Is supra-ventricular arrhythmia a reason for the bad performance of the FlowTrac device?

In their recent paper evaluating arterial waveform analysis as a tool to measure cardiac output (CO), Michael Sander and colleagues do not provide data on the heart rhythms of their cardiac surgery patients [1]. As the FloTrac (Flowtrac/Vigileo, Edwards Lifescience, Munich, Germany) device calculates CO from an arterial pressure-based algorithm, integrating vessel compliance and peripheral resistance effects, it seems plausible that these measurements may be influenced by cardiac arrhythmia. We recently treated a septic patient with atrial fibrillation who, in addition to monitoring with the FlowTrac device, received a pulmonary artery catheter because of a suspicion of right ventricular failure. The patient was on pressure-controlled mechanical ventilation. We found no significant correlation between simultaneous measurements performed with the pulmonary artery catheter and measurements performed with the FlowTrac device (r = 0.297, P = 0.405). Bland-Altman analysis showed a mean bias of -0.43 l/min and limits of agreement of -4.5 and 3.6 l/min (Figure ​(Figure1).1). This finding is in keeping with the results of a pilot study assessing the FloTrac system, which found worse correlations between waveform-based measurements of CO and thermodilution-derived CO for patients with atrial fibrillation, as compared to patients with sinus rhythm [2]. Figure 1 Measurements of cardiac output performed with the pulmonary artery catheter and with the FlowTrac device. Scatter plot of cardiac output (CO) measurements by FloTrac versus measurements by pulmonary artery catheter (PAC). In Sanders and colleagues' study, sinus rhythm is not mentioned among the prerequisites for measurements to be included in the analysis. We wonder whether the FloTrac device could provide meaningful data in patients with regular rhythms. Given the scarce and unfavourable data on the validity of this system, we believe that it should not be used at present, especially not in a medical intensive care unit setting where supra-ventricular arrhythmia is common.