Effect of aerosolized milrinone during drug-induced pulmonary hypertension in lambs

We tested whether aerosolized milrinone in lambs selectively reduces drug-induced acute pulmonary hypertension without reducing the mean systemic blood pressure (MSBP). Seven, 2-3-week-old lambs were anesthetized (50 mg/kg ketamine), paralyzed (0.1 mg/kg vecuronium bromide) and mechanically ventilated. A femoral artery, pulmonary artery, and jugular vein were catheterized for continuous monitoring of MSBP, mean pulmonary artery pressure, periodic gas-exchange analyses, and determination of cardiac output by thermodilution technique. An Airlife Misty nebulizer was used in a dry state to establish a stable baseline of an inspired fraction of oxygen (FiO(2)) at 0.21. Acute pulmonary hypertension with hypoxemia was then induced by increasing the mean pulmonary artery pressure up to 30-35% of the MSBP using 2-6 microg/kg/min of Thromboxane A(2) mimetic (U-46619), intravenously. The lambs were then subjected to 15 min of saline nebulization without milrinone followed by 30 min saline nebulization with a relatively high concentration of milrinone (10 mg/ml, total dose of 40 mg). Aerosolized milrinone had no effect on systemic or pulmonary artery pressure during combined acute pulmonary hypertension and hypoxemia. We speculate that our nebulization procedures failed to deliver sufficient amount of milrinone for producing a detectable hemodynamic effect.