Thermodilution technique for measurement of cardiac output during artificial ventilation

The feasibility of using the thermodilution method to monitor cardiac output during artificial ventilation was studied in anesthetized pigs. Normal saline (0.5 ml) at room temperature was injected into the left ventricle or the right atrium. The dilution curves were detected in the aortic arch and the pulmonary artery, respectively. The ventilation rate was 10 cycles/min at end-expiratory pressures of 0, 5, 10, and 15 cmH2O. For each level, 50 measurements of cardiac output were performed at regular intervals over the ventilatory cycle. The order of measurements were randomly selected. The average of each series of 50 measurements showed excellent correlation with the estimates of cardiac output based on the direct Fick method for oxygen. The maximum difference between the values of cardiac output randomly measured by the thermodilution method was 40% for the left side of the heart and 70% for the right side. However, when the values of cardiac output were sorted according to the specific phases of the respiratory cycle, there was a systematic variation with a small random error. For the left side of the heart, a satisfactory moment of injection for estimation of mean cardiac output appeared to be at the end of the spontaneous expiration. On the other hand, the analysis of cardiac output values at the right side did not reveal any satisfactory moment for injectate administration under changing circumstances, e.g., positive end-expiratory pressure.