STUDY OBJECTIVE:

To assess concentrations of morphine and its metabolites after patient-controlled analgesia (PCA).

DESIGN:

Pilot pharmacokinetic study of morphine and pharmacokinetic simulation.

SETTING:

Post-anesthesia care room and ward of an academic teaching hospital.

PATIENTS:

10 ASA physical status I, II, and III postoperative surgical patients.

INTERVENTIONS:

Patients received morphine via PCA by routine hospital protocols.

MEASUREMENTS:

The population mean plasma and effect-site concentrations of morphine, morphine-6-glucuronide (M6G), and morphine-3-glucuronide (M3G) was simulated in 4 patient group scenarios: morphine PCA used alone, morphine PCA used with continuous background morphine infusion of 0.5 mg/hr, morphine PCA used with continuous background morphine infusion of 1.0 mg/hr, and morphine PCA used with continuous background morphine infusion of 2.0 mg/hr.

MAIN RESULTS:

The 4 groups exhibited simulated peak morphine, M6G, and M3G effect-site concentrations at 8 to 24 hours post-infusion. The highest peak morphine, M6G, and M3G effect-site concentrations decreased in the following order by group: 2.0 mg/hr morphine infusion + PCA group, 1.0 mg/hr morphine infusion + PCA group, and 0.5. mg/hr morphine infusion + PCA group.

CONCLUSIONS:

Patients receiving morphine PCA should be monitored closely from 8 to 24 hours postoperatively. Morphine PCA given with background infusion rates up to 1.0 mg/hr does not offer distinct pharmacokinetic advantages over morphine PCA alone. Morphine PCA with background infusion rate of 2.0 mg/hr is associated with the greatest risk of respiratory depression.

Copyright © 2011 Elsevier Inc. All rights reserved.