Thirty-nine patients undergoing lumbar spinal fusion , stratified for sex and one - or two-level fusion , were randomized to receive a continuous postoperative epidural infusion of either 0.1 % bupivacaine with 15gmL ( -1 ) hydromorphone ( LA group ) or 0.9 % saline ( NS group ) at 6mLhr ( -1 ) for 48hr through an epidural catheter placed intraoperatively .