Background: The feasibility of thermodilution cardiac output measurements with the more convenient room temperature thermal indicator instead of cold injectates has been repeatedly investigated. However, the issue has not been addressed with the appropriate statistical approach advocated by Altman and Bland. Furthermore, we wished to determine if the incorporation of a second thermistor in the thermodilution catheter, to measure the temperature of the thermal indicator where it is delivered into the right atrium/superior caval vein, would result in more precise cardiac output measurements.
Methods: Fifty patients were randomized to receive a single or dual thermistor pulmonary artery thermodilution catheter. Cardiac output was calculated as the average of four injections of 10 ml of isotonic saline. Precision (2 x SD of differences in replicate measurements) for the two catheters and injectate temperatures, and bias and limits of agreement between measurements, with cold and room temperature injectates, were determined.
Results: Precision was (0 degrees C) 0.42 l/min and (20 degrees C) 0.90 l/min, and bias and limits of agreement -0.83 l/min and -1.93-0.27 l/min for the single thermistor catheter. For the dual thermistor system precision was (0 degrees C) 0.34 l/min and (20 degrees C) 0.58 l/min. Bias and limits of agreement were -0.03 l/min and -0.61-0.55 l/min.
Conclusion: The second thermistor is redundant if cold injectates are used. If one wishes to use room temperature injectates the single thermistor system is inadequate. A dual thermistor catheter is, on the other hand, acceptable.