VENTILATION PERFUSION INDEXES DO NOT CORRELATE WITH THE DIFFERENCE BETWEEN OXYGEN-CONSUMPTION MEASURED BY THE FICK PRINCIPLE AND METABOLIC MONITORING SYSTEMS IN CRITICALLY ILL PATIENTS

Objective: To determine whether the difference between oxygen consumption (Vo2) measured by metabolic gas monitoring systems and by the Fick principle is related to venous admixture, deadspace/tidal volume ratio, or alveolar-arterial oxygen tension gradient in critically ill patients. Design: A prospective study. Setting: An 11-bed general ICU in a 900-bed teaching hospital. Patients: Twenty critically ill patients admitted to the ICU who required mechanical ventilation, right heart catheterization, and arterial and mixed venous gas measurements for normal clinical management. Results: Thirty-three recordings were analyzed. The mean Vo2 measured by the metabolic gas monitoring system was 308 +/- 63.9 (SD) mL/min and was significantly greater than the mean Vo2 measured by the Fick principle of 284 +/- 72.0 mL/min. The difference between the two measurements of 24.3 +/- 47.6 mL/min correlated poorly with venous admixture (r2 = .0009), dead-space/tidal volume ratio (r2 = .0064) and alveolar-arterial oxygen tension gradient (r2 = .017). Conclusions: If the difference in Vo2 measured by metabolic gas monitoring systems and the Fick principle is due to intrapulmonary Vo2 then in critically ill patients the ventilation/perfusion indices of venous admixture, deadspace/tidal volume ratio and alveolar-arterial oxygen tension gradient correlate poorly with intrapulmonary Vo2.