Identifying critical DO₂ with compensatory reserve during simulated hemorrhage in humans

Background Based on previous experiments in nonhuman primates, we hypothesized that DO 2 crit in humans is 5–6 ml O 2 ·kg −1 min −1 . Study Design and Methods We measured the compensatory reserve (CRM) and calculated oxygen delivery (DO 2 ) 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, DO 2 and CRM at baseline. The CRM‐DO 2 relationship was best fitted to a logarithmic model in HT subjects (amalgamated R 2 = 0.971) and a second‐order polynomial model in the LT group (amalgamated R 2 = 0.991). Average DO 2 crit for the entire subject cohort was estimated at 5.3 ml O 2 ·kg −1 min −1 , but was ~14% lower in HT compared with LT subjects. The reduction in DO 2 from 40% CRM to 20% CRM was 2‐fold greater in the LT compared with the HT group. Conclusions Average DO 2 crit in humans is 5.3 ml O 2 ·kg −1 min −1 , but is ~14% lower in HT compared with LT subjects. The CRM‐DO 2 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 DO 2 from 40% CRM to 20% CRM for LT subjects is located on the steepest part of the CRM‐DO 2 relationship.