Effect of sodium nitroprusside on hypoxic pulmonary vasoconstriction in the dog

The objective of the study was to determine the direct actions of sodium nitroprusside (SNP) on the pulmonary vasculature actively constricted by hypoxic ventilation in the closed-chest dog. In 21 supine anesthetized (pentobarbital sodium, 25 mg/kg) dogs the left lower lobe was catheterized with a 20 F wedged-flow catheter. With constant flow to this lung lobe, lobar pulmonary artery pressure (PAP), left ventricular end-diastolic pressure (LVEDP), and lobar pulmonary vascular resistance (PVR) were studied during room air and 10% O2 mechanical ventilation. With lobar flow and LVEDP constant, observed changes of lobar PAP and PVR represent direct effects on vascular tone unaffected by passive factors. Ventilation with 10% O2 resulted in a significant increase in lobar PAP [12.2 +/- 1.6 to 17.2 +/- 1.2 (SE) Torr] and PVR [42.2 +/- 13.3 to 61.2 +/- 15.0 (SE) Torr X 1(-1) X min]. Then during continued hypoxia in 12 animals, femoral vein infusion of SNP at doses of 1, 2, and 5 microgram X kg-1 X min-1 resulted in significant decreases in lobar PAP to 14.8 +/- 1.5, 14.8 +/- 1.5, and 13.4 +/- 1.6 Torr, respectively, and in PVR to 56.7 +/- 14.1, 54.0 +/- 13.9, and 47.8 %/- 13.2 Torr X 1(-1) X min, respectively. In six animals the sequence of drug infusion was then reversed with similar results. It is concluded that the actively constricted pulmonary vascular bed is directly dilated by SNP infusion.