Assessing Skin Blood Flow and Interface Pressure in Patients With Spinal Cord Injury Provided an Alternating Pressure Overlay: A Cross-sectional Study

The effects of an alternating pressure (AP) overlay on the skin are not fully understood. Purpose This study was conducted among persons with spinal cord injury (SCI) to examine skin blood flow (SBF) and interface pressure (IP) during and after AP overlay use. Methods In this cross-sectional, repeated measures study, persons eligible for participation were clinic outpatients from a large metropolitan area in the midwest United States who were 18 to 65 years old with a SCI with a neurologic level of injury at T10 or above for more than 1 year and used a wheelchair for primary mobility. Persons with a current pressure injury, diabetes mellitus, and/or hypertension or other vascular or pulmonary diseases were excluded. Data regarding age, gender, body mass index (BMI), duration of SCI, and American Spinal Injury Association Impairment Scale scores were collected. The experimental study involved 3 protocols: the AP protocol (participants lay supine for 40 minutes on an operating room [OR] pad with a low-profile AP that used a 10-minute inflation-deflation cycle); the post-AP protocol (participants lay on the 2-inch foam OR pad for 40 minutes), with 30 minutes of rest in between; and the control protocol, comprised of 40-minutes of laying supine on the OR pad. Each participant served as his/her own control. Outcome variables included 1) peak IP (the highest value among adjoining sensors located at the highest pressure point); 2) averaged IP (the averaged value of the sensors), calculated from pressure mapping system data from the sacrum and left heel; and SBF, measured using a laser Doppler flowmetry system. Descriptive analyses were performed for all variables to determine need for parametric or nonparametric analyses. The mean value of peak IP, averaged IP among inflation and deflation cycles of AP, and post-AP and control protocols were compared using repeated measures analysis of variance (ANOVA). Mean SBF among inflation and deflation cycles of AP and post-AP and control protocols were compared using the nonparametric Friedman test, and Wilcoxon signed rank tests were used to compare the SBF responses during the post-loading period. If the results of repeated measures ANOVA or Friedman tests were statistically significant, paired t tests and Wilcoxon signed rank tests were used for pairwise comparison with Bonferroni correction at alpha level 0.0125, respectively. Results Among the 15 participants (11 men, 4 women; age 41.77 ± 14.49 [range 20-62] years; BMI 26.81 ± 4.13 [range 22-37]; injury duration 17 ± 14.62 [range 1-48] years; mostly (11) African American), peak IP decreased during the AP deflation at sacrum (51.47 ± 30.18 mm Hg vs. 114.13 ± 60.97 mm Hg; P = .002) and heel (26.79 ± 12.91 mm Hg vs. 53.05 ± 18.22 mm Hg; P = 0 .001), and SBF increased at the heel (27.92 ± 32.15 vs. 10.43 ± 11.16 au; P = .006) but was not significant at the sacrum (15.54 ± 15.33 au vs. 11.96 ± 10.26 au, P = .023). Peak IP decreased during post-AP at the sacrum (104.62 ± 58.17 mm Hg; P = .002) but not at the heel (47.69 ± 16.21 mm Hg; P = .097). SBF increased during post-AP at the sacrum (15.78 ± 15.82 au; P = .012) but not at the heel (16.31 ± 29.18 au, P = .427). Conclusion An AP overlay redistributed IP and increased SBF at the sacrum and heel during use, and its effect 40 minutes after removal was observed only at the sacrum. Studies, including evaluating the lasting effect of AP on weight-bearing tissue protection at different anatomical locations, are needed.