The Two-Gasometry Indirect Fick Method: An Inconsistent Method for Measuring Cardiac Output in Critical Patients

Background

The two-gasometry indirect Fick method (iFM) is commonly applied to estimate cardiac output (CO) and cardiac index (CI) in low-resource settings, but validation against accessible methods such as transthoracic echocardiography (TTE) is lacking. We evaluated agreement and clinical interchangeability of CO and CI measurements from TTE (reference test) and two-gasometry iFM (index test) in critically ill patients.

Methods

Cross-sectional study conducted in a low-resource critical care unit. CO and CI were measured by intensive care medicine residents using TTE (3 measurements per patient-event) and two-gasometry iFM (1 measurement per patient-event). Agreement was evaluated using mean absolute difference, linear mixed-effects models, and Bland-Altman analysis for repeated measures with 95% limits of agreement (LoA). Precision, bias, and variability were quantified with coefficients of variation, error, and least significant change (LSC), with 95% confidence intervals (95%CI) using bootstrapping.

Results

A total of 243 measurements were performed across 81 time points in 52 participants. The two-gasometry iFM showed poor correlation with TTE for both CO (ICC = 0.05) and CI (ICC = –0.04). Mean bias was −2.48 L/min (95% LoA: −8.93 to 3.98) for CO and −1.53 L/min/m² (95% LoA: −5.27 to 2.22) for CI. Mean absolute percentage error was 57.5% (95%CI: 45.5 to 74.6). The LSC was 11.9% for TTE and 80.4% for two-gasometry iFM.

Conclusions

The two-gasometry indirect Fick method has poor precision, high variability, and significantly overestimates CO by ∼2.5 L/min and CI by ∼1.5 L/min/m² compared to TTE. Therefore, it is unsuitable for clinical decision-making in critical patients.

Key points

• Question: Is there agreement and clinical interchangeability of CO and CI measurements from transthoracic echocardiography (TTE) and the two-gasometry indirect Fick method (iFM) in critical care patients?

• Finding: The two-gasometry indirect Fick method shows poor agreement with transthoracic echocardiography for measuring cardiac output and cardiac index due to systematic bias, high variability, and lack of precision.

• Meaning: A minimal change of 80.5% in CO with the two-gasometry iFM is needed to be considered a clinically informative change, making this method unsuitable for clinical decision-making in critically ill patients.