Unpredictability of regression of analgesia during the continuous postoperative extradural infusion of bupivacaine.

Twenty-four otherwise healthy patients scheduled for elective major abdominal surgery received general anaesthesia plus lumbar extradural analgesia. A loading dose of 0.5% plain bupivacaine was given to produce sensory analgesia (pin prick) from T4 to S5 and followed by a continuous infusion of 0.5% plain bupivacaine 8 ml h-1. Pain, scored on a 5-point scale, and sensory analgesia were assessed hourly for 16 h after skin incision. If sensory analgesia decreased by more than 5 segments from its preoperative level, or if the pain score reached 2 (moderate pain), the patients were removed from the study, and pain was treated otherwise. Only three patients maintained their initial levels of sensory analgesia and a pain score of less than 2. In the remaining patients sensory analgesia decreased at least 5 segments or pain score reached 2 between 4 and 16 h after skin incision. We found a weak correlation between increasing age and the duration of sensory analgesia (r = 0.46, P less than 0.05), but no significant correlations between duration of sensory analgesia and sex, weight, height, body surface area, serum albumin concentration, duration or site of operation.