Dressings can prevent pressure ulcers: fact or fallacy? - Wounds ...

Clinical REVIEW8 Pressure duration over the pressurepoints which can result in damagefrom high pressure for short intenseperiods, which can be as damaging aslow pressure for prolonged periods(Bell, 2005)8 Collagen function protecting themicrocirculation helps to maintainthe pressures inside and outsidethe cells preventing cell bursting.Collagen levels vary from personto person with lessening protectivequalities with aging (Russell, 1998)8 Autoregulatory processes initiatedwhen external pressure is sensed,leading to increased internal capillarypressure, reduced blood flow andreactive hyperaemia to counteractthe pressure loading.These mechanisms can fail when theexternal pressure exceeds the person’sdiastolic pressure rather than the32mmHg often quoted (Nixon, 2001).The response of tissue to externalforces varies greatly, being dependenton a large number of factors. It istherefore not possible to establish a‘safe level of pressure’. In addition,tolerance to pressure can vary greatlyfrom individual to individual due tothe interplay of external factors listedin Table 1. Given the highly variablenature of pressure transmission, capillaryclosure and the individual’s normal andadaptive responses to pressure stress,the production of time/pressure curves(mathematical models for predicting thetime likely to cause tissue damage whentissue is exposed to specific levels ofpressure [Reswick and Rogers, 1976])may be of little practical benefit to theclinician in everyday practice (Sharp andMcLaws, 2005; Grefen, 2009).Shear is a mechanical stress appliedparallel to the skin. The SFI describes itas: ‘An action or stress resulting fromapplied forces which causes or tends tocause two contiguous internal parts ofthe body to deform in the transverseplane (i.e. “shear strain”)’ (SFI, 2006).This sliding or twisting force occurscontinuously within soft tissues evenwhen perpendicular pressure is applied,but increases greatly when combinedwith lateral movement, as seen when84 Wounds uk, 2009, Vol 5, No 4the body is adapting to the inclinationof the bed or when sitting in a chair. Ifthe skin adheres to the surface support(which is more likely if the skin is moistor wet from environmental factorsor intrinsically from incontinence orsweating) (Weir, 2007; Beldon, 2008), thetissues attached to the gradually movingskeletal frame become distorted which,in turn, distort the blood vessels leadingto their collapse or rupture.Shear forces are generated as aresult of the interplay of friction andpressure (Collier and Moore, 2006).When applied, shear increases theeffects of pressure resulting in vascularocclusion at only half the pressure ofnon-stressed tissues (Bennett and Lee,1986). Shear forces may also have asignificant role in the development ofdeep tissue damage, although this isdifficult to measure in the clinical setting(Russell, 1998). Potentially, shearing isthe most serious extrinsic risk factordue to the rapidity with which it canresult in tissue damage (Sharp andMcLaws, 2005). This is more likely tooccur in the elderly as a result of loose,fragile skin and the ease with which thedifferent tissue types can be sheared offtheir respective attachments (Allman etal, 1995).The edges of ulcers caused by shearforces appear to be ragged with moreuneven wound margins, often withsurrounding epidermal scuffing. Bruisingmay also be a feature (Figure 4).The mechanisms of shear damagehave important consequences for theplanning and delivery of preventativecare interventions, even though thereare few clinical methods to estimateshearing forces or their resultant effectson tissues (Verluysen, 1985). It is hopedthat the work of the SFI will add to thisbody of knowledge.Friction is a complex phenomenonwhich depends on complex physicalscience and engineering concepts. Insimplistic terms, within the context offriction-induced tissue damage, we arereferring to kinetic friction. Kinetic (ordynamic) friction occurs when twoobjects are moving relative to eachFigure 3. Sacral area showing clinical features ofblanching hyperaemia.Figure 4. Shear pressure damage occurring in ayoung woman during childbirth.other and rub together. Bergman-Evanset al (1994) define it as the resistanceto lateral movement. Kinetic friction isdependent on mass, force applied andthe friction co-efficients of the surfacesinvolved. Clinically, the effect of frictionbetween the skin and a support surfacehas important dynamics that can initiatepressure ulcer formation:8 It can cause excessive wear to thecornified layers of the skin withresultant exposure of the underlyingstructures (Read, 2001)8 It can cause the formation of blistersas separation occurs between thelayers of the epidermis leading to