IDENTIFICATION OF THE CRITICAL OXYGEN DELIVERY FOR ANAEROBIC METABOLISM IN CRITICALLY ILL SEPTIC AND NONSEPTIC HUMANS

Objectives.-To determine the critical oxygen delivery threshold for anaerobic metabolism and to compare its value between septic and nonseptic critically ill patients. Design.-Cohort analytic study, consecutive sample. Setting.-Two tertiary care medical and surgical intensive care units in university hospitals. Patients.-Nine septic and nine nonseptic critically ill humans. A diagnosis of sepsis was established by the presence of sepsis syndrome, positive cultures obtained within 48 hours of study, and autopsy evidence of a source of infection. Methods and Interventions.-The O2 consumption (determined by indirect calorimetry), O2 delivery (calculated from the Fick equation), and concentration of arterial plasma lactate were simultaneously determined at 5- to 20-minute intervals while life support was discontinued. Main Outcome Measures.-Critical O2 delivery, critical O2 extraction ratio, and maximal O2 extraction ratio. Results.-In all septic and eight nonseptic patients, O2 delivery and O2 consumption displayed a biphasic relationship over the range of O2 delivery studied. There were no differences in critical O2 delivery threshold (3.8+/-1.5 vs 4.5+/-1.3 mL.min-1.kg-1; P>.28), Critical O2 extraction ratio (0.61+/-0.05 vs 0.59+/-0.16; P>.64), and maximal O2 extraction ratio (0.74+/-0.08 vs 0.80+/-0.11; P>.29) between septic and nonseptic patients. These data have greater than 90% power to detect a difference of 2 mL.min-1.kg-1 in the critical O2 delivery and 0.1 in the critical and maximal O2 extraction ratios between the septic and nonseptic groups. Conclusions.-The critical O2 delivery for anaerobic metabolism was identified from the biphasic relationship between O2 delivery and O2 consumption in individual humans. The critical O2 delivery is considerably lower than previously reported in humans with the use of pooled group data. Sepsis does not alter the critical O2 delivery for anaerobic metabolism or tissue O2 extraction ability. Interventions to increase O2 delivery to supranormal levels in critically ill humans in the hope of increasing O2 consumption may be inappropriate.