Pharmacokinetics of meperidine in spinal anaesthesia.

Five male patients undergoing haemorrhoidectomy received intrathecal meperidine 1 mg X kg-1 as the sole anaesthetic agent. Plasma concentration-time profiles were investigated. The peak plasma concentration of meperidine was 175 +/- 78.8 ng X ml-1 (mean +/- SD) and this occurred 90 minutes after intrathecal injection. The plasma concentrations generally were lower than those necessary for systemic analgesic effects. The terminal elimination half life of meperidine (t 1/2 beta) in the plasma was 198 minutes. Intrathecal meperidine produced good surgical anaesthesia in all patients studied. The mean duration of sensory and motor block was 77 +/- 18.8 and 47 +/- 7.4 minutes respectively. Four patients did not require any analgesic supplement during the postoperative period. No patient developed clinically evident respiratory depression or neurological sequelae. The pharmacokinetic data suggests that intrathecal meperidine provides prolonged postoperative analgesia through a regional effect on opioid receptors in the spinal cord.