Neonates born @ weeks postmenstrual age received either tramadol -LSB- T -RSB- @ mgkg ( -@ ) or placebo -LSB- P -RSB- @-hourly for up to @ days postthoracoabdominal surgery in addition to morphine ( commenced at @ mcgkg ( -@ ) h ( -@ ) ) and @-hourly i.v. acetaminophen .