Answer:
The pressure distribution in a sitting patient is much more concentrated than one lying supine. Pressure over the ischial tuberosities is around 130 mm Hg sitting and 60 mm supine. Wheelchair patients are usually not hospitalized, and are thus usually unsupervised. To diminish pressure, gel flotation pads have been devised. They may be made of silastic, silicone, or polyvinyl chloride. The concept of the gel is to disperse the pressure over a larger surface area. Kosiak has studied pressures on these pads and found >75mm Hg over the ischial tuberosities. This is compared to the capillary perfusion pressure of 32 mm Hg. Thus if patients do not lift themselves every 10 minutes, pressure ischemia will result.