Identification of the Critical Oxygen Delivery for Anaerobic Metabolism in Critically III Septic and Nonseptic Humans

Abstract

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 O₂consumption (determined by indirect calorimetry), O₂delivery (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 O₂delivery, critical O₂extraction ratio, and maximal O₂extraction ratio. Results. —In all septic and eight nonseptic patients, O₂delivery and O₂consumption displayed a biphasic relationship over the range of O₂delivery studied. There were no differences in critical O₂delivery threshold (3.8±1.5 vs 4.5±1.3 mL·min^-1·kg^-1;P>.28), critical O₂extraction ratio (0.61 ±0.05 vs 0.59±0.16;P>.64), and maximal O₂extraction 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^-1in the critical O₂delivery and 0.1 in the critical and maximal O₂extraction ratios between the septic and nonseptic groups. Conclusions. —The critical O₂delivery for anaerobic metabolism was identified from the biphasic relationship between O₂delivery and O₂consumption in individual humans. The critical O₂delivery is considerably lower than previously reported in humans with the use of pooled group data. Sepsis does not alter the critical O₂delivery for anaerobic metabolism or tissue O₂extraction ability. Interventions to increase O₂delivery to supranormal levels in critically ill humans in the hope of increasing O₂consumption may be inappropriate. (JAMA. 1993;270:1724-1730)