Decrease in vascular resistance in the isolated canine hindlimb after graded doses of alfentanil, fentanyl, and sufentanil.

Under halothane anesthesia five dogs were prepared with both hindlimbs isolated from the systemic circulation to allow intermittent placement on extracorporeal perfusion at constant flow. One limb of each dog was surgically denervated. In this relatively anesthetic-free preparation, graded equivalent doses of alfentanil, fentanyl, and sufentanil were infused over 30 s, and vascular resistance was measured. Increasing opioid administration caused a progressive diminution in peripheral resistance. By the high dose level, alfentanil (500 micrograms/kg), fentanyl (50 micrograms/kg), and sufentanil (6 micrograms/kg) caused equal and significant decreases of 48%, 48%, and 44% in resistance, respectively. There was no difference among the opioids in effects on resistance at equivalent dosages. Neither pretreatment with naloxone nor denervation changed the response to the narcotics. We conclude that the three synthetic opioids produce vasodilation by direct action on the peripheral vascular smooth muscle.