Purpose: Pressure ulcers at the ischial tuberosities are a significant cause of morbidity and mortality in persons with spinal cord injuries. Current prevention strategies are not fully adequate and surgical treatment is not always effective. In order to develop a novel method of ulcer prevention using magnetic repulsion, we needed to establish the relationship between pressure developed at the level of the ischial tuberosities while seated and the associated changes in blood flow in the sub-ischial tissues. Our hypothesis was that a threshold pressure or “pressure-prescription” would emerge as a target we could subsequently aim for with magnetic offloading.

Method: We performed a physiologic study comparing incremental pressure applied at the ischial tuberosities with alterations in the local cutaneous perfusion. Persons with spinal cord injuries were compared to uninjured controls in order to detect any significant differences in the pressure-perfusion relationship between these two groups. Subjects were progressively lowered onto a seat where perfusion was measured with laser Doppler perfusion imaging and pressure was recorded with a pressure mapping system. The mean perfusion vs. pressure curve was determined from a zero loaded position to a maximally loaded position.

Results: Healthy controls exhibited stable cutaneous ischial tissue perfusion up to sitting pressures of 150mm Hg followed by a 10% increase in blood flow at higher peak pressures. In contrast, subjects with spinal cord injuries underwent an early decrease in perfusion of 20% up to pressures of 150mm Hg, with a subsequent leveling off of their flow at higher pressures. The spinal cord injured also demonstrated lower reperfusion values indicative of a weaker reactive hyperemia in response to pressure.

Conclusion: Compared to uninjured people, individuals with spinal cord injuries appear to have inherent blood flow regulation differences in response to pressure at the sub-ischial tissues, possibly due to dysfunction of an autonomic phenomenon termed pressure-induced vasodilation. Furthermore, a threshold pressure for maintaining optimal perfusion remains elusive for both healthy subjects and persons with spinal injuries.

CLINICAL RELEVANCE: Targeting the altered blood flow response to pressure in patients with spinal cord injuries, pharmacologically or mechanically, may lead to a reduction in the incidence of decubital ulcers.