Pressure ulcers result from increased pressure on the skin
and subcutaneous tissues, which compromises blood flow
and renders the tissues ischaemic (Colin et al. 1996). Oxygen
delivery declines when tissue pressure exceeds the capillaryclosing pressure. Estimates of capillary pressures usually
range from 10 to 30 mmHg (Guyton & Hall 2000), but may
be lower for individuals in poor health (Dealey 1995). Thus,
when the interface pressure between the skin and supporting
surface exceeds capillary closing pressure, commonly assumed to be at 32 mmHg (Landis 1930, Bouten et al. 2003,
Swain 2005), tissue hypoperfusion occurs (Reddy 1990,
Rithalia & Gonsalkorale 1998, Gebhardt 2005). The longer
that pressure exceeds the local capillary closing pressure in
one area, the more likely tissue injury and a subsequent
pressure ulcer will result