VALIDATION OF AN INDIRECT CALORIMETER TO MEASURE OXYGEN-CONSUMPTION IN CRITICALLY ILL PATIENTS

We tested a system of indirect calorimetry (Deltatrac Metabolic Monitor) for accuracy to measure oxygen consumption and sensitivity to detect small changes in oxygen consumption, in vitro, using a lung model simulation of ventilating patients who have stiff lungs. In vivo, we assessed reproducibility of the system to measure oxygen consumption in patients who had adult respiratory distress syndrome (ARDS) or sepsis. In vitro, oxygen consumptions of 100, 200, 300, and 500 mL/min were simulated by injecting N2 into the lung model at 3-minute ventilations (10, 15, and 20 L/min) and at four levels of FIO2 (0.40, 0.60, 0.70, and 0.80). After each baseline measurement, N2 flow was increased to simulate an increase in VO2 of 10% to 15%. At FIO2 of 0.6 and 0.8, 15 cm H2O of positive end-expiratory pressure (PEEP) was applied to the lung model and measurements repeated. The Deltatrac had an average error of -2.1% (range, -7% to 3%). After simulated increases in oxygen consumption, the Deltatrac detected the change with an error less than 1% in all combinations tested. Although adding PEEP to the lung model altered accuracy of measurement, error was always less than 5%. We measured oxygen consumption in 27 ventilated and sedated patients who had severe ARDS or sepsis (FIO2, 0.60 ± 0.1; PEEP, 9 ± 3 cm H2O; static compliance, 27 ± 12 mL/cm H2O [mean ± SD]). Coefficient of variation for oxygen consumption was 4.6% ± 1.6%. Even at FIO2, 0.80 coefficient of variation was less than 6%. We conclude that this system of indirect calorimetry is accurate, sensitive, and reproducible to measure VO2 in critically ill patients. © 1991.