Measurement of cardiac output in small animals by aortic thermodilution.

The accuracy of cardiac output determination in small animals by aortic thermodilution was assessed. In eight Sprague-Dawley rats a thermistor probe was inserted into the ascending aorta via the right carotid artery, and saline indicator (0.04 ml) at room temperature was injected into the right atrium. The mean cardiac index was 282.7 ± 35.2 ml/min/kg (mean ± SD). To verify technical accuracy, in vitro models were created using glass or plastic tubing in a water bath. The flow measured by thermodilution correlated well with the actual flow (r = 0.989 at 0–200 ml/min; r = 0.971 at 200–500 ml/min). To further assess the accuracy of aortic thermodilution, we compared simultaneous measurements of cardiac output in New Zealand white rabbits using the aortic thermistor method vs a Swan-Ganz catheter. The systemic flows measured by the two techniques were within ±15%, with correlations of 0.949 for 0.5 ml indicator and 0.956 for 1.0 ml indicator at 0–4°C. The measurement of cardiac output in small animals by thermodilution with an aortic thermistor is simple and accurate. This technique appears suitable for use in the study of shock in small animals.