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 an aerobic metabolism and to compare its value between septic and nonsept ic critically ill patients. Design.-Cohort analytic study, consecutive sample. Setting.-Two tertiary care medical and surgical intensive car e units in university hospitals. Patients.-Nine septic and nine nonsep tic critically ill humans. A diagnosis of sepsis was established by th e presence of sepsis syndrome, positive cultures obtained within 48 ho urs of study, and autopsy evidence of a source of infection. Methods a nd Interventions.-The O2 consumption (determined by indirect calorimet ry), O2 delivery (calculated from the Fick equation), and concentratio n of arterial plasma lactate were simultaneously determined at 5- to 2 0-minute intervals while life support was discontinued. Main Outcome M easures.-Critical O2 delivery, critical O2 extraction ratio, and maxim al O2 extraction ratio. Results.-In all septic and eight nonseptic pat ients, O2 delivery and O2 consumption displayed a biphasic relationshi p 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>.6 4), 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 bet ween the septic and nonseptic groups. Conclusions.-The critical O2 del ivery for anaerobic metabolism was identified from the biphasic relati onship between O2 delivery and O2 consumption in individual humans. Th e 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.