Wavelet analysis of sacral tissue oxygenation oscillations by near-infrared spectroscopy in persons with spinal cord injury

The objective of this study is to assess the blood oxygenation oscillations in the tissue over the sacrum, a high risk area for pressure ulcer, using spectral analysis of near-infrared spectroscopy (NIRS) signals based on wavelet transform. A total of twenty subjects were recruited for this study, of which ten were persons with spinal cord injury (SCI) (3 women, 7 men, age=34.5±5.2years) and the other ten were healthy subjects (3 women, 7 men, age=36.5±6.2years). Waterlow Scale was used for the pressure ulcer risk assessment. External pressure of 26.6kPa (200mm Hg) was applied to the sacrum via a specifically designed indentor. The loading duration was 3min. The subjects were examined lying face-down. Tissue oxygenation signal was monitored for 20min prior to and after the loading period from the tissue over the sacrum area using NIRS. With spectral analysis based on wavelet transform, five frequency intervals were identified (I, 0.005-0.02Hz, II, 0.02-0.06Hz, III, 0.06-0.15Hz, IV, 0.15-0.40Hz and V, 0.40-2.0Hz) corresponding to endothelial related metabolic, neurogenic, myogenic, respiratory and cardiac activities, respectively. The results showed that the amplitude of the [HbO2] and [Hb] component in intervals I, II and III for persons with SCI was significantly lower during the resting conditions than that for normal subjects (p<0.05). During the post-loading period, the response of [HbO2] and [Hb] oscillatory activities in intervals II and III were significantly lower in the tissue over the sacrum for persons with SCI than that for normal subjects (p<0.05). Also significant negative correlation was found between oscillatory activities and Waterlow scale in persons with SCI. The results indicated that the low oscillatory activities might be related to early tissue injury in persons with SCI.