Comparison of a modified Fick method with thermodilution for determining cardiac output in critically ill patients on mechanical ventilation.

We compared a modified Fick method for measuring cardiac output against the thermodilution method in 11 critically-ill patients on mechanical ventilation. Oxygen consumption (VO2) was calculated indirectly, by measuring the carbon dioxide elimination (VCO2) during steady state and by assuming an average respiratory quotient of 0.9. For a total of 129 measurements, the mean difference in cardiac output between both methods was 0.03 +/- 1.3 l/min (95% confidence interval, -0.19 to 0.25 l/min, p = 0.7) standard deviation, with the largest differences being measured in the low cardiac output range (less than 5 l/min). No statistically significant difference was found between the cardiac output values obtained with either method. These data showed a good correlation between the two methods and suggest that the modified Fick method may be useful in determining cardiac output in seriously ill patients on mechanical ventilation not requiring pulmonary arterial catheterisation, or where facilities for undertaking metabolic measurements are not available.