The pulmonary vasodilator effect of increased rate of mechanical ventilation, with and without respiratory alkalosis, was studied in chronically instrumented newborn lambs. Pulmonary hypertension was first induced by ventilating with a hypoxic gas mixture. Subsequent respiratory alkalosis caused significant decreases in pulmonary arterial pressure and pulmonary vascular resistance. When normocarbia was re-established by adding carbon dioxide to the inspired gas, with the ventilator rate being held constant, the pressure and resistance returned to the baseline hypertensive state. Therefore, mechanical factors, either direct or indirect, appear to be of minor importance in the mechanism of pulmonary vasodilation secondary to frequency-induced hyperventilation.

Alkalosis, Respiratory↗

PubMedDate not supplied

Effect of pulmonary hypertension on lung compliance in newborn lambs.

We studied the effects of increased pulmonary arterial pressure and vascular resistance on lung compliance in 12 anesthetized newborn lambs. Lung compliance was unaffected by large changes in pulmonary artery pressure and pulmonary vascular resistance (80% increase) that were induced by either hypoxia or acidosis. We conclude that contraction of the pulmonary vascular smooth muscle in normal newborn lambs does not affect the stiffness of the lung in the parenchymal regions undergoing volume change during ventilation.

Animals↗