Indirect Fick and Thermodilution Cardiac Output Measurements Have Significant Variance Compared to Direct Fick in LVAD Patients

Purpose Direct Fick is the gold standard for measuring cardiac output (CO). Due to direct Fick complexity, indirect Fick (iFICK) and Thermodilution (TD) are most commonly used to assess CO during right heart catheterization (RHC). However, a small previous study found a 45% discordance in CO estimations when comparing TD with iFick in LVAD patients. This study aimed to compare TD and iFick estimations to direct Fick in order to compare which common method was most accurate. Methods We included 16 LVAD patients referred for clinically indicated RHC. Concomitant with obtaining mixed venous oxygen saturation, resting oxygen consumption was obtained via the MGC diagnostics calorimeter system. Three common formulae were used to calculate iFick for comparison - Lafarge, Dehmer, and Bergstra. Thermodilution was measured in triplicate and averaged with removal of outliers. Correlation between each method and direct Fick was assessed with linear regression. Bias between each method and direct Fick was compared using Bland-Altman Plot analysis. Greater than 25% percent error was used a priori as clinically unacceptable. Results Moderate correlation was present between all methods and direct Fick (TD: R=0.45; Lafarge: R=0.61; Dehmer: R=0.51; Bergstra: R=0.4). However, while absolute bias was lowest between TD and direct Fick (+14.5 ± 30%), all methods tended to overestimate CO compared to direct Fick and had poor overall agreement (Figure). By any method, more than 30% of measurements carried significant discrepancy (>25%) compared to direct Fick. Conclusion Though TD CO carried the least bias, both it and all methods of iFick calculation tend to overestimate CO and have poor agreement when compared to the direct Fick method. These imprecise CO measurements could lead to poor clinical decisions in VAD patients and misjudgment of transplant candidacy due to inaccurate calculation of pulmonary vascular resistance. Where possible, direct Fick should be used to measure CO in LVAD patients.