Pharmacokinetics of fentanyl in the elderly

The pharmacokinetics of fentanyl were determined in seven elderly (71-82 yr) and seven younger adults (18-41 yr) anaesthetized with thiopentone, nitrous oxide in oxygen and morphine. Fentanyl was administered as a 2-min i.v. infusion at doses of 15 micrograms kg-1 for elderly patients and 20 micrograms kg-1 for the younger patients. Plasma samples were obtained for 4 h and fentanyl concentrations determined by radioimmunoassay. Fentanyl concentration, per microgram kg-1 administered, was higher in elderly than in young patients at 2 min (7.73 +/- 3.14 v. 4.54 +/- 1.83 ng ml-1 (mean +/- SD), respectively) and at 4 min after the start of infusion (3.26 +/- 1.44 v. 1.78 +/- 0.72 ng ml-1, respectively). Concentrations were similar at all other sampling times. Pharmacokinetic variables were determined by non-compartmental techniques. Total plasma clearance was similar for the two age groups. Volume of distribution at steady-state (VDss) was smaller in elderly patients (1.36 +/- 0.44 v. 2.27 +/- 0.82 litre kg-1). Despite age-related changes in VDss, plasma fentanyl concentrations for the two groups were similar throughout the 4-h sampling period except immediately following administration. These results suggest that, if there are age-related differences in response to fentanyl, the likely pharmacokinetic explanation is the higher concentration of fentanyl in the elderly immediately following its administration.