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

  1. Daniela Cuellar-Mendoza
  2. Javier Mancilla-Galindo
  3. Esmeralda Garza-Santiago
  4. Liliana Aline Fernández-Urrutia
  5. Gabriela Alejandra Bautista-Aguilar
  6. Ashuin Kammar-García
  7. Ernesto Deloya-Tomas
  8. Orlando Rubén Pérez-Nieto
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Abstract

Background

The two-gasometry indirect Fick method (iFM) is commonly applied to estimate cardiac output (CO) and cardiac index (CI) in low-resource settings. Validation against other accessible methods such as transthoracic echocardiography (TTE) is lacking.

Objective

To determine agreement and clinical interchangeability of CO and CI measurements from TTE (reference test) and two-gasometry iFM (index test) in critical care patients from a low-resource hospital.

Methods

Cross-sectional study including critical patients. CO and CI were measured by intensive care medicine residents with TTE (3 per patient-event) and the two-gasometry iFM (1 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 243 measurements were performed across 81 time points in 52 participants. Bland-Altman analysis and linear mixed-effects models revealed poor correlation for both CO (ICC = 0.09) and CI (ICC = -0.09). The mean bias between TTE and iFM was -1.59 L/min (95 LoA: -8.1 to 4.92) for CO and -1 L/min/m² (95% LoA: -4.8 to 2.8) for CI. The mean absolute percentage error of the two-gasometry iFM method compared to TTE 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 performs poorly compared to TTE and should not be used for decision making in critical patients.

Highlights

  • The two-gasometry indirect Fick method shows poor agreement with transthoracic echocardiography for measuring cardiac output and cardiac index.

  • Bland-Altman analysis showed significant bias, high variability, and lack of precision of the two-gasometry indirect Fick method measurements.

  • Mean absolute percentage error of the two-gasometry indirect Fick method was 57.5%, far exceeding acceptable clinical limits.

  • The two-gasometry indirect Fick method is unsuitable for clinical decision-making in critically ill patients.

Article activity feed

  1. Version published to 10.1101/2025.03.05.25323438v1 on medRxiv