Accuracy of conventional oximetry for flow estimation in patients with superior cavopulmonary connection a comparison with phase-contrast cardiac MRI

Background-Cardiac catheterization is routinely used as a diagnostic tool in single ventricle patients with superior cavopulmonary connection. This physiology presents inherent challenges in applying the Fick principle to estimate flow. We sought to quantitatively define the error in oximetry-derived flow parameters using phase-contrast cardiac MRI (CMR) as a reference. Methods and Results-Thirty patients with superior cavopulmonary connection who underwent combined CMR and catheterization between July 2008 and June 2012 were retrospectively analyzed. Estimates of flow and resistance calculated using the Fick equation were compared with CMR measurements. Oximetry underestimated CMR-measured pulmonary blood flow (Q_p) by an average of 1.1 L/min per m² or 32% of the CMR value (P<0.0001). Oximetry overestimated systemic blood flow (Q_s) by an average of 0.5 L/min per m² or 15% of the CMR value (P=0.009). There was no correlation between the Q _p:Q_s ratio derived by Fick and that measured by CMR (ρ_c=0.01). The error in the Fick Q_p correlated moderately with the measured systemic-to-pulmonary arterial collateral flow (r=0.39). The median total oxygen consumption calculated using combined CMR and oximetry data was 173 mL/min per m², higher than the assumed values used to calculate flows by the Fick equation. The upper body circulation received on average 51% of systemic blood flow while conducting only 39% of total body metabolism. Conclusions-Fick-derived estimates of flow are inherently unreliable in patients with superior cavopulmonary connections. Integrating flows measured by CMR and pressures measured by catheter will provide the best characterization of superior cavopulmonary connection physiology.