Milrinone decreases both pulmonary arterial and venous resistances in the hypoxic dog

We have studied the effect of milrinone on pulmonary vascular resistance (PVR) in dogs with hypoxic pulmonary vasoconstriction (HPV). Using a pulmonary arterial occlusion method, we measured effective pulmonary capillary pressure (Pcap) by which total PVR was partitioned into arterial (PVRa) and venous (PVRV) components. Hypoxic ventilation (FIO2=0.11-0.13) produced significant increases in mean pulmonary arterial pressure (PAP) and Pcap (P<0.01) associated with increases in PVRa and PVRv (P<0.01). During the hypoxic period, milrinone significantly decreased mean PAP and Pcap (P<0.01), reflected in decreases in PVRa and PVRv (P<0.01). The longitudinal distribution of PVR (PVRa/PVRv) remained unchanged throughout the experiment, indicating that HPV occurred equally in the arterial and venous segments and that milrinone-induced vasodilatation occurred equally in both segments. During hypoxia, milrinone did not produce an increase in cardiac output or a decrease in Pa-O2. Milrinone also produced significant decreases in mean systemic arterial pressure (P<0.01) and systemic vascular resistance (P<0.05) to a similar extent to the decreases in mean PAP and PVR, suggesting no selective dilating effect of milrinone on the pulmonary vasculature. These results indicate that in HPV, milrinone decreased the vascular tone of both pulmonary arterial and venous segments without increasing cardiac work or impairing pulmonary oxygenation. This suggests a potential for use in patients suffering from hypoxic pulmonary hypertension.