Additive effects of inhaled nitric oxide and intravenous milrinone in experimental pulmonary hypertension

Objective: To determine whether inhaled nitric oxide (INO) and intravenous milrinone have additive pulmonary vasodilator effects in a rat model of pulmonary hypertension. Design: Prospective, experimental study. Setting: Animal laboratory of a university medical center. Subjects: Male New Zealand White rabbits. Interventions: Anesthetized rabbits were mechanically ventilated and instrumented for measurement of systemic mean arterial pressure (MAP), pulmonary artery pressure (PAP), left atrial pressure, and cardiac output (CO). After baseline measurements, the nitric oxide synthase inhibitor N-G-nitro-L-arginine methyl ester (30 mg/kg iv) was administered. Pulmonary hypertension was produced by the continuous infusion of U46619, a thromboxane A(2) mimetic. INO (40 ppm) was added to the inspired gas, and hemodynamic measurements were obtained before and after INO. Milrinone was administered sequentially as a 30-mg/kg bolus followed by a 3-mu g/kg/min infusion, a 100-mg/kg bolus followed by a 10-mu g/kg/min infusion, and a 300-mg/kg bolus followed by a 30-mu g/kg/min infusion (M3). Hemodynamic measurements were obtained with and without INO at each dose of milrinone. Measurements and Main Results: During U46619-induced pulmonary hypertension, INO decreased PAP and pulmonary vascular resistance (PVR) but did not affect MAP, systemic vascular resistance (SVR), or CO. Milrinone dose dependently decreased PAP, PVR, MAP, and SVR and increased CO. At each dose of milrinone, INO further decreased PVR but not SVR. M3 decreased PVR 49%, and the addition of INO decreased PVR an additional 19% so that PAP and PVR decreased to baseline values. Conclusions: Milrinone and INO both decrease pulmonary hypertension individually, end the combination produces additive effects. Combination therapy may produce potent and selective pulmonary vasodilation during the treatment of pulmonary hypertension.