Impact of tricuspid regurgitation severity on the agreement between Fick and thermodilution cardiac output measurements

Background/Introduction Right heart catheterisation (RHC) is generally used for evaluation and monitoring of the hemodynamic status in patients with significant tricuspid regurgitation (TR). In RHC, in cardiac output is usually measured using one of the two methods: the direct Fick method or thermodilution (TD). However, there are studies showing that these two methods may lead to diverting results in patients with significant TR, with overestimation of CO when TD is used. Purpose The aim of the study was to determine the extent of differences between the CO measurements in patients with TR, made by either Fick or TD. Methods 804 patients aged between 17 and 91 years underwent RHC with CO measurements by Fick and TD, and also echocardiography, with grading of TR and stratifying the patients into two clinically relevant groups by TR severity: TR 0-2 (none/mild/mild-moderate, n=519) and TR 3-5 (severe/massive/torrential, n=285). Results Patients with severe TR (grades 3-5) were significantly older (71.5±14.0 vs. 61.7±16.2 years) and exhibited a more compromised hemodynamic profile, with lower CO (Fick: 4.2±1.5 vs. 4.8±1.6 L/min), higher right atrial pressure (12.1±5.9 vs. 6.6±4.6 mmHg), higher arteriovenous oxygen difference (58.9±14.8 vs. 54.2±14.9 mL/L), and reduced right ventricular function (tricuspid annular plane systolic excursion: 16.4±5.0 vs. 18.2±5.5 mm). Despite these marked hemodynamic differences, the agreement between the two CO measurement methods was remarkably similar between groups. The mean absolute difference was nearly identical (0.76±0.75 L/min in TR 0-2 vs. 0.77±0.73 L/min in TR 3-5, p=0.93), as was the median relative difference (4.3% vs. 5.4%, p=0.70, Figure 1). Both groups demonstrated a small positive bias (Fick higher than TD) of similar magnitude (+0.17 vs. +0.23 L/min). The correlation between methods was slightly higher in the TR 0-2 group (r=0.78 vs. r=0.73), though this difference did not reach statistical significance (p=0.13). The 95% limits of agreement were comparable between groups, with widths of 4.12 L/min (TR 0-2) and 4.06 L/min (TR 3-5), and the percentage of measurements falling within these limits was high in both groups (95.2% vs. 96.5%, Figure 2). When defining good clinical agreement as an absolute difference of less than 0.5 L/min between methods, 41.8% of patients in the TR 0-2 group and 41.1% in the TR 3-5 group achieved this threshold (p=0.89), indicating that approximately two-fifths of patients demonstrate excellent agreement regardless of TR severity. Conclusion(s) These findings demonstrate that the agreement between Fick and TD CO measurements is not adversely affected by increasing TR. Both methods demonstrate comparable agreement across the entire spectrum of TR severity, supporting the use of TD for CO assessment even in patients with the most advanced grades of tricuspid regurgitation where hemodynamic compromise is most pronounced.Figure 1Figure 2