[No loss of analgesic effect by peridural low volume morphine administration]

The risk of respiratory depression after epidural bolus injections of morphine can be reduced by lowering the injection volume of bolus epidural injections. The aim of the investigation was to evaluate the analgesic effectiveness of low-volume epidural injections of morphine and to determine the relationship between morphine consumption and the time of commencement of treatment by epidural, on-demand, low-dose infusion of morphine for constant pain relief after abdominal operations. Two mg morphine were administered immediately prior to the epidural infusion at the initial basal rate of 0.16 mg/0.06 ml per hour. The sixteen patients of Group I received an initial morphine bolus injection in a 10 ml volume of saline after complaining of worst post-surgical pain. The 34 patients that comprised Groups II and III in a randomised investigation received the initial 2 mg morphine in 1 ml saline. The treatment of Group II (n = 14) commenced when the patients complained of worst pain, in Group III (n = 20) after the patients had been extubated and were awake but before they complained of pain. Mean morphine consumption of the three groups until 8 a.m. on the first postoperative day did not differ significantly. We may, therefore, conclude that reduction of the injection volume does not result in loss of analgesic effectiveness, and that the treatment should commence as early as possible so as to spare the patient's pain. If the epidural catheter's tip is placed in the spinal area of the maximum nociceptive input, low-volume administration of morphine should be definitely employed.