MEASURING CARDIAC-OUTPUT IN CRITICALLY ILL PATIENTS - DISAGREEMENT BETWEEN THERMODILUTION-BASED, CALCULATED-BASED, EXPIRED GAS-BASED, AND OXYGEN CONSUMPTION-BASED METHODS

Calculated values of oxygen consumption have been used to calculate a Fick cardiac output when thermodilution measurements are unreliable an d when oxygen consumption measurements are unavailable. To determine t he accuracy of these calculations, we measured cardiac output in 20 pa tients by four methods: (1) a reference Fick cardiac output calculated from metabolic oxygen consumption measurements and arterial-venous ox ygen content difference (COmet); (2) thermodilution cardiac output (CO therm), (3) an estimated Fick cardiac output based on calculated oxyge n consumption using standard equations (COcalc), and (4) an estimated Fick cardiac output using a bedside measurement of expired carbon diox ide production (COexp). The mean difference +/-95% limits of agreement between COtherm and COmet was 1.71+/-5 liters/min. The mean differenc e between COcalc and COmet was -0.04+/-3.33 liters/min. The mean diffe rence between COexp and COmet was 0.31+/-3.01 liters/min. On the basis of these wide confidence intervals, we conclude that (1) thermodiluti on and metabolic measurements of cardiac output frequently differ in c ritically ill patients, and (2) estimates of oxygen consumption, based on either standard equations or on expired carbon dioxide production measurements, are poor substitutes for metabolic measurements of oxyge n consumption in critically ill subjects and may provide inaccurate es timates of cardiac output.