Sodium Nitroprusside Increases Qs/Qt in Dogs with Regional Atelectasis

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Abstract
The investigation of the effects of sodium nitroprusside (SNP) on arterial O2 tension (PaO2), pulmonary shunt (.ovrhdot.Qs/.ovrhdot.Qt) and pulmonary vascular resistance (PVR) in the presence of atelectasis of 1 lung was made. Ten dogs were anesthetized, their tracheas intubated with a bronchial divider and their lungs ventilated with IPPB with pure O2. 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 < 0.05) with SNP infusion. .ovrhdot.Qs/.ovrhdot.Qt increased from 30 .+-. 7.0 to 39 .+-. 6.0% (P < 0.05); cardiac output did not change significantly. PVR of the atelectatic lung decreased; PVR of the ventilated lung was unchanged. Decrease in PVR in the atelectatic lung suggests that SNP decreases PaO2 and increases .ovrhdot.Qs/.ovrhdot.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.