The Fick principle underestimates pulmonary blood flow in pre-Fontan patients

<h2>Abstract</h2><h3>Background</h3> Pulmonary blood flow (Qp) measured by oximetry and Fick's principle (FP) is unreliable if there is multifocal pulmonary blood supply with differing oxygen contents. This often occurs in single ventricle patients with a bidirectional cavopulmonary connection (BCPC) and aortopulmonary collaterals (APC). We aimed to compare the difference between cardiac magnetic resonance (CMR) measured and Fick-estimated Qp (Fick-Qp) and pulmonary vascular resistance (PVRI) at pre-Fontan assessment. <h3>Methods</h3> A retrospective analysis of BCPC patients who underwent combined CMR and catheterization under the same general anesthetic. CMR flows were measured in the superior vena cava (CMR-Qsvc), pulmonary veins (CMR-Qpv) and the difference between these was APC flow (Qapc). Oximetric Fick-Qp was compared to CMR-Qsvc and CMR-Qpv. <h3>Results</h3> Ninety-five patients were included, median (IQR): age 2.92 (2.32 - 3.38) years and weight 12.8 (11.95 - 14.65) kg. The rank order of flows was CMR-Qpv > Fick-Qp > CMR-Qsvc, median values (IQR): 3.37 (2.97 - 3.98) > 2.39 (2.09 - 2.83) > 1.91 (1.67 - 2.25) L/min/m². Hence PVRI derived from these flows were in the reverse rank order, PVRI-CMR-Qpv < PVRI-Fick-Qp <PVRI-CMR-Qsvc, mean values (95%CI): 1.37 (1.04 - 1.67) < 1.8 (1.49 - 2.18) < 2.38 (1.81 - 2.84). <h3>Conclusions</h3> In comparison to the reference method of CMR-Qpv, the oximetric Fick method underestimates pulmonary blood flow and overestimates PVRI. In patients with high Fick based PVRI, a reassessment using CMR based measurement of pulmonary blood flow should be considered for more accurate measurement of PVRI and risk stratification.