Lengthy surgeries often expose bony prominences to loading conditions associated with high risk of pressure injuries. Of 1.6 million pressure injuries that develop in acute care settings annually, roughly 25% are acquired intra-operatively during surgeries that last more than three hours.1 Prolonged ischemia may be one of the factors increasing risk.2 Alternating pressure (AP) has been shown to increase skin blood flow (SBF).3 This study compared the response of sacral skin blood flow on a foam operating room (OR) pad with and without an AP overlay. An experimental, crossover research design was conducted in the laboratory with and without an AP overlay on a foam OR pad. Healthy participants (n=10) with mean age 27.5±5.7 years and body mass index (BMI) 26.8±7.0 kg/m2 laid supine for sixty minutes in each condition while sacral SBF data was collected using a laser Doppler optic probe. Mean SBF measurements were tested for significant differences between conditions. The loaded SBF data was normalized to the mean baseline SBF. The ratio of the mean normalized SBF of the last 10 minutes to the mean SBF of the first 10 minutes represented the SBF response to each test condition. The difference in this measure between test conditions quantified the relative effectiveness. Post hoc analyses examined the relationships between the relative effectiveness and BMI and age. Mean SBF was not significantly different between test conditions (p=0.53). Participant BMI had a strong negative correlation to difference in SBF response between conditions (r = -0.89, p<0.001). Relative blood flow increase with the AP overlay was higher for participants with lower BMI. The relationship between SBF response and age was weak (r = 0.11, p=0.74). The AP overlay was more effective at increasing sacral SBF over time than the operating room pad alone in participants with normal BMI (< 25).