Mechanisms of hypercapneic pulmonary hypertension

The mechanisms and potential mediator of hypercapneic pulmonary hypertension are incompletely understood. We studied 18 dogs, anaesthetised and spontaneously breathing both room air and after the inhalation of a gas mixture containing 10% CO2, 20.9% O2, and 69.1% N2, to determine the role of histamine, serotonin, and acidaemia in pulmonary hypertension produced by hypercapnia. Hypercapnia increased the mean pulmonary artery pressure by 0.33 kPa (2.5 mmHg) while wedge pressure and pulmonary arteriolar resistance did not change. Cardiac output significantly increased, indicating that the pulmonary hypertensive effect of hypercapnia is mainly flow related. Neither chlorpheniramine nor methysergide had significant effects on hypercapneic pulmonary hypertension. The infusion of sodium bicarbonate corrected the pH; pulmonary artery pressure and cardiac output increased while pulmonary arteriolar resistance dropped, suggesting that the increased cardiac output masked the effect of pH on pulmonary arteriolar resistance. The lack of effect of chlorpheniramine or methysergide on pulmonary resistances indicates that the vasoconstrictive effect of increased hydrogen ion concentration which accompanies hypercapnia is attributable neither to histamine nor to serotonin release.