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

We have studied the effect of milrinone on pulmonary vascular resistance (P VR) in dogs with hypoxic pulmonary vasoconstriction (HPV). Using a pulmonar y arterial occlusion method, we measured effective pulmonary capillary pres sure (Pcap) by which total PVR was partitioned into arterial (PVRa) and ven ous (PVRV) components. Hypoxic ventilation (FIO2=0.11-0.13) produced signif icant 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 pe riod, milrinone significantly decreased mean PAP and Pcap (P<0.01), reflect ed in decreases in PVRa and PVRv (P<0.01). The longitudinal distribution of PVR (PVRa/PVRv) remained unchanged throughout the experiment, indicating t hat HPV occurred equally in the arterial and venous segments and that milri none-induced vasodilatation occurred equally in both segments. During hypox ia, milrinone did not produce an increase in cardiac output or a decrease i n Pa-O2. Milrinone also produced significant decreases in mean systemic art erial pressure (P<0.01) and systemic vascular resistance (P<0.05) to a simi lar extent to the decreases in mean PAP and PVR, suggesting no selective di lating effect of milrinone on the pulmonary vasculature. These results indi cate that in HPV, milrinone decreased the vascular tone of both pulmonary a rterial and venous segments without increasing cardiac work or impairing pu lmonary oxygenation. This suggests a potential for use in patients sufferin g from hypoxic pulmonary hypertension.