Combining low inspired oxygen and carbon dioxide during mechanical ventilation for the Norwood procedure

Background: Staged reconstruction has become the preferred approach for treating neonates with hypoplastic left heart syndrome (HLHS). The haemodynamic instability of a single ventricle providing blood flow in parallel to the systemic and the pulmonary circulation, combined with the effects of cardiopulmonary bypass (CPB), results in precarious perioperative conditions. The two ventilatory manoeuvres commonly used for increasing pulmonary vascular resistance are (i) decreasing the fraction of inspired oxygen to < 0.21 and (ii) adding carbon dioxide (CO₂) to the ventilatory circuit. Whether molecular nitrogen (N₂) or CO₂ is used in these situations is a matter of physician and institutional preference. The effect of the two modalities in combination has not been studied in depth. Methods: This prospective observational study was designed to look at the effects of adding inhaled CO₂ to children with HLHS who were already on a hypoxic mixture during the immediate perioperative period. Results: Twelve suitable neonates were enrolled in the study. Combining the two ventilatory modalities had an additive effect. The effect was more significant in the prebypass (83% of patients) compared with the postbypass period (25% of patients). Conclusions: Low inspired oxygen and CO₂(have an additive vasoconstrictive effect on the pulmonary vessels. The use of both of these ventilatory manoeuveres is less effective postoperatively compared with the prebypass period.