Identifying critical DO2 with compensatory reserve during simulated hemorrhage in humans

Background Based on previous experiments in nonhuman primates, we hypothesized that DO2crit in humans is 5–6 ml O2·kg−1 min−1. Study Design and Methods We measured the compensatory reserve (CRM) and calculated oxygen delivery (DO2) in 166 healthy, normotensive, nonsmoking subjects (97 males, 69 females) during progressive central hypovolemia induced by lower body negative pressure as a model of ongoing hemorrhage. Subjects were classified as having either high tolerance (HT; N = 111) or low tolerance (LT; N = 55) to central hypovolemia. Results HT and LT groups were matched for age, weight, BMI, and vital signs, DO2 and CRM at baseline. The CRM‐DO2 relationship was best fitted to a logarithmic model in HT subjects (amalgamated R2 = 0.971) and a second‐order polynomial model in the LT group (amalgamated R2 = 0.991). Average DO2crit for the entire subject cohort was estimated at 5.3 ml O2·kg−1 min−1, but was ~14% lower in HT compared with LT subjects. The reduction in DO2 from 40% CRM to 20% CRM was 2‐fold greater in the LT compared with the HT group. Conclusions Average DO2crit in humans is 5.3 ml O2·kg−1 min−1, but is ~14% lower in HT compared with LT subjects. The CRM‐DO2 relationship is curvilinear in humans, and different when comparing HT and LT individuals. The threshold for an emergent monitoring signal should be recalibrated from 30% to 40% CRM given that the decline in DO2 from 40% CRM to 20% CRM for LT subjects is located on the steepest part of the CRM‐DO2 relationship.