Mechanisms of action of morphine in the treatment of experimental pulmonary edema

The hemodynamic effects of morphine were correlated with the course of acute pulmonary edema in 18 dogs. Pulmonary edema was produced by various technics designed to simulate the clinical cardiovascular disorders in which pulmonary edema most commonly occurs. Morphine (0.5 mg./kg.) resulted in subsidence of pulmonary edema in all animals, and improvement was concomitant with striking and parallel decreases in pulmonary arterial flow and pressure and in left atrial and left ventricular enddiastolic pressures. Total pulmonary vascular resistance was measured before and after morphine administration in 12 normal dogs. A preparation was utilized in which left atrial pressure and pulmonary and systemic arterial flows were maintained constant. A slight to moderate decrease in resistance usually occurred, but the changes were variable and not of significance. The experiments indicate that the beneficial effects of morphine in pulmonary edema may be principally attributed to the effects of the drug on the capacitance vessels of the peripheral circulation. Capacity of the peripheral vascular bed is increased, systemic venous return is reduced, and improvement results from a "pharmacologic phlebotomy.".