port on the device. 
     Aptly, the exhaust port from the fluid path through the apparatus is provided with filter means to prevent offensive odours from being ejected into the atmosphere. 
     In general terms the device unit comprises an aspirant pump; means for monitoring pressure applied by the aspirant pump; a flowmeter to monitor fluid flow through the aspirant pump; a control system which controls the aspirant pump in response to signals from sensors such as the pressure monitoring means and the flowmeter, for example, and which control system also controls a power management system with regard to an on-board battery pack and the charging thereof and lastly a user interface system whereby various functions of the device such as pressure level set point, for example, may be adjusted (including stopping and starting of the apparatus) by a user. The device unit may contain all of the above features within a single unified casing. 
     When the dressing is applied to the wound a volume of air is trapped under the sealing drape in the wound cavity. When the vacuum source such as a vacuum pump, for example, is started the wound cavity is evacuated to a predetermined vacuum such as by 0.1 of an atmosphere (i.e. about 76 mmHg below atmospheric pressure). In this case 0.1 of the volume of air in the wound cavity is removed and which is measured by the flow sensor means such as a flowmeter, for example, in the control and monitoring system. The volume of air removed will be proportional to the actual volume of the wound. The volumes of other parts of the apparatus such as the aspiration conduit, the waste canister, the vacuum pump and the flow conduits in the apparatus leading to the flow sensor may be measured and/or are known and constant factors for which a correction factor may be incorporated in memory means in the control system software. 
     Once the desired vacuum level is achieved and is steady at that level, the various factors may be computed by the software in the control system to calculate the wound volume. It is important that the desired vacuum is at a steady state and the vacuum source such as a vacuum pump may be running slowly or intermittently to achieve such a steady state due, for example, to a leak into the wound cavity through or around the sealing drape. The software may contain appropriate data relating to the vacuum pump operating regime to calculate the steady state leak rate and to enable the control and monitoring system to apply a suitable correction factor to allow for a steady state leak rate. Thus, when the wound cavity has achieved a steady state when the vacuum level pressure is at the desired value, what the flow sensor is reading is the actual leak rate which may be used by the software to compute the correction factor applicable. 
     Different wound filling materials may have different compressibility from each other. Gauze, for example, is made from material fibres which themselves are virtually incompressible at the levels of vacuum under consideration in the present invention; foam materials, however, are much more compressible and thus, correction factors need to be made to allow for the type of wound filling material in use. This may be easily achieved by test cavities of known volumes used to calibrate the apparatus for different filling materials and calculate appropriate correction factors. 
     As noted above it is desirable that at a time of dressing change the waste canister is empty since its empty volume forms part of the correction factor incorporated into the software. However, it is not necessary that the waste canister be empty so long as its free space is known so that a suitable factor may be entered into the control and monitoring system such as by a key pad associated with the apparatus device, for example. 
     In the present invention the device may have means such as LED display means, for example, of displaying and storing the volume of the wound as measured at each stage such as at each dressing change time, for example, so that a record exists of the progress of wound healing. Alternatively, the device may have an output to a separate display and/or recording device where data may be held and/or displayed. 
     In view of the fact that the device unit contains the majority of the intrinsic equipment cost therein ideally it will also be able to survive impact, tolerate cleaning in order to be reusable by other patients. 
     In terms of pressure capability the aspiration means may be able to apply a maximum pressure drop of at least −200 mmHg to a wound site/dressing. The apparatus is capable of maintaining a predetermined negative pressure even under conditions where there is a small leak of air into the system and a high exudate flow. 
     The pressure control system may prevent the minimum pressure achieved from exceeding for example −200 mmHg so as not to cause undue patient discomfort. The pressure required may be set by the user at a number of discreet levels such as −50, −75, −100, −125, −150, −175 mmHg, for example, depending upon the needs of the wound in question and the advice of a clinician. Thus suitable pressure ranges in use may be from −25 to −80 mmHg, or −50 to −76 mmHg, or −50 to −75 mmHg as examples. The control system may also advantageously be able to maintain the set pressure within a tolerance band of +/−10 mmHg of the set point for 95% of the time the apparatus is operating given that leakage and exudation rates are within expected or normal levels. 
     Aptly, the control system may trigger alarm means such as a flashing light, buzzer or any other suitable alarm means when various abnormal conditions apply such as, for example: pressure outside set value by a large amount due to a gross leak of air into system; duty on the aspiration pump too high due to a relatively smaller leakage of air into the system; pressure differential between wound site and pump is too high due, for example, to a blockage or waste canister full. 
     The apparatus of the present invention may be provided with a carry case and suitable support means such as a shoulder strap or harness, for example. The carry case may be adapted to conform to the shape of the apparatus comprised in the joined together device and waste canister. In particular, the carry case may be provided with a bottom opening flap to permit the waste canister to be changed without complete removal of the apparatus form the carry case. 
     The carry case may be provided with an aperture covered by a displaceable flap to enable user access to a keypad for varying the therapy applied by the apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: 
         FIG. 1  shows a generalised schematic block diagram showing a general view of an apparatus and the constituent apparatus features thereof; 
         FIG. 2  shows a similar generalised schematic block diagram to  FIG. 1  and showing fluid paths therein; 
         FIG. 3  shows a generalised schematic block diagram similar to  FIG. 1  but of a device unit only and showing power paths for the various power consuming/producing features of the apparatus; 
         FIG. 4  shows a similar generalised schematic block diagram to  FIG. 3  of the device unit and showing control system data paths for controlling the various functions and components of the apparatus; 
         FIG. 5  shows a perspective view of an apparatus; 
         FIG. 6  shows a perspective view of an assembled device unit of the apparatus of  FIG. 5 ; 
         FIG. 7  shows an exploded view of the device unit of  FIG. 6 ; 
         FIG. 8  shows a partially sectioned side elevation view through the interface between a waste canister and device unit of the apparatus; 
         FIG. 9  shows a cross section through a waste canister of the apparatus of  FIGS. 5 to 8 ; 
         FIG. 10  shows a schematic cross section of a wound, the volume of which is to be measured, the wound having apparatus for the application of TNP therapy thereto; 
         FIG. 11  shows a graph of flow vs time of an example of wound volume measurement; and 
         FIG. 12  which shows a graph of flow against pressure in the wound cavity. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1 to 4  of the drawings and where the same or similar features are denoted by common reference numerals. 
       FIG. 1  shows a generalised schematic view of an apparatus  10  of a portable topical negative pressure (TNP) system. It will be understood that embodiments of the present invention are generally applicable to use in such a TNP system. Briefly, negative pressure wound therapy assists in the closure and healing of many forms of “hard to heal” wounds by reducing tissue oedema; encouraging blood flow and granular tissue formation; removing excess exudate and may reduce bacterial load (and, therefore, infection). In addition the therapy allows for less disturbance of a wound leading to more rapid healing. The TNP system is detailed further hereinafter but in summary includes a portable body including a canister and a device with the device capable of providing an extended period of continuous therapy within at least a one year life span. The system is connected to a patient via a length of tubing with an end of the tubing operably secured to a wound dressing on the patient. 
     More particularly, as shown in  FIG. 1 , the apparatus comprises an aspiration conduit  12  operably and an outer surface thereof at one end sealingly attached to a dressing  14 . The dressing  14  will not be further described here other than to say that it is formed in a known manner from well know materials to those skilled in the dressings art to create a sealed cavity over and around a wound to be treated by TNP therapy with the apparatus of the present invention. The aspiration conduit has an in-line connector  16  comprising connector portions  18 ,  20  intermediate its length between the dressing  14  and a waste canister  22 . The aspiration conduit between the connector portion  20  and the canister  22  is denoted by a different reference numeral  24  although the fluid path through conduit portions  12  and  24  to the waste canister is continuous. The connector portions  18 ,  20  join conduit portions  12 ,  24  in a leak-free but disconnectable manner. The waste canister  22  is provided with filters  26  which prevent the escape via an exit port  28  of liquid and bacteria from the waste canister. The filters may comprise a 1 μm hydrophobic liquid filter and a 0.2 μm bacteria filter such that all liquid and bacteria is confined to an interior waste collecting volume of the waste canister  22 . The exit port  28  of the waste canister  22  mates with an entry/suction port  30  of a device unit  32  by means of mutually sealing connector portions  34 ,  36  which engage and seal together automatically when the waste canister  22  is attached to the device unit  32 , the waste canister  22  and device unit  32  being held together by catch assemblies  38 ,  40 . The device unit  32  comprises an aspirant pump  44 , an aspirant pressure monitor  46  and an aspirant flowmeter  48  operably connected together. The aspiration path takes the aspirated fluid which in the case of fluid on the exit side of exit port  28  is gaseous through a silencer system  50  and a final filter  52  having an activated charcoal matrix which ensures that no odours escape with the gas exhausted from the device  32  via an exhaust port  54 . The filter  52  material also serves as noise reducing material to enhance the effect of the silencer system  50 . The device  32  also contains a battery pack  56  to power the apparatus which battery pack also powers the control system  60  which controls a user interface system  62  controlled via a keypad (not shown) and the aspiration pump  44  via signals from sensors  46 ,  48 . A power management system  66  is also provided which controls power from the battery pack  56 , the recharging thereof and the power requirements of the aspirant pump  44  and other electrically operated components. An electrical connector  68  is provided to receive a power input jack  70  from a SELV power supply  72  connected to a mains supply  74  when the user of the apparatus or the apparatus itself is adjacent a convenient mains power socket. 
       FIG. 2  shows a similar schematic representation to  FIG. 1  but shows the fluid paths in more detail. The wound exudate is aspirated from the wound site/dressing  14  via the conduit  12 , the two connector portions  18 ,  20  and the conduit  24  into the waste canister  22 . The waste canister  22  comprises a relatively large volume  80  in the region of 500 ml into which exudate from the wound is drawn by the aspiration system at an entry port  82 . The fluid  84  drawn into the canister volume  80  is a mixture of both air drawn into the dressing  14  via the semi-permeable adhesive sealing drape (not shown) and liquid  86  in the form of wound exudates. The volume  80  within the canister is also at a lowered pressure and the gaseous element  88  of the aspirated fluids is exhausted from the canister volume  80  via the filters  26  and the waste canister exhaust exit port  28  as bacteria-free gas. From the exit port  28  of the waste canister to the final exhaust port  54  the fluid is gaseous only. 
       FIG. 3  shows a schematic diagram showing only the device portion of the apparatus and the power paths in the device of the apparatus embodying the present invention. Power is provided mainly by the battery pack  56  when the user is outside their home or workplace, for example, however, power may also be provided by an external mains  74  supplied charging unit  72  which when connected to the device  32  by the socket  68  is capable of both operating the device and recharging the battery pack  56  simultaneously. The power management system  66  is included so as to be able to control power of the TNP system. The TNP system is a rechargeable, battery powered system but is capable of being run directly from mains electricity as will be described hereinafter more fully with respect to the further figures. If disconnected from the mains the battery has enough stored charge for approximately 8 hours of use in normal conditions. It will be appreciated that batteries having other associated life times between recharge can be utilised. For example batteries providing less than 8 hours or greater than 8 hours can be used. When connected to the mains the device will run off the mains power and will simultaneously recharge the battery if depleted from portable use. The exact rate of battery recharge will depend on the load on the TNP system. For example, if the wound is very large or there is a significant leak, battery recharge will take longer than if the wound is small and well sealed. 
       FIG. 4  shows the device  32  part of the apparatus embodying the present invention and the data paths employed in the control system for control of the aspirant pump and other features of the apparatus. A key purpose of the TNP system is to apply negative pressure wound therapy. This is accomplished via the pressure control system which includes the pump and a pump control system. The pump applies negative pressure; the pressure control system gives feedback on the pressure at the pump head to the control system; the pump control varies the pump speed based on the difference between the target pressure and the actual pressure at the pump head. In order to improve accuracy of pump speed and hence provide smoother and more accurate application of the negative pressure at a wound site, the pump is controlled by an auxiliary control system. The pump is from time to time allowed to “free-wheel” during its duty cycle by turning off the voltage applied to it. The spinning motor causes a “back electro-motive force” or BEMF to be generated. This BEMF can be monitored and can be used to provide an accurate measure of pump speed. The speed can thus be adjusted more accurately than can prior art pump systems. 
     According to embodiments of the present invention, actual pressure at a wound site is not measured but the difference between a measured pressure (at the pump) and the wound pressure is minimised by the use of large filters and large bore tubes wherever practical. If the pressure control measures that the pressure at the pump head is greater than a target pressure (closer to atmospheric pressure) for a period of time, the device sends an alarm and displays a message alerting the user to a potential problem such as a leak. 
     In addition to pressure control a separate flow control system can be provided. A flow meter may be positioned after the pump and is used to detect when a canister is full or the tube has become blocked. If the flow falls below a certain threshold, the device sounds an alarm and displays a message alerting a user to the potential blockage or full canister. 
     Referring now to  FIGS. 5 to 9  which show various views and cross sections of a preferred embodiment of apparatus  200  embodying the present invention. The preferred embodiment is of generally oval shape in plan and comprises a device unit  202  and a waste canister  204  connected together by catch arrangements  206 . The device unit  202  has a liquid crystal display (LCD)  208 , which gives text based feedback on the wound therapy being applied, and a membrane keypad  210 , the LCD being visible through the membrane of the keypad to enable a user to adjust or set the therapy to be applied to the wound (not shown). The device has a lower, generally transverse face  212  in the centre of which is a spigot  214  which forms the suction/entry port  216  to which the aspiration means (to be described below) are connected within the device unit. The lower edge of the device unit is provided with a rebated peripheral male mating face  218  which engages with a co-operating peripheral female formation  220  on an upper edge of the waste canister  204  (see  FIGS. 8 and 9 ). On each side of the device  202 , clips  222  hinged to the canister  204  have an engaging finger (not shown) which co-operates with formations in recesses  226  in the body of the device unit. From  FIG. 7  it may be seen that the casing  230  of the device unit is of largely “clamshell” construction comprising front and back mouldings  232 ,  234 , respectively and left-hand and right-hand side inserts  236 ,  238 . Inside the casing  230  is a central chassis  240  which is fastened to an internal moulded structural member  242  and which chassis acts as a mounting for the electrical circuitry and components and also retains the battery pack  246  and aspiration pump unit  248 . Various tubing items  250 ,  252 ,  254  connect the pump unit  248  and suction/entry port  216  to a final gaseous exhaust via a filter  290 .  FIG. 8  shows a partially sectioned side elevation of the apparatus  200 , the partial section being around the junction between the device unit  202  and the waste canister  204 , a cross section of which is shown at  FIG. 9 . Theses views show the rebated edge  218  of the male formation on the device unit co-operating with the female portion  220  defined by an upstanding flange  260  around the top face  262  of the waste canister  204 . When the waste canister is joined to the device unit, the spigot  214  which has an “O” ring seal  264  therearound sealingly engages with a cylindrical tube portion  266  formed around an exhaust/exit port  268  in the waste canister. The spigot  214  of the device is not rigidly fixed to the device casing but is allowed to “float” or move in its location features in the casing to permit the spigot  214  and seal  264  to move to form the best seal with the bore of the cylindrical tube portion  266  on connection of the waste canister to the device unit. The waste canister  204  in  FIG. 9  is shown in an upright orientation much as it would be when worn by a user. Thus, any exudate  270  would be in the bottom of the internal volume of waste receptacle portion  272 . An aspiration conduit  274  is permanently affixed to an entry port spigot  278  defining an entry port  280  to receive fluid aspirated from a wound (not shown) via the conduit  274 . Filter members  282  comprising a 0.2 μm filter and  284  comprising a 1 μm filter are located by a filter retainer moulding  286  adjacent a top closure member or bulkhead  288  the filter members preventing any liquid or bacteria from being drawn out of the exhaust exit port  268  into the pump and aspiration path through to an exhaust and filter unit  290  which is connected to a casing outlet moulding at  291  via an exhaust tube (not shown) in casing side piece  236 . The side pieces  236 ,  238  are provided with recesses  292  having support pins  294  therein to locate a carrying strap (not shown) for use by the patient. The side pieces  230  and canister  204  are also provided with features which prevent the canister and device from exhibiting a mutual “wobble” when connected together. Ribs (not shown) extending between the canister top closure member  288  and the inner face  300  of the upstanding flange  260  locate in grooves  302  in the device sidewalls when canister and device are connected. The casing  230  also houses all of the electrical equipment and control and power management features, the functioning of which was described briefly with respect to  FIGS. 3 and 4  hereinabove. The side piece  238  is provided with a socket member  298  to receive a charging jack from an external mains powered battery charger (both not shown). 
       FIG. 10  shows a schematic cross section of a wound  400  and apparatus  402  connected thereto to apply TNP therapy to the wound and to measure its volume. The apparatus comprises wound filling material  404 ; an aspirant conduit  406  having one end  408  sealed within the wound cavity  410  by an overlying sealing drape  412  at point  414  and a distal end  416  of the conduit  406  attached to a waste canister  418  for the collection of exudate from the wound. The waste canister is operably connected to a pressure sensor  420  by a conduit portion  422  and to the inlet side of a vacuum pump  426  by a conduit portion  428 . The vacuum pump output side is connected by a conduit portion  430  to a flow sensor  432  and exhaust is taken away via a conduit  434 . A control and monitoring system is provided at  440  and receives signals  442  from the pressure sensor  420 ; receives signals  444  from the vacuum pump; and, receives signals  446  from the flow sensor  432 . The control system  440  sends control signals  448  to the vacuum pump  426  in order for it to maintain a steady state pressure as set by instructions  450  entered by a clinician/user on a data entry keypad  452 . The control system  440  outputs data  454  to a display  456  such as an LCD display, for example, as required by the clinician/user in response to instructions  450  but includes data relating to the volume of the wound  400 . The sealing drape  412  is sealed to the patient&#39;s sound flesh at  460  surrounding the wound  400  generally by a layer of pressure sensitive adhesive (not shown) on the flesh contacting side of the drape  412 . 
     In operation a clinician/user enters a required pressure, consistent with calibration procedures previously employed, to be achieved in the wound cavity  410  on the keypad  452 . The vacuum pump  426  is initiated and pumps the wound cavity  410  down to the required pressure and continues to operate as appropriate to maintain the required pressure at a steady state. The apparatus  402  has been calibrated in that the volumes of the conduit  406 ; empty waste canister  418 ; conduits  422 ,  428 ,  430 ; and vacuum pump  426  are known and a correction factor has been entered in the memory of the control system  440 . 
       FIG. 11  shows a graph of fluid flow, in this case air against time to achieve steady state conditions.  FIG. 11  indicates time taken to reach a steady state pressure as set in the control system at start up and which time to reach a steady state pressure is indicated by the line  500 . Initially the rate of air flow rises rapidly to a maximum at  502  after which the flow rate falls off as the set pressure is approached and the steady state is achieved at line  502 . Part of the flow rate under the curve  504  may probably be due to an inward leak of air, most probably into the wound cavity  410  (the joints between the component parts of the apparatus from the conduit  406  in the direction of fluid flow should all be sound and leak-free) between the sealing drape  412  and the patient&#39;s sound flesh  460 . The point  508  indicates the steady state flow rate to maintain the desired set pressure and the area of the triangle  510 ,  508 ,  512  indicates the portion or volume of air aspirated from the wound cavity attributable to the leak and may be deducted from the total area under the curve  504  up to the line  500 . Thus, the control and monitoring system  440  computes the volume of the wound from the area under the curve  504  bounded by the line  508 - 510  and will be displayed on the display  456  and/or in suitable recording apparatus (not shown). 
     A similar graph to that of  FIG. 11  and shown in  FIG. 12  may be drawn but showing flow against pressure in the wound cavity  410 . Steady state is reached when the set pressure has been achieved. In the graph of  FIG. 12  the set pressure  600  and steady state coincide at the line  602  and the total area under the curve  604  is comprised of: a factor relating to leak rate as before and is the triangular area  600 ,  606 ,  608 ; a factor relating to system volume indicated by the vertically hatched area  610  and the wound volume indicated by the area  612 . When the system is at a steady state indicated by the line  614  the flow through the flow sensor relates only to the leakage into the system. 
     The volume of the wound may be computed from the expression: 
     
       
         
           
             
               V 
               wound 
             
             = 
             
               Area 
               ⁢ 
               
                   
               
               ⁢ 
               under 
               ⁢ 
               
                   
               
               ⁢ 
               curve 
               × 
               
                 
                   ( 
                   
                     760 
                     - 
                     
                       Set 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       pressure 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       in 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mmHg 
                     
                   
                   ) 
                 
                 760 
               
             
           
         
       
     
     If the dressing drape  412  is completely leak-free then the steady state flow rate when the set required pressure is reached will be zero and the point  508  will lie on the time axis. 
     The flow meter  432  is shown as situated after the vacuum pump  426  but may in other embodiments of apparatus according to the present invention lie between the waste canister  418  and the vacuum pump  426 . Suitable correction factors relating to the exact arrangement of apparatus used will need to be determined. 
     The method of the present invention may be carried out by the apparatus described and discussed with reference to  FIGS. 1 to 9  above which also constitutes apparatus according to the present invention. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
     Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.