Effect of respiratory cycle on measurements of cardiac output by thermodilution

Current recommendations for obtaining reproducible measurements of cardiac output by thermodilution are contradictory. Several investigators have shown that reproducibility of measurements can be improved by initiating injections at specific moments in the respiratory cycle. We prospectively studied 31 patients, comparing three thermodilution measurements of cardiac output at peak-inspiration, end-exhalation and random in mechanically ventilated patients. Patients were separated into two groups: group 1 was composed of intubated patients on intermittent mandatory ventilation (IMV) without spontaneous overbreathing, and group 2 included patients on IMV with spontaneous overbreathing. The results from this study demonstrated no statistical difference in the ranges or standard deviations of the measurements of cardiac output among injections initiated at random, peak-inspiration or end-exhalation in either group. We conclude that the reproducibility of measurements cannot be enhanced by initiating injections at specific moments in the respiratory cycle.