Identifying Critical DO with Compensatory Reserve During Simulated Hemorrhage in Humans

Background: Based on previous experiments in nonhuman primates, we hypothesized that DO crit in humans is 5-6 ml O ·kg  min .

Study Design And Methods: We measured the compensatory reserve (CRM) and calculated oxygen delivery (DO ) 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 and CRM at baseline. The CRM-DO relationship was best fitted to a logarithmic model in HT subjects (amalgamated R  = 0.971) and a second-order polynomial model in the LT group (amalgamated R  = 0.991). Average DO crit for the entire subject cohort was estimated at 5.3 ml O ·kg  min , but was ~14% lower in HT compared with LT subjects. The reduction in DO from 40% CRM to 20% CRM was 2-fold greater in the LT compared with the HT group.

Conclusions: Average DO crit in humans is 5.3 ml O ·kg  min , but is ~14% lower in HT compared with LT subjects. The CRM-DO 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 from 40% CRM to 20% CRM for LT subjects is located on the steepest part of the CRM-DO relationship.