Meperidine, Serum or Plasma

For therapeutic monitoring only (not for forensic purposes). Meperidine, a phenylpiperidine derivative, is a synthetic opioid analgesic. Many of its pharmacologic properties and indications are similar to those of morphine, but meperidine has no effect on cough and is much less constipating. It is ¹/₈ as potent as morphine on a weight basis when administered parenterally. Meperidine is about ¹/₃ to ¹/₄ as potent orally as parenterally.

This analgesic is widely used in anesthetic premedication, in balanced anesthesia, and in obstetric analgesia. Meperidine is preferred to morphine for obstetric use because its rapid onset of action and shorter duration usually permit greater flexibility in maternal analgesia, possibly with less effect on neonatal respiration. Nevertheless, it can produce significant respiratory depression in the newborn infant proportional to the fetal blood concentration. This can be minimized by giving small incremental doses of 25 mg intravenously during labor.

The maximal analgesic effect occurs 30 to 50 minutes after intramuscular injection. The duration of action (two to four hours) is shorter than that of morphine. Results of studies on the relationship between blood concentration of meperidine and analgesic response have indicated that 700 ng/mL would relieve severe pain in 95% of nontolerant patients. Because absorption varies among individuals following intramuscular administration, blood concentrations adequate to produce analgesia may not be achieved with usual doses in about 5% of patients. It has been suggested that intravenous infusion with blood concentration determinations may be an effective approach to assure pain relief more rapidly, but there are practical restrictions to this. These observations are invaluable, however, for having shown the need for dose adjustment based on the response of the individual.