Effect of continuous tolazoline infusion on cardiopulmonary response to dopamine in unanesthetized newborn lambs.

We evaluated the cardiopulmonary interaction of tolazoline and low-, medium-, and high-dose dopamine infusion (2.7, 27, and 270 micrograms/kg/min) in 11 normal, chronically instrumented, unsedated neonatal lambs. Concomitant tolazoline and dopamine infusions caused tachycardia at all dopamine doses and blocked alpha-adrenergic-mediated increases in systemic resistance and left atrial pressure caused by medium and high doses of dopamine. Moderate doses of dopamine alone increased cardiac output. During simultaneous tolazoline infusion, dopamine increased cardiac output at both moderate and high doses. The effect of dopamine on pulmonary artery pressure was not modified by tolazoline; pulmonary pressure increased similarly with dopamine alone and with dopamine plus tolazoline. Calculated pulmonary vascular resistance rose with dopamine alone, but not with infusion of tolazoline and dopamine, because pulmonary blood flow rose in proportion to pulmonary artery pressure.