Relationship between pain severity and serum beta-endorphin levels in postoperative patients

To examine the relationship among postoperative pain severity, serum beta-endorphin level, and serum morphine level in pediatric patients after posterior spinal fusion with instrumentation.

A prospective study.

University-based medical center.

Ten patients age 13-17 years admitted for posterior spinal fusion with instrumentation.

Each subject was administered an initial dose of intravenous morphine 100 micrograms/kg, followed by a constant infusion of 50 micrograms/kg/hour. The primary physician was allowed to titrate the dosage as required to meet the patient's requirement for analgesia. Whole blood was obtained for the analysis of serum morphine and beta-endorphin levels preoperatively, after the initial morphine dose, 24 hours after initiation of the infusion, and before any change in dosage. At each blood sampling time, pain severity ratings were obtained from the subject, nurse, and parent using a 10-point linear scale.

No statistical difference between serum beta-endorphin values preoperatively and after the initial dose of morphine was observed; mean values were 68 and 60 pg/ml, respectively. The relationships between serum beta-endorphin level and pain scores were statistically significant only for self (subject) pain scores (p = 0.014, r = 0.30). Mean serum morphine level was 21.9 ng/ml for patients with self pain scores of 4 or less.

The clinical usefulness of serum beta-endorphin as a measure of pain severity was not established under the experimental conditions of this study.