Lack of agreement between thermodilution and carbon dioxide-rebreathing cardiac output

A continuous, accurate, non-invasive monitor of cardiac output would represent a major step forward in patient management. A cardiac output computer, NICO2, based on the Fick principle and an automatic partial carbon dioxide (CO2)-rebreathing technique has just become available. We compared the performance of this monitor with the standard thermodilution method.

Thirty patients were investigated after cardiac surgery. Replicate measurements were performed simultaneously with the thermodilution and NICO2 techniques. An Altman-Bland analysis was used to assess repeatability of each of the two methods and to determine the agreement between the two techniques.

The repeatabilities of thermodilution and CO2-rebreathing cardiac output were excellent, with coefficients of repeatability of 0.35 l/min and 0.60 l/min. Mean thermodilution and NICO2 cardiac output were 4.4 l/min (SD 0.9, range 2.7-6.1) and 4.6 l/min (SD 1.3, range 1.6-6.9). A comparison of the methods, however, revealed excessive limits of agreement (+/-1.80 l/min).

The agreement between the NICO2 derived cardiac output and the de facto standard - thermodilution cardiac output - is poor. The methods are not interchangeable with the present version of the NICO2. The repeatability of the partial CO2-rebreathing technique holds promise that a sufficient accuracy may be obtained by suitable modifications of the monitor's algorithms.