Wavelet analysis of skin blood oscillations in persons with spinal cord injury and able-bodied subjects

OBJECTIVE: To assess the blood oscillations in the skin over the ischial tuberosity (high-risk area for pressure ulcer) using spectral analysis of laser Doppler flowmetry signals based on wavelet transform. DESIGN: Wavelet analysis of skin blood oscillations in persons with spinal cord injury (SCI) and able-bodied subjects. SETTING: Seating and body support interface laboratory. PARTICIPANTS: Ten men were recruited for this study, of whom 5 were able-bodied subjects (age, 31.2+/-3.3 y) and 5 were persons with SCI (age, 37.2+/-7.3 y). INTERVENTIONS: External pressure of 16.0 kPa (120 mmHg) was applied to the ischial tuberosity via 1 specifically designed pneumatic indentor. The loading duration was 30 minutes. MAIN OUTCOME MEASURES: Skin blood flow was monitored for 10 minutes prior to loading and 20 minutes after the prescribed loading period. With spectral analysis based on wavelet transform, 5 frequency intervals were identified (.01-.02, .02-.06, .06-.15, .15-.40, .40-2.0 Hz) corresponding to endothelial related metabolic, neurogenic, myogenic, respiratory, and cardiac activities, respectively. RESULTS: The relative amplitude of the metabolic component for persons with SCI was significantly lower (F=5.26, P=.032) during the resting conditions as compared with able-bodied subjects. During the postloading period, the response of oscillatory activities was evidently lower in the skin over the ischial tuberosity for persons with SCI when compared with able-bodied subjects. In addition, the relative amplitude of the neurogenic component (.02-.06 Hz) during postloading was significantly lower for persons with SCI (F=5.44, P=.029). CONCLUSIONS: These findings suggest that the contributions of endothelial related metabolic and neurogenic activities to the blood perfusion regulation become relatively less for persons with SCI during the resting and postloading periods, respectively.