Vascular Mechanisms

Morphine selectively impairs certain sympathetic reflexes involving peripheral veins. This response is due to a CNS action of the drug with reduction of sympathetic tone. Vasodilatation after morphine may also result from a direct effect of morphine on vascular smooth muscle. At high doses, morphine inhibits the presynaptic release of norepinephrine in saphenous vein rings and results in a decreased contractile response to electrical stimulation. Pharmacologic studies evaluating alfentanil, fentanyl, and sufentanil in the dog demonstrate direct peripheral vessel smooth muscle relaxation.^[216]