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 apulmonary arterial occlusion method, we measured effective pulmonarycapillary 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 meanPAP and Pcap (P < 0.01), reflected in decreases in PVRa and PVRv (P <0.01). The longitudinal distribution of PVR (PVRa/PVRv) remained unchangedthroughout the experiment, indicating that HPV occurred equally in thearterial and venous segments and that milrinone-induced vasodilatationoccurred equally in both segments. During hypoxia, milrinone did notproduce an increase in cardiac output or a decrease in PaO2. Milrinone alsoproduced significant decreases in mean systemic arterial pressure (P <0.01) and systemic vascular resistance (P < 0.05) to a similar extent tothe decreases in mean PAP and PVR, suggesting no selective dilating effectof milrinone on the pulmonary vasculature. These results indicate that inHPV, milrinone decreased the vascular tone of both pulmonary arterial andvenous segments without increasing cardiac work or impairing pulmonaryoxygenation. This suggests a potential for use in patients suffering fromhypoxic pulmonary hypertension.