The impaired response of spinal cord injured individuals to repeated surface pressure loads

Three cyclic pressure loads (10 minutes "on" and 5 minutes "off") of 30 or 75mmHg were applied to the flat anterior surface of the tibia on 11 able bodied (AB) and nine spinal cord injured (SCI) subjects. Transcutaneous PO₂ (TcPO₂) and PCO₂ (TcPCO₂), laser Doppler (LD) skin blood flow, velocity, and volume, and the depth of leg indentation (LI) were recorded. In the AB subjects the mean TcPO₂ decreased from 31.5 to 27.2mmHg with the 30mmHg load, In contrast for the SCI subjects TcPO₂ decreased from 32.3 to 7.3mmHg (significantly different from AB group at p < 0.05). Means of the LD values for volume, velocity, and flow were normalized, giving the highest value of each of the parameters including both the AB and SCI subject during the entire time period a value of 100%. At 30mmHg loading, the laser LD normalized volume in the AB subjects were 72.5% and 97.0% for the on and off periods, respectively, whereas for the SCI subjects the values were 43.7% and 69.7% for the on and off periods, respectively (significantly different from the AB group at p < 0.05). At 75mmHg loading, the volume values for the AB subjects were 59.1% and 95.3% for the on and off periods, respectively, whereas for the SCI subjects, the values were 31.6% and 65.4% for the on and off periods, respectively (significantly different from the AB group at p < 0.05). The mean off values for the LI after the 30mmHg load were 2.7 and 5.8mm for the AB and SCI subjects, respectively, whereas after the 75mmHg load the values were 4.4 and 6.6mm for the AB and SCI subjects, respectively (at both loads the AB and SCI groups were significantly different p < 0.05). The combination of the TcPO₂, LD volume, and LI data support the hypothesis that the SCI subjects have a reduced vascular response to loading and thus may be more susceptible to develop pressure sores for physiological reasons. The greatest difference is in the 30mmHg load range where autoregulation can be observed.