Agreement of thermodilution and direct Fick methods for cardiac output across varying haemodynamic conditions

INTRODUCTION: The direct Fick method is the gold standard for measuring cardiac output (Q). However, thermodilution (TD) is more widely used in clinical practice due to its feasibility. As the accuracy of TD-derived Q (QTD) may vary by flow state, it can influence the estimation of pulmonary vascular resistance (PVR) and pulmonary hypertension (PH) classification. This study aimed to compare QTD with Fick-derived Q (QF) across haemodynamic states and evaluate the clinical reliability of TD-derived PVR (PVRTD). METHODS: We retrospectively analysed 852 right heart catheterizations (2014-22) with simultaneous QTD and QF measurements. RESULTS: 64% of patients had heart failure, and 20% had pulmonary arterial hypertension. QTD correlated strongly with QF (r = 0.78, P < .001) but demonstrated a flow-dependent bias-overestimating Q in low-flow states (+0.1 L/min) and underestimating it in normal/high flow (-0.48 L/min). Despite this, PVRTD accurately identified elevated PVRF at both 2 and 5 Wood unit thresholds (area under the curve = 0.97, P < .001). Agreement between QTD and QF was unaffected by tricuspid regurgitation severity. CONCLUSION: Although TD shows a predictable bias across flow states, it remains a clinically reliable method for estimating elevated PVR. These findings support the clinical utility of TD in PH assessment, while emphasizing the need for direct Fick measurement in low-output states-particularly in patients evaluated for advanced therapies such as transplantation.