The pulmonary vasoconstrictor response to the unilateral administration of 7% oxygen was studied in 10 anaesthetized dogs in which the lungs were mechanically ventilated. The distribution of blood flow between the two lungs was measured continuously by recording the radioactivity in the mixed expired gas from each lung during the infusion i.v. of xenon-133. Infusions of nitroglycerine and sodium nitroprusside which produced the same decrease in mean aortic pressure produced similar decreases in hypoxic pulmonary vasoconstriction. The reduction in arterial PO2 during unilateral hypoxia was greater when the hypoxic vasoconstrictor response was depressed by drugs than it was during the control periods, although mixed venous PO2 was unchanged. It is concluded that both drugs may cause hypoxaemia by increasing the blood flow to alveoli with a low ventilation/perfusion ratio.

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