The Estimated Fick Method Systematically Over-Estimates Cardiac Index Compared to Thermodilution in Children

Cardiac index (CI) may be derived from the Fick method, using measured or estimated oxygen consumption (VO₂), or from thermodilution. In children, LaFarge VO₂ estimates correlate poorly with measured VO₂ values. In a large adult cohort, there was only modest correlation between estimated Fick CI (eFick CI) and thermodilution CI (TDCI). We evaluated the extent of agreement between eFick CI using LaFarge estimates of VO₂ and TDCI in a pediatric cohort. A retrospective, single-center chart review of patients 3-18 years of age who underwent cardiac catheterization with documented eFick CI and TDCI from 2004 to 2020 included 201 catheterizations from 161 unique patients. The mean patient age at catheterization was 12.2y (SD 4.4y). The most frequent diagnosis was cardiomyopathy, followed by congenital heart disease and pulmonary hypertension. TDCI and eFick CI differed by > 20% in 49% of catheterizations. eFick CI systematically exceeded TDCI by a mean percentage difference of 24% (SD 31%). Higher mean CI ((eFick CI + TDCI)/2) and older age were predictive of greater percent difference between eFick CI and TDCI. For each increase in mean CI by 1.0 L/min/m², the expected percent difference in CI increased by 9.9% (p < 0.001). In pediatric patients undergoing cardiac catheterization, eFick CI with LaFarge VO₂ systematically exceeds TDCI. The difference between methods is frequently > 20%, which may have clinically significant implications. Discrepancies between eFick CI and TDCI increase at higher mean CI.