Topical negative pressure therapy: mechanisms and indications

Topical negative pressuretherapy: mechanisms andindicationsPaul E Banwell, Melinda Musgrave

Banwell PE, Musgrave M. Topical negative pressure therapy: mechanisms and indications. Int Wound J2004;1:95—106.

ABSTRACTTopical negative pressure (TNP) therapy has emerged as a high-technology, microprocessor-controlled physicalwound-healing modality. Complex effects at the wound—dressing interface following application of a controlledvacuum force have been documented. These include changes on a microscopic, molecular level and on amacroscopic, tissue level: interstitial fluid flow and exudate management, oedema reduction, effects on woundperfusion, protease profiles, growth factor and cytokine expression and cellular activity, all leading to enhancedgranulation tissue formation and improved wound-healing parameters. Primary indications for clinical use havebeen documented and include traumatic wounds, open abdominal wounds, infected sternotomy wounds, woundbed preparation, complex diabetic wounds and skin-graft fixation. Whilst this therapy now forms an essentialpart of the wound healing armamentarium, extensive clinical trials are recommended to confirm efficacy anddelineate its optimum use.

Key words: Acute . Chronic . Evidence . Exudate . Fluid . Granulation tissue . Healing . Surgical . Topical negative pressure .Wound

INTRODUCTIONTopical negative pressure (TNP) therapy is theapplication of a local subatmospheric pressure(Figure 1) across a wound (1). It is a novel nonpharmacological, physical method of pro-moting wound healing and has now beenincorporated into the wound managementarmamentarium as a standard of care. Althoughguidelines for clinical use have been recom-mended (2), reports in the literature suggestan evolution in wound-type utilisation.Specifically, TNP has found a routine role inacute wound management, the down-stagingand temporisation of complex trauma prior tosurgery, wound bed preparation, and in the

management of patients with a poor operativerisk and those with chronic wounds. How-ever, controversy still remains regarding thebenefits of such treatment in certain clinicalscenarios, e.g. open tibial fractures.Whilst there is an expanding evidence-base

for clinical efficacy and effectiveness, parallelresearch programmes have been instituted tounderstand the scientific basis and mech-anisms for this innovative wound therapyand justify its clinical use. Many questionsstill remain unanswered regarding the relativeimportance of the multimodality mechanismsof action including mechanical transductionand fluid dynamics within the wound—foaminterface.Thus, this article attempts to critique the pro-

posed mechanisms of action for TNP therapyand reviews the current indications for use.

HISTORICALIt has long been recognised that surgical heal-ing is promoted by removing fluid collections

Key Points

. topical negative pressure (TNP)therapy is the application oflocal subatmospheric pressureacross the wound

. TNP has a role in acute woundmanagement

Authors: PE Banwell, Department of Plastic Surgery, RadcliffeInfirmary, Oxford and Odstock Burns, Wound Healing andReconstructive Surgery Research Trust, Laing Laboratory,Salisbury District Hospital, Salisbury, UK; M Musgrave,Department of Plastic Surgery, Radcliffe Infirmary, Oxford, UKAddress for correspondence: PE Banwell, Department ofPlastic Surgery, Radcliffe Infirmary, Oxford, UKE-mail: [email protected]

� Blackwell Publishing Ltd and Medicalhelplines.com Inc 2004 . International Wound Journal . Vol 1 No 2 95

REVIEW&

from a wound site in the postoperative period.In addition, the observation that tissue growthresponds positively to applied mechanicalforces (a permutation of Wolff’s Law), as inosteogenic distraction and soft tissue expan-sion, has led to a variety of negative pressurewound therapy systems being designed (4—9).The philosophies underpinning the earlydevelopment of TNP therapy differ subtlybetween North American and Europeangroups. The North American approach wasprincipally concerned with developing atherapy for managing difficult, complexwounds, with an emphasis on improvinggranulation tissue formation, exudate man-agement, infection control and wound closure(10,11). The European concept of TNP therapycan be characterised as a therapy designed formanaging acute, high-energy, contaminatedinjuries where haemostasis and infectioncontrol are at a premium (12). From themid-1990s, the majority of published researchexamining TNP has focused on the use ofpurpose-built programmable microprocessor-controlled vacuum pumps. These devicesdeliver a predetermined negative pressureacross a wound surface in a controlled fashion.This facility has encouraged a more structuredand scientific examination of TNP and

produced a body of research with negativepressure devices at its core. Within the litera-ture and clinical practice, a number ofsynonyms for TNP (1) are presently in use,including: subatmospheric pressure (10),vacuum-sealing technique (VST) (12), sealedsurface wound suction (SSS), vacuum-assistedclosureTM (VACTM), vacuum pack technique(13), negative pressure dressing and foamsuction dressing (2,14).

MECHANISMSA systematic analysis of the mechanisms ofaction of TNP is required to support the ratio-nale for its use in clinical practice. Untilrecently, the proposed mechanisms of actioncentred around the following: an increase inwound perfusion, a reduction in oedema, sti-mulation of granulation tissue formation, adecrease in bacterial colonisation and removalof wound exudate (Figure 2).

SPECTRUM OF TNP EFFECTSThe above model may now be improved, asour understanding of the differential effects ofTNP improves. Alternative models of TNPeffects have since been suggested (15) to incor-porate a cascade of putative actions on cellsand tissues (Figure 3). Thus, one can viewTNP effecting change from a microscopic,molecular level to a macroscopic, tissue level.Vogt (16), Morykwas (17) and others (2) have

Key Points

. this article reviews proposedmechanisms of action of TNP andreviews current indications for use

. it has long been recognised thatsurgical healing is promoted byremoving fluid for a wound site

. North American and Europeandevelopment of TNP therapy hasbeen subtly different

Atmospheric pressure is the pressure exerted bythe whole atmosphere on the Earth’s surface(760 mmHg or 100kPa).

Figure 1. Definition of atmospheric pressure.

Figure 2. Postulated mechanisms of action of topical negative pressure.

Topical negative pressure therapy

� Blackwell Publishing Ltd and Medicalhelplines.com Inc 2004 . International Wound Journal . Vol 1 No 296

argued that mechanical transduction at thefoam—wound interface contributes to enhancedfluid flow through tissues, extracellular matrix(ECM) stretching and activation of signaltransduction pathways ultimately resulting incell recruitment and activation. TNP is there-fore able to modulate the balance betweeninternal cytoskeletal forces and ECM forceswhich are critical for the control of cellshape, migration, differentiation and tissuepatterning.

Interstitial fluid flow and exudatemanagementFundamental to TNP is the direct removal offluid from the wound (1). By definition, it hasbeen inferred that this induces an interstitialfluid gradient shift (10) which has a numberof positive effects: a reduction in oedema, asecondary increase in dermal perfusion andthe removal of wound fluid.The importance of this alteration in gradient

interstitial fluid pressures has probably beenunderestimated, although it is well knownthat interstitial fluid flow can modify ECMcomponents and organisation. Specifically,interstitial fluid can also modulate otherfundamental cellular wound-healing processesincluding growth factor expression and cellularmitotic activity (18).

Oedema reductionOedema occurs secondary to the direct andindirect effects of the humoral and cellularinflammatory response. The net result of thisis increased fluid accumulation and decreased

interstitial flow within the wound matrix.Application of TNP has a direct effect oninterstitial fluid egress and creates an intersti-tial fluid gradient (1). The removal of pro-inflammatory mediators which contribute tooedema formation also facilitates resolution ofthis process. Whilst this is clinically evident inmany wounds, sceptics have argued that theeffect of TNP on oedema is purely anecdotal.However, quantitative assessment is notori-ously difficult and results from on-going trialsare not yet available.

Wound perfusionThe effect of TNP on the vascular biology ofwounds is complex and not fully understood(2). It is likely that the changes in interstitialfluid pressures described above have an indirecteffect on wound perfusion by decompressingsmall blood vessels. In addition, Shakespeare(personal communication) has argued forother indirect effects on the microvasculature:mechanical forces exerted on the ECM willinevitably affect the microvasculature containedwithin it, and it is therefore mechanical stressthat may be the principal effector. In contrast,direct effects on the dermal vasculature arethought to be mediated by influencing vasomo-tor tone and vasoactive mediators.To date, the following data has been pro-

duced to support the quantitative effects ofTNP on wound perfusion: using needleprobe laser Doppler flowmetry, subatmos-pheric pressures of 125mmHg resulted in afourfold increase in blood flow (10) in an exci-sional porcine wound model (n= 10 wounds).

Key Points

. proposed mechanism of actioncentres around mechanical stressfluid flow and increase in woundperfusion

. fundamental to TNP is the directremoval of fluids from the wound

. interstitial fluid flow can modifyextracellular matrix componentsand organisation

Molecular

Macroscopic

Fluid FlowECM Effects

MechanicalTransduction Gene Activation Signal

Transduction

Cellularand

Humoral Effects

MacroscopicWoundEffects

Cascade of TNP effects

Figure 3. Differential effects of topical negative pressure.



Furthermore, simple volunteer studies havedemonstrated an immediate increase inblood flow in uninjured forearms using atranscutaneous ultrasonic Doppler flow velo-city meter (19,20) and laser Doppler imaging(21). TNP was also used to treat 32 burns in adeep dermal burn wound model and despitea delay in treatment of 6 hours, still resulted ina statistically significant increase in dermalblood flow in those burns treated for 72hours (22). Using a polyurethane foam inter-face dressing, the changes in blood flowappear to be pressure-dependent (10), becauseincreasing pressures to 400mmHg across thewounds actually decreased blood flow in exci-sional wounds. However, increasing pressuresin patients does not necessarily have adeleterious effect on wound outcome, asmany clinics (12,23) use high pressures effec-tively with a polyvinylalcohol foam. Theseresults imply that the interface foam dressingmay be critical in the transmission of pressureand hence final outcome parameters.

Wound fluid and proteasesCentral to current thought on acute andchronic wound healing is the concept of aprotease imbalance. Elevated proteolyticactivity is a feature of chronic wounds. Pro-teolytic matrix degradation products of fibro-nectin, vitronectin and tenascin-C have beendemonstrated in the fluid of chronic wounds.Chronic wound fluids contain significantlyhigher levels of collagenolytic activity than isfound in fluids from surgical wounds, healingopen dermal wounds or skin-graft donor sites(24). In particular, matrix metalloproteinaseMMP-2 and MMP-9 levels are higher inchronic wounds compared to surgical wounds(25). MMP-1 protein levels, as measured byenzyme-linked immunosorbent assay, werethreefold higher in chronic wounds than inhealing dermal wounds (24). However, it hasbeen shown that MMP-8 predominates interms of molar amounts of collagenase andactivity in chronic wounds (26).High levels of proteolytic enzymes, cytokines

and acute phase proteins have been found insuction-treated wound fluid and serum (27—29)following TNP treatment, although thesereduce sequentially over time. In particular,several groups have demonstrated a reductionin metalloproteinase expression following treat-ment (30—32). In an experimental rabbit model

of crush/ischaemia, TNP was shown to signifi-cantly reduce serum myoglobin over timewhen compared to controls (33). Removal oftissue fluid may also help reduce the systemicshock response in burns (34). In extravasationinjury following injection of chemotherapeuticagents, TNP has been shown to attenuate tissuedamage and physically remove the contrastagent from tissues (35).

Effect on cellular activityCellular components and debris may be iden-tified in the fluid removed by TNP or indeedin foam dressings. An interesting finding sup-ported by several groups of investigators isthat TNP has an effect on cellular inflamma-tion. TNP may reduce the inflammatory infil-trate in both acute (36,37) and chronic wounds(38). Interestingly, following treatment of burnwounds, this effect predominantly involvedneutrophil extravasation rather than otherinflammatory cell types (39).

Mechanical stressOne of the major effects on the wound occurssecondary to the mechanical stress mediatedat the foam—wound interface. Within a closedwound environment created by the hermeti-cally sealed drape, application of TNP imposesequally distributed mechanical forces acrossthe wound. These mechanical forces haveeffects on several levels although principallythought to be via mechanical stretch andfluid-shear stress (17).Mechanical stress has direct effects on cellu-

lar activity (40) and, in particular, angiogen-esis (41—43). Recent work has suggested thatgene activation is a critical precursor step inthe wound-healing process and an in vitro

modelling system of TNP therapy has demon-strated up-regulation of gene groups relatedto cell proliferation and tissue growth includ-ing (44) mitogen-activated protein kinases,CDK4, immediate early protein, interleukin(IL) enhancer-binding protein, IL-6, MMP-11and MMP-13. This mechanical transduction-related activation of specific genes has beensupported by others (45,46). Interestingly,however, mechanical stress also down-regulates certain other genes including theheat shock protein group and IL growth factors(44). Similar in vitro work using variableloading forces on collagen lattices has also

Key Points

. effect of TNP on oedema ispoorly understood

. again, effect of TNP on vascularbiology needs to be clarified

. some studies have indicated,however, that TNP increasesblood flow

. tests have indicated that thefoam may be crucial to the trans-mission of pressure

. protease imbalance is central tocurrent theories on acute andchronic wound healing

. some studies have shown TNP tobe effective in reducing MMPexpression



demonstrated increased fibroblast activity andcollagen turnover (47); a similar mechanism isimplicated following TNP application.

Growth factors and cytokine expressionfollowing TNP therapyOn a cellular product level, TNP has also beenshown to up-regulate growth factor expres-sion. Kopp et al. demonstrated a threefold tofourfold increase in transforming growthfactor-b-1 and vascular endothelial growthfactor expression and a 2�5-fold increase inplatelet-derived growth factor (PDGF) expres-sion (48), although previously other investiga-tors revealed no effect on insulin growthfactor-1 expression (49). Several groups havenow also shown a significant reduction ininflammatory cytokine profiles followingapplication of TNP (15,50).

Granulation tissue formationOn a macroscopic level, one of the mostobvious effects of TNP treatment is thegeneration of granulation tissue formation(Figure 4). Clinically, this has not yet beencorroborated by quantitative studies, althoughtwo studies have addressed this issue in

animal models. In an excisional full-thicknesswound model, alginate impressions weretaken on a daily basis following treatmentwith TNP (10). Volume displacement of thesecasts demonstrated that TNP-treated woundsincreased granulation tissue formation whencompared to controls by 63�3% and 103�4%(continuous and intermittent suction, respect-ively). This is a significant increase in granu-lation formation compared to conventionaldressings or compared with the use of growthfactor derivatives such as PDGF and basicfibroblast growth factor. A further variable ingranulation tissue formation appears to bepressure (51). Another group recently studiedthe skin-excised rabbit ear wound model and,using a lens micrometer, demonstrated a sig-nificant increase in granulation tissue forma-tion (52). In a randomised, controlled trial,Joseph et al. (53) also studied granulation tis-sue formation and made comment on newvessel growth and fibroblast morphology(which together with macrophages form thedominant constituents of granulation tissue),but no attempt to quantify this was made.

Reverse tissue expansionThe reverse tissue expansion effect (1) demon-strates the powerful way in which TNP util-ises the natural visco-elastic forces of theskin adjacent to open wounds (Figure 5). In aclosed system using an open-pore, reticulateddressing, a centripetal effect is exerted on theskin edges when a vacuum is applied. Ananalogy has been made to tissue expansionoccurring in a reverse fashion; the mechanicalstretch in this technique has been shown toincrease vascularity of the skin as well asmitotic activity (54,55).

Tissue salvageTNP appears to salvage compromised woundtissue in the zones of trauma. Experimentalmodels using random pattern skin flaps ofexcessive length to breadth ratio revealedthat those flaps treated with TNP both beforeand after raising of the flap survived signifi-cantly better than controls (10). Likewise, TNPapplied within 12 hours following a partial-thickness burn, inflicted upon a swine model,significantly reduced the depth of cell deathwithin the wound (36). Less inflammatorycells and cellular debris were also found inthe TNP-treated wounds. In an animal study

Key Points

. some groups found TNP to havean effect on cellular inflammation

. TNP therapy imposes equally dis-tributed mechanical forces acrossthe wound

. mechanical stress has directeffects on cellular activity and inparticular angiogenesis

Skin Foam/woundinterface

Wound bed

V.A.C.®PU Foam

Compressive loading

Wound bed

Islands ofgranulation

Tensileloading

Figure 4. Topical negative pressure stimulates granulation

tissue formation.



mimicking doxorubicin extravasation injury,modelled by injecting doxorubicin repeatedlyintradermally into pigs, TNP prevented chroniculcer formation in all treated wounds, whileulcers occurred in 10 of 16 control wounds (35).Tissue salvage is considered to be effected byremoving toxic (12,56) and pro-inflammatoryfactors, reducing oedema and inhibiting pro-gression to a deteriorating wound environment.

Bioburden effectsOne of the primary uses of TNP therapy isin the management of infected wounds.Fleischmann has been a major proponent ofhis vacuum-sealing technique for this purposeand has reported extensive series in both theEnglish and German literature (12). Microbio-logical and Gram staining of TNP fluid showslarge number of bacteria. Experimentally, asignificant reduction in the wound bioburdenhas been demonstrated: experimental woundsin swine innoculated with a human isolate ofS. aureus and a swine isolate of S. epidermidis

were subjected to treatment with either TNPor controlled moist saline dressings (n= 5).Daily biopsies were taken for a total of 2weeks. Analysis of incubated agar platesrevealed a reduction from 108 organisms to105 organisms between day 4 and 5 in TNP-treated wounds compared to a mean of 11days in control wounds (10). A diminishedwound bioburden has also been shown inhuman wounds (12,31), although there arereports of adverse effects in some instancesof wound flora (57). Nethertheless, TNP hasbeen used successfully to treat many differenttypes of infected wounds (29,58—61).

TNP has further become an effective carriersystem for the delivery of antibiotics withimproved healing rates over conventionalwound antisepsis/biosis (62). The FleischmanInstillation Technique uses an antibiotic deliv-ery system to wounds by utilising an adaptedTNP apparatus. The technique involves theprovision of negative pressure to a wound atnormal pressures (75—175mmHg). This suc-tion is interrupted at several points throughsuccessive 24-hour periods. At this stage, thevacuum line is blocked and in alternatingfashion antiseptic or antibiotic solutioninstilled in the line and the vacuum released.The solution remains in the wound for 30minutes and the vacuum then reestablished.This technique has lead to infected tissueundergoing definitive skin closure at day 7.The Fleischmann technique provides greaterantibiotic delivery to infected tissue and hasallowed the successful salvage of chronicosteomyelitis.

Moist wound environmentTNP maintains a moist wound environmentwhich provides optimal conditions for epithe-lialisation, prevention of tissue desiccation,angiogenesis and enhanced cellular meta-bolism (1). Furthermore, it also maintainswound temperatures in a normothermicfashion which is critical for optimal enzymaticprocesses (21).

CLINICAL INDICATIONSThe clinical indications for use of this therapyare constantly evolving. Similarly, as clinicalexperience increases and series reported, the

Key Points

. TNP has been shown to up-regulate growth factor expression

. one of the most obvious effectsof TNP treatment is the gener-ation of granulation tissue

. TNP utilises the natural visco-elastic forces of the skin

. TNP appears to salvage compro-mised wound tissue in the zonesof trauma

. one of the primary uses of TNPtherapy is the management ofinfected wounds

Dehiscedabdominal

wound

‘Crinkle’ effect

Centripetal forceson wound edges

(Creep, stress relaxation)

Delayedprimaryclosure

Figure 5. Reverse tissue expansion effect of topical negative pressure.



contraindication boundaries will also change.A brief overview of some of primary indica-tions for use are discussed (Table 1).

Acute woundsTNP is highly effective in the management oftrauma (2,11,12,23). It creates a sealed environ-ment, minimising contamination, maximisingoxygenation of tissues by improving localblood flow and stimulates granulation tissuevia mechanical stress. Exposed bone, ten-don and neurovascular bundles are rapidlyencroached upon and covered with granula-tion tissue (64). Dressings may be appliedimmediately following injury or debridement(63).Early wound debridement and coverage of

exposed bone with well-vascularised tissue,within 72 hours, remains the ‘gold standard’management of open fractures. However,TNP may permit temporisation of open frac-tures beyond 72 hours pending definitive softtissue cover and, in some cases, avoid morecomplex surgery (2). However, this practice ofdelayed soft tissue cover remains contro-versial and long-term studies are awaited.Complex soft tissue injuries in the absence ofexposed bone are ideally suited to treatment.This converts a technically challenging woundinto one that requires only split skin graft orlocal flap cover of open granulation tissue.Tissue loss from the foot, exposed tendons,tissue loss in gunshot wounds and deglov-ing injuries are good candidates (2,64—67).Prolonged TNP therapy with resultant down-staging of the wound also presents analternative to flap cover in patients withsignificant comorbidity, in situations wherethe necessary surgical skills are unavailableor where patient beliefs (e.g. Jehovah’sWitnesses) render major surgery potentiallylife threatening.

Burns may also be treated with TNP. In theacute phase, it is of particular benefit inunstable patients where definitive coveragemay be delayed and repeated dressingchanges interrupt intensive care therapy.Burn-wound progression described by Jackson(68) may also be attenuated (22,36).

Surgical Wounds

The open abdomenAlthough uncommon, the open abdomen isassociated with significant morbidity and amortality rate in excess of 25%. In addition,there are considerable socio-economic costsassociated with prolonged hospital admission.Long-term sequelae include enterocutaneousfistula formation, ventral hernia development,unstable wounds liable to breakdown andaesthetic considerations. They may arise dueto abdominal dehiscence or following a lapa-rostomy for blunt and penetrating trauma,infection, abdominal compartment syndromeor necrotising fasciitis.Therapeutic approaches to the open

abdomen by necessity initially control thewound, containing abdominal contents andpreventing infection, and must subsequentlypromote wound closure without hernia form-ation. TNP has revolutionised management(69—74). It obviates the need for deep tensionsutures or the use of prosthetic mesh, andunlike previous therapies, such as the BogotaBag, prosthetic mesh closure, Wittman patch orvacuum pack, TNP actively removes exudateand provides quantitative analysis of thirdspace fluid losses. It also facilitates fascialclosure with robust wound healing reducinghernia occurrence. In superficial wounds,dressing foam is applied directly to theintact fascia, while in deep wounds withexposed viscera, or complex cases where afistula is present, an interposed dressinginserted between foam and wound bed isrequired (2).

Cardiac wound infectionPoststernotomy infection remains as a signifi-cant cause of morbidity following elective car-diac surgery. A number of series support theuse of TNP with debridement in successfullymanaging deep infections, poststernotomymediastinitis and sternal osteomyelitis, either

Key Points

. TNP has also been adapted as asystem to deliver antibiotics(instillation therapy)

. TNP provides a moist woundenvironment

. clinical indications for TNP areconstantly evolving

. TNP has been shown to be effec-tive in the management of trauma

. TNP therapy has been shown topresent an alternate technique tosurgical flaps. This has beentermed ‘down-grading the recon-structive ladder’

. TNP therapy has improved themanagement of abdominal surg-ical wounds

. patients treated with TNP havebeen reported to require fewerdressing change and reducedhospital stays

Table 1 Clinical indications for use of TNP

Traumatic wounds

Open abdominal wounds

Infected sternotomy wounds

Wound bed preparation

Skin-graft fixation

Pressure sores

Complex diabetic wounds

Miscellaneous



eventual healing through secondary intent orwith delayed definitive flap closure (75—78).Patients treated with TNP are reported tohave significantly fewer dressing changes,fewer final flap procedures and reducedhospital stays as against those undergoingconventional treatment (79). Furthermore, thepartially collapsed foam of the TNP dressingacts to splint the two elements of the dividedsternum, improving ventilation and reducingpain. However, prolonged use of TNP insuch cases is not recommended and radicaldebridement of all non viable tissue and flapclosure should be performed if wounds fail torespond to therapy.

Skin-graft fixationTNP dressings provide firm fixation of a skingraft to its bed following the wound contour,eliminating shearing, removing fluid collec-tions and inhibiting infection. These are theoptimal conditions for graft-take, and graftsurvival rates of more than 90% are reportedusing this technique (80—85). TNP is also effect-ive in managing graft donor site complicationssuch as exposed tendons following free radialforearm flap harvest (86).

Chronic Wounds

Pressure ulcersPressure ulcers commonly present in agedand infirm patients and those sufferingfrom debilitating neurological disease. Insuch patients, significant comorbidity andassociated impaired wound healing oftenprecludes reconstructive surgery. TNP is aneffective, and safe, definitive treatment of pel-vic and trochanteric pressure ulcers. It deliversenhanced wound healing with less hospitaladmissions and expense when compared toprevious non surgical therapies (88—92). Twiceweekly dressing changes reduce patientdiscomfort and nursing demands (2) andencourage extended management within thecommunity. TNP is also valuable in optimisingulcer wound beds in those selected caseswhere surgery is planned (92).

Complex diabetic woundsAn increasing body of evidence is beingamassed that TNP is more effective than trad-

itional dressings in managing diabetic ulcers,in terms of nursing demands, costs, patientcomfort and in improving outcome (93—95)(Figure 6).

Vascular ulcersLarge venous ulcers will respond to TNP,although results vary and prolonged treat-ment is to be expected. TNP arguably has noimpact on arterial ulcers and those withsignificant arterial deficiency and persistentlocal ischaemia (93). Furthermore, the foamdressing may cause localised necrosis of anyunderlying skin unless surrounding flangedressings are used. Experimentally, ischaemichuman limbs show no improvement in skinmicrocirculation during or following negativepressure (115mmHg) therapy (96).

CONTRAINDICATIONSThere are no established absolute contraindi-cations to TNP. However, it should not beapplied to slough or necrotic tissue. The appli-cation of TNP over open joints is controversialand results may be varied. Application over atumour is theoretically hazardous, although itmay be considered useful in palliative woundcare. There is no contraindication to applica-tion over an oncologically safe resection site(1). Particular caution is recommended whenconsidering TNP in cases with coagulopathyand over an open peritoneal or pleural space.In the latter cases, interposed dressings aremandatory to minimise damage to underlyingstructures. Complications seen with this ther-apy are listed in Table 2.

Key Points

. TNP has been reported to providehealing in pressure ulcers

. evidence is mounting regardingits success in the treatment ofcomplex diabetic wounds

. large venous ulcers respond toTNP but results so far are varied,with prolonged use of TNP beingrequired

. TNP should not be applied toslough or necrotic tissue

. emergence of telemedicine inwound healing will also impactthe proliferation of TNP

. there is a continued need tostrengthen the evidence-base forwound treatment modalities,including TNP

Figure 6. Example of an anatomical dressing for use in

complex diabetic foot wounds.



FUTUREAdvances in technology have had a profoundimpact on TNP therapy, both in the deliveryof more powerful and compact mobile suctiondevices and in the provision of advancedmicroprocessor-driven systems with modemfacilities. These features may well be har-nessed in the future to incorporate telemedi-cine capabilities which is an expanding fieldof interest.The structure and composition of the TNP

foam dressing appears to influence woundbehaviour. A better understanding of foammaterial science and foam/wound interac-tions will lead to new materials for dressings.Impregnated foams containing growth factorsor antibacterial agents, e.g. silver, offer afurther attractive method of altering thewound environment.The novel field of combination therapy

involving TNP is only beginning to beexplored but also holds out the possibilityfor new treatments that include tissue engin-eering, direct delivery of drugs and growthfactors and micromanagement of the woundenvironment (2).Lastly, we should consider criticisms of the

therapy. Undoubtedly, this technique has hada significant impact on clinical wound-healingperceptions, but it should not be viewed as apanacea; as with the laparoscopic revolutionof the 1980s, there was initial enthusiasmfor all procedures to be performed endo-scopically. Subsequently, with extensive pub-lications and expert opinion, acceptedindications have been realised. A similar ana-logy can be used with TNP: it should not beused indiscriminately and it is inevitable thatclear patterns will emerge for use. Manyauthorities critique the fact that the level of

evidence for its use in clinical practice is low.Whilst we agree that further research isrequired to strengthen the level of evidence,the number of articles published in the litera-ture is enormous and underpins a genuineinterest to understand optimal use. We hopethat the results of multiple, large, on-goingrandomised controlled trials help clarify thesituation. As with all other wound-healingmodalities, there is a need for continuedresearch and an expanding evidence-base toallow clinicians to make informed choices foroptimal use of TNP therapy.

ACKNOWLEDGEMENTSWe are grateful to the Oxford Wound HealingSociety for permission to use Figures 2, 4 and 5.

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Table 2 Complications of TNP therapy

Dressing sensitivity

Pain

Pressure necrosis on skin

Haemorrhage

Haematoma

Infection

Toxic shock

Failure of wound to respond

Psychological intolerance



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Topical negative pressure therapy: mechanisms and indications