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

BACKGROUND: 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. METHODS: 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. RESULTS: 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). CONCLUSION: 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.