Pulmonary Shunt and Cardiovascular Responses to CPAP during Nitroprusside-induced Hypotension

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Abstract
The effects of continuous positive airway pressure (CPAP) on cardiovascular dynamics and pulmonary shunt (.ovrhdot.QS/.ovrhdot.QT) were investigated in 12 dogs before and during sodium nitroprusside infusion that decreased mean arterial blood pressure 40-50%. Before nitroprusside infusion, 5 cm H2O CPAP significantly, P < .05, decreased arterial blood pressure, but did not significantly alter heart rate, cardiac output, systemic vascular resistance or .ovrhdot.QS/.ovrhdot.QT. H2O CPAP (10 cm) before nitroprusside infusion produced a further decrease in arterial blood pressure and significantly increased heart rate and decreased cardiac output and .ovrhdot.QS/.ovrhdot.QT. Nitroprusside caused significant decreases in arterial blood pressure and systemic vascular resistance and increases in heart rate, but did not change cardiac output or .ovrhdot.QS/.ovrhdot.QT. H2O CPAP (5 cm) during nitroprusside did not further alter any of the variables, but 10 cm H2O CPAP decreased arterial blood pressure, cardiac output, and .ovrhdot.QS/.ovrhdot.QT. Nitroprusside infusion rates that decrease mean arterial blood pressure by 40-50% do not change cardiac output or .ovrhdot.QS/.ovrhdot.QT. During nitroprusside infusion low levels of CPAP do not markedly alter cardiovascular dynamics, but high levels of CPAP (10 cm H2O), while decreasing .ovrhdot.QS/.ovrhdot.QT, produce marked decreases in arterial blood pressure and cardiac output.