Wavelet analysis of sacral skin blood flow oscillations to assess soft tissue viability in older adults

The objective of this study was to investigate age-related changes of microvascular functions using wavelet analysis of blood flow oscillations (BFO), and to develop a new quantification method to present wavelet coefficients for studying the contribution of each blood flow control mechanism to the skin vasodilatory response. 20 older people between 65 and 85 years of age were recruited into the study. The heater temperature at the sacral skin was raised to 42 degrees C in 2 min and maintained at that temperature for the duration of the 50 minute heating period to induce a biphasic vasodilatory response: axon reflex mediated and nitric oxide mediated vasodilation. Wavelet-based spectrum analysis was used to calculate sacral skin blood flow and frequency bands including metabolic (0.008-0.02 Hz), neurogenic (0.02-0.05 Hz), myogenic (0.05-0.15 Hz), respiratory (0.15-0.4 Hz), and cardiac (0.4-2.0 Hz) frequencies. Biphasic Thermal Index is defined as ratios of first peak, nadir, and second peak to baseline blood flow. The results showed that older people have a biphasic blood flow response with lower vasodilatory function. Increased age is associated with an increase in baseline blood flow and a decrease of skin vasodilatory function (p<0.05). Biphasic Thermal Index is (5.5, 4.5, 10.1) for the 65-75 years group and is (3.7, 3.2, 6.7) for the 75-85 years group. During local heating, the metabolic frequency appears to become the most dominant frequency in power during the second peak, and the power of neurogenic frequency does not seem to significantly increase during the axon reflex mediated vasodilation. Wavelet coefficients averaged during the one-minute period are able to characterize time-series changes of frequencies. This study has shown the feasibility of using wavelet-based spectrum analysis of skin blood flow oscillations to assess skin microvascular function for detection of the specific conditions which constitute the highest risk factors for pressure ulcers among older people. Aging is associated with a decrease of skin blood flow function, which results in increased risk for ischemic damage and pressure ulcers.