Background: Hemodynamic stability after Norwood palliation often requires manipulation of pulmonary vascular resistance to alter the pulmonary-to-systemic blood flow ratio (Qp:Qs). Qp:Qs is often estimated from arterial saturation (SaO2), a practice based on 2 untested assumptions: constant systemic arteriovenous O2 difference and normal pulmonary venous saturation.
Methods and results: In 12 patients early (</=3 days) after Norwood palliation, simultaneous arterial, superior vena caval (SsvcO2), and pulmonary venous (SpvO2) oximetry was used to test whether SaO2 accurately predicts Qp:Qs. Stepwise multiple regression assessed the contributions of SaO2, SsvcO2, and SpvO2 to Qp:Qs determination. SaO2 correlated weakly with Qp:Qs (R2=0.08, P<0.05). Inclusion of SsvcO2 and SpvO2 improved Qp:Qs prediction accuracy. Pulmonary venous desaturation (SpvO2 <95%) was observed frequently (30%), especially at FiO2 </=0.21, but normalized with higher FiO2 or PEEP in all patients. In 6 patients, FiO2 was increased incrementally from 0.17 to 0.50 to determine whether this was an effective means to manipulate Qp:Qs. Qp:Qs failed to change predictably with increased FiO2. In 5 of 6 patients, however, higher SpvO2 and SaO2 enhanced systemic oxygen delivery, as demonstrated by improvement in oxygen extraction.
Conclusions: SaO2 correlated poorly with Qp:Qs because of variability in SsvcO2 and SpvO2. A novel observation was that pulmonary venous desaturation occurred frequently early after Norwood palliation but normalized with higher FiO2 or PEEP. Because unrecognized pulmonary venous desaturation confounds p:s assessment and compromises SaO2 and oxygen delivery, judicious use of inspired oxygen and PEEP may be beneficial in selected patients early after Norwood palliation.