Arterial O₂ saturation alone does not predict pulmonary to systemic blood flow ratio in the post-operative norwood patient

Introduction: This study tests the hypothesis that arterial O₂ saturation (SaO₂) is a good predictor of pulmonary flow (Qp) and pulmonary to systemic flow ratio (Qp/Qs) after Norwood palliation. Methods: Hemodynamic data were collected in 7 patients (3-5 days post Norwood palliation) who underwent controlled, prospective manipulation of inspired O₂ and CO₂ concentration. Using indwelling Unes placed intra-operatively, 43 complete oximetric studies were obtained. Qp and Qs were calculated from simultaneously measured O₂ content in the aorta (Ao), superior vena cava (SVC) and pulmonary vein (PV). O₂ Consumption was measured (n=3) or was assumed (n=4) at 12cc/kg/min. Results: Multiple regression was used to predict Qp/Qs and Qp (dependent variables) from O ₂ saturation data. SaO₂ alone correlated weakly with Qp/Qs or Qp (R² 0.17 and 0.21, respectively; p<0.01). Estimation of Qp/Qs and Qp improved substantially with inclusion of SVC and PV O₂ saturation data. This was explained by variability of O₂ saturation observed in SVC (range: 32-68%) and PV (range: 76-99%) blood. Of note, PV saturation <95% was observed in 13 of 43 (30%) studies, including all 5 studies in sub-ambient oxygen. Nine measurements in 6 patients were <90%. Conclusions: This study demonstrates for the first time that pulmonary venous desaturation is common after the Norwood procedure; it also confirms that variations in SVC saturation occur routinely. Therefore, SaO₂ alone is a poor predictor of Qp and Qp/Qs after the Norwood operation. Coefficient of Determination (R²) Qp/Qs Qp Ao alone .17 .21 O₂ Saturation Ao+SVC .65 .21 Data Ao+PV .43 .84 Ao+PV+SVC .93 .88.