Morphine requirements using patient-controlled analgesia: influence of diurnal variation and morbid obesity

The influence of diurnal variation and morbid obesity on the morphine requirements of patients using patient-controlled analgesia (PCA) was studied. Forty-six patients undergoing either elective (Group I) or gastric bypass (Group II) surgery composed the study group. Patients were allowed to use the PCA machine for 36 to 72 hours postoperatively to deliver 0.6 mg/sq m doses of morphine sulfate intravenously. Counters on the device indicating the number of doses received were monitored every two hours along with vital signs and a pain and sedation rating. Dosing rates were studied with regard to the time of day. Morbid obesity had no significant effect on dosing rate requirements. Analyses revealed a significant diurnal rhythm in morphine dosing rate requirements despite a 27-fold interpatient variability. Peak analgesic use was at 0900, and the nadir was at 0300. Patient pain rankings did not indicate any changes in pain status, demonstrating PCA's use of analgesia as an endpoint for dosing. Sedation status showed a higher degree of sedation at night when compared with the daytime values, documenting that sedation was not drug-induced. There is a circadian variation in narcotic analgesic need in the postoperative patient that is met appropriately by PCA.