Effects of milrinone on pulmonary vasculature in normal dogs and in dogs with pulmonary hypertension.

OBJECTIVE: To study the effects of milrinone on pulmonary vasculature.

BACKGROUND: It has been suggested that bipyridines or their derivatives may have a selective pulmonary vasodilation effect.

METHODS: Preliminary study: milrinone administration to 12 normal dogs (low dose [bolus 75 micrograms/kg for 5 min followed by a continuous infusion at 0.75 micrograms/kg.min, n = 6]; high dose [bolus 150 micrograms/kg for 5 min followed by continuous infusion at 1.5 micrograms/kg.min, n = 6]). Main study: milrinone administration to 18 dogs with pulmonary hypertension due to pulmonary embolism induced by a massive injection of autologous muscle cubes. The pulmonary hypertension dogs were divided into three groups: a) group E (n = 6) received embolization only, as control; b) group L (n = 6) received low-dose milrinone; and c) group H (n = 6) received high-dose milrinone, equivalent to the preliminary study group. Hemodynamic measurements and blood samplings were obtained at baseline and at 15, 30, and 60 min after start of milrinone infusion.

RESULTS: Milrinone did not change mean pulmonary artery pressure (MPAP) in normal dogs. Milrinone decreased MPAP significantly in dogs with pulmonary hypertension. Pulmonary vascular resistance index remained at an almost constant level in normal dogs, but decreased significantly in dogs with pulmonary hypertension. Mean arterial pressure was maintained at a constant level in all groups. High-dose milrinone administration decreased systemic vascular resistance index (SVRI) significantly; low-dose milrinone administration decreased SVRI slightly.

CONCLUSIONS: Milrinone may have a selective pulmonary vasodilatory effect only in dogs with pulmonary hypertension. The mechanism that produced a selectivity on pulmonary vasculature in dogs with pulmonary hypertension is unknown. However, an inhibition of platelet aggregation may decrease the MPAP resulting from an increase in cAMP caused by milrinone. Further studies are needed to resolve the pulmonary vasodilatory effect of milrinone in dogs with pulmonary hypertension.