Hypoxia is/is not the optimal means of reducing pulmonary blood flow in the preoperative single ventricle heart.

COMMENT ON POINT:COUNTERPOINT To the Editor: Optimal may depend on the circumstances and both points have utility (1, 3). We found carbon dioxide manipulation more useful and less dangerous than alveolar hypoxia in acute transitional settings when mechanical controlled ventilation is required in unrepaired ductal or shuntdependant pulmonary blood flow (2). The technical aspects of transitioning from spontaneous to controlled mechanical ventilation (anesthetic induction, bag/mask, endotracheal intubation) includes inability to precisely control minute ventilation and requires variable, albeit brief, apnea. Alveolar hypoxemia prior to apnea quickly leads to systemic desaturation and bradycardia and attempts to then quickly deliver oxygen to the alveoli, resulting in hypocarbia, decreasing pulmonary vascular resistance, and transferring of volume to the pulmonary circulation. In the operating room, body temperature and carbon dioxide production decline and hypothermia-induced systemic vasoconstriction combine to shift blood flow to the lung. These observations resulted in using added CO2 along with normoxic inspired gas, which maintained circulatory stability and avoided metabolic acidosis and desaturation-related events. We also observed that the circulatory response to inspired CO2, both increase and decrease, was much more rapid than changing inspired oxygen. This allows more precise control to affect a balanced circulation in acute situations. Understanding that both oxygen and CO2 will independently and synergistically affect pulmonary vascular resistance will optimize management of these patients as they transition through the environments and reconstructive treatments that are required.