The effects of morphine on the resistance and capacitance vessels of the peripheral circulation

The effects of morphine on total peripheral vascular resistance and the capacity of the peripheral vascular compartment were determined in normal dogs. In 13 animals, in which systemic flow was maintained constant, morphine (1 mg. per kilogram) caused immediate decreases in resistance averaging 46 ± 20 per cent of the control values. The decrease in resistance was transient, and normal or increased resistance was noted 30 minutes after the drug was administered. This effect of morphine on resistance vessels was indirect, however, since no immediate change in pressure occurred when morphine was injected into an isolated perfused vascular bed. In 13 dogs, studied during cardiopulmonary bypass, the capacity of the total peripheral vascular compartment increased after the administration of morphine. Blood volume rose an average of 11 ± 6 ml. per kilogram, and systemic venous return, measured with a flowmeter, rapidly decreased after the injection. Venous tone, determined by major-vessel occlusion in 8 dogs, decreased from 6.8 ± 1.3 to 3.5 ± 1.1 cm. H₂O after morphine (0.5 mg. per kilogram) was given. The measured changes in both capacitance and venous tone occurred promptly and persisted throughout a 30-minute period of observation. The experiments indicate that increased capacitance and venous pooling are important effects of morphine on the circulation which have not been well recognized.