Background and Objectives The purpose of the study is to determine the ideal concentration of morphine when given with bupivacaine as a continuous high thoracic epidural infusion for postthoracotomy pain.

Methods In a prospective study, 60 patients scheduled for thoracic surgery received a high thoracic epidural catheter. Postoperative analgesia was provided by a continuous epidural infusion for 3 days. The patients were randomly divided into two groups: group 1 (loading dose 1 mg morphine epidurally and continuous infusion of bupivacaine 0.75% + 0.2 mg/mL morphine at an infusion rate of 0.8 mL/hr); group 2 (loading dose 0.5 mg morphine epidurally and continuous infusion of bupivacaine 0.75% + 0.1 mg/mL morphine at an infusion rate of 0.8 mL/hr).

Results The visual analog scales were not different at rest but with exercise in group 1 there was better pain relief than in group 2. The number of patients requiring supplementation of analgesia in group 2 (n = 42) was six times that of group 1 (n = 7). PaCO₂ increased in both groups during the first postoperative day. There was no difference in the incidence of side effects between the two groups.

Conclusions Continuous high thoracic epidural administration 0.2 mg/mL morphine in bupivacaine 0.75% at an infusion rate of 0.8 mL/hr with a loading dose of 1 mg morphine is an effective dose for postthoracotomy pain relief in rest, and more important, during exercise.