Intravenous fentanyl kinetics

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Abstract
Fentanyl is considered to be a short-acting narcotic analgesic but prolonged and recurrent ventilatory depression was reported. Fentanyl kinetics and excretion in 7 healthy male subjects who were given a 3.2 or 6.4 .mu.g/kg dose of 3H-fentanyl i.v. was studied. Arterial blood and urine samples were analyzed for unchanged fentanyl and total radioactivity. Fentanyl concentrations fell rapidly and 98.6% of the dose was eliminated from plasma in 60 min, but the terminal elimination phase of fentanyl from the body was slow (t 1/2.beta. [elimination half life] = 219 min) due to te slow return of the unchanged drug from a peripheral compartment to the central compartment where elimination occurred primarly by biotransformation. Of the dose, 85% was recovered in urine and feces in 72 h; < 8% was recovered as unchanged fentanyl. There were fluctuations in plasma fentanyl levels during the elimination phase in all cases. The long t 1/2.beta. and fluctuations in plasma levels may contribute to prolonged and recurrent ventilatory effects of fentanyl.