Effects of dopamine and nifedipine infusions on the pulmonary circulation of the lamb.

The purpose of this study was to test the hypothesis that nifedipine when given with dopamine will lower pulmonary vascular resistance in hypoxic lambs without altering systemic vascular resistance. We studied six unanesthetized lambs (ranging in age from 13 to 35 days) as they breathed air or on a separate day as they breathed 10% O2 and 3% CO2 in nitrogen. First, we infused dopamine at progressively higher rates (10, 20, 40, 80, and 160 micrograms/kg/min) while measuring mean aortic, pulmonary arterial, and left atrial pressures and heart rate continuously and cardiac output and arterial blood gas tensions at frequent intervals. Then, while maintaining the dopamine infusion at 160 micrograms/kg/min, we infused boluses of nifedipine intravenously (10 micrograms/kg) every 5 min until a cumulative dose of 50 micrograms/kg had been administered. In both groups of lambs, cardiac output increased with increasing rates of dopamine infusion (baseline to maximum dopamine: 260 +/- 20 ml/kg/min to 420 +/- 60 ml/kg/min for normoxic lambs and 400 +/- 50 ml/kg/min to 560 +/- 80 ml/kg/min for hypoxic lambs). While systemic vascular resistance and pulmonary vascular resistance did not change significantly in either group during dopamine infusion, the ratio of pulmonary vascular resistance to systemic vascular resistance increased at low rates of infusion and decreased at high rates. The peak in this ratio occurred at a rate of infusion of 20-40 micrograms/kg/min in normoxic lambs and 40-80 micrograms/kg/min in hypoxic lambs. Infusion of nifedipine did not affect cardiac output in normoxic lambs but decreased it significantly in hypoxic lambs. Nifedipine infusion did not affect pulmonary vascular resistance in the normoxic lambs and increased pulmonary vascular resistance in the hypoxic lambs. We conclude that nifedipine, even when given with high doses of dopamine, is not a specific pulmonary vasodilator.