Accuracy of indirect Fick method for systemic flow calculation in adults with a Fontan circulation

Indirect Fick calculation of systemic blood flow (Qs) relies on estimated oxygen consumption (VO₂), but empirical VO₂ equations have not been validated in adults with a Fontan circulation.

We retrospectively studied 109 adults with a Fontan circulation who underwent clinically indicated exercise catheterization at Mayo Clinic (2018-2026). We compared resting VO₂ measured with a metabolic cart against the LaFarge & Miettinen, Dehmer et al., and Bergstra et al. equations. Agreement for VO₂, Fick Qs, and cardiac index (CI) category (low <2.5, normal 2.5-4.0, high >4.0 L/min/m²) was assessed with Bland-Altman analysis and weighted κ.

Measured VO₂ was 223 ± 53 mL/min. Correlation with the estimates was moderate (r = 0.72-0.75). Mean bias was -0.7 (LaFarge & Miettinen), +12.3 (Dehmer et al.), and +47.8 mL/min (Bergstra et al.), and the 95% limits of agreement were approximately ±33%. Median absolute percentage errors were 10.1%, 11.1%, and 22.1%, respectively. All equations showed significant proportional bias: they overestimated low VO₂ and underestimated high VO₂. Higher BMI was associated with greater underestimation. Indirect Fick Qs correlated strongly with direct Fick Qs (r = 0.86-0.87), but the limits of agreement remained wide (∼±37%). CI category was misclassified in 25.7%, 27.5%, and 44.0% of patients (weighted κ 0.60, 0.57, and 0.39).

Empirical VO₂ equations approximate resting VO₂ at the cohort level, but they carry clinically relevant individual error in adults with a Fontan circulation. This error carries through to Fick-derived flow and CI classification. Direct VO₂ measurement is preferable when feasible. When it is not, LaFarge & Miettinen performed best of the equations tested.