Pharmacokinetics of i.v. and rectal pethidine in children undergoing ophthalmic surgery.

We have studied the pharmacokinetics of i.v. and rectal pethidine in 20 children age 4-8 yr undergoing ophthalmic surgery. After i.v. administration, the clearance of pethidine was mean 10.4 (SD 1.7) ml kg-1 min-1, volume of distribution at steady state 2.8 (0.6) litre kg-1 and elimination half-life 3.0 (0.5) h. After rectal administration, plasma pethidine concentrations varied greatly and peak concentrations appeared late, at 147 (44) min. The mean systemic bioavailability after rectal administration was approximately 55%. Because the bioavailability of rectal pethidine varies greatly, this route is not encouraged in the management of acute pain.