Hemodynamic responses to angiotensin II in the newborn lamb

Angiotensin II is known to increase both pulmonary and systemic arteriolar tone in adult animals. Its influence on these vascular beds shortly after birth is less well understood. Therefore, we studied the effects of infusions of angiotensin II (0.1 microgram/kg/min) on pulmonary and systemic hemodynamics in 13 anesthesized newborn lambs. Systemic vascular resistance increased significantly from a mean of 0.079 +/- 0.03 to 0.094 +/- 0.04 mm Hg/ml/min during angiotensin II infusion while the mean pulmonary vascular resistance was unchanged at 0.024 +/- 0.01 mm Hg/ml/min. Interestingly, cardiac output increased significantly by 18.9% during angiotensin II infusion. During hypoxemia produced by ventilating with 10-12% oxygen, the responses to angiotensin II infusion were similar to those obtained during normoxia. The absence of an effect on pulmonary vascular resistance and the increase in cardiac output were not predicted based on results reported for older animals. The mechanisms responsible for these age-related differences are unknown. Our findings have implications regarding the potential clinical use of angiotensin II as a modulator of blood pressure in the hypotensive newborn.