Primary pulmonary hypertension: Effect of unilateral pulmonary artery occlusion and infusion of acetylcholine

A case of primary pulmonary hypertension is presented with evidence of increased vascular tone. Temporary unilateral occlusion of right middle and lower lobes caused a decrease in estimated arteriolar resistance of unoccluded lung. Infusion of acetylcholine caused a profound decrease in pulmonary artery pressure and pulmonary arteriolar resistance without any systemic effects, suggesting direct action of the drug on the pulmonary arterioles. It is suggested that, in this case, increased tone is the primary defect and the histologic vascular changes secondary.