Sodium nitroprusside increases Qs/Qt in dogs with regional atelectasis

This study investigated the effects of sodium nitroprusside (SNP) on arterial oxygen tension (Pao2), pulmonary shunt (Qs/Qt), and pulmonary vascular resistance (PVR) in the presence of atelectasis of one lung. Ten dogs were anesthetized, their tracheas intubated with a bronchial divider, and their lungs ventilated with IPPB with pure oxygen. Atelectasis of the left lung was produced by occluding the left side of the bronchial divider and ventilating the right lung. SNP was infused to decrease mean arterial blood pressure by 25%. Pao2 decreased from (mean value+/-1 SD) 134+/-75 to 77+/-23 torr (P less than 0.05) with SNP infusion. Qs/Qt increased from 30+/-7.0 to 39+/-6.0% (P less than 0.05), while cardiac output did not change significantly. PVR of the atelectatic lung decreased, while PVR of the ventilated lung was unchanged. The decrease in PVR in the atelectatic lung suggests that SNP decreases Pao2 and increases Qs/Qt by reversing the hypoxic pulmonary vasoconstriction. As a result, during SNP infusion, perfusion of the atelectatic lung was maintained while perfusion of the ventilated lung decreased.