Objective: To compare measurement of cardiac output by the CO2 rebreathing method vs. the thermodilution cardiac output technique in the setting of acute respiratory failure in patients with chronic obstructive pulmonary disease.
Design: Prospective, comparative study of two methods in a consecutive sample.
Setting: Intensive care unit.
Patients: Twenty-five patients with chronic obstructive pulmonary disease with acute respiratory failure were studied. The patients were being mechanically ventilated and monitored with systemic and pulmonary artery catheters.
Measurements and main results: Cardiac output was determined, using both the thermodilution technique and an indirect CO2 Fick method. Veno-arterial CO2 content difference was calculated from an estimated mixed venous PCO2 obtained by an equilibrium CO2 rebreathing method and measured PaCO2. PCO2 was converted to content using the equation of the CO2 dissociation curve described by McHardy. A wide range of cardiac output was studied. There was a significant correlation between thermodilution and CO2 rebreathing methods (r2 = .92, p < .001). The mean difference between thermodilution and CO2 rebreathing methods was -0.06 L/min/m2, standard deviation for the bias was 0.028 L/min/m2, and 95% confidence interval for the bias was -0.120 to -0.001 L/min/m2.
Conclusion: Our results suggest that the CO2 rebreathing method may be a reliable non-invasive technique to determine cardiac output in mechanically ventilated patients with chronic obstructive pulmonary disease.