Pulmonary Arterial Hypertension Induced by Distention of the Main Pulmonary Artery in Conscious Newborn, Young, and Adult Sheep

Abstract

SummaryDistention of the main pulmonary artery by balloon inflation in sheep results in presumably reflex elevation of pulmonary arterial pressure and resistance distal to the balloon. This response to main pulmonary artery distention is significantly greater in newborn lambs than in older lambs or adult sheep. In several of the newborn lambs, pulmonary artery pressure was raised to suprasystemic levels. Further, in some of the newborn animals, these increases in pulmonary artery pressure and resistance were sustained after deflation of the balloon for periods up to 2 hr. The functional significance of this pulmonary hypertension reflex was not elucidated. However, the data strongly suggest that this reflex may contribute to the maintenance of high pulmonary vascular resistance during fetal and early neonatal life. SpeculationProximal distention of the main pulmonary artery induces a positive feedback reflex which significantly elevates pulmonary artery pressure and resistance. We postulate that this reflex operates in conjunction with hypoxia to maintain high pulmonary vascular resistance in the fetus. It is possible that at birth this reflex mechanism operates in reverse to help lower the pulmonary vascular resistance smoothly, thus avoiding volume overload of the neonatal lungs and left ventricle. Finally, we postulate that this reflex may play a role in the pathogenesis of the persistant fetal circulation syndrome (7).In the conscious adult dog distention of the proximal main pulmonary artery, near the bifurcation, by a partially inflated balloon, results in a significant elevation of pulmonary artery pressure distal (as well as proximal) to the balloon (19,20,22). This elevation in pressure is produced by an increase in pulmonary vascular resistance which occurred without significant change in right ventricular end-diastolic pressure, aortic pressure, and cardiac output. Left atrial pressure either did not change or decreased. We have reported that stepwise balloon distention of the main pulmona artery results in step increases in pulmonary artery pressure abwnstream from the balloon (22) and that this phenomenon is most likely due to sympathetic reflex pulmonary vasoconstriction (19,20).A prominent feature of this pulmonary artery distention reflex is its presumably positive feedback nature. The term "positive feedback" is used to describe this pulmonary vascular pressor reflex because it appears to depend on the excitation of stretch receptors upstream from the pulmonary vascular bed to cause the constriction of the downstream pulmonary vessels such that the greater the distention of the main pulmonary artery the greater the downstream vasoconstriction. Consideration of the implications and significance of the existence of such a pulmonary arterial stretch receptor system to raise pulmonary vascular resistance, via positive feedback mechanisms, as shown in the dog, poses the following questions: (I) is this pulmonary hypertension reflex present in other species? (2) is this reflex more active in the newbo...

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