Intrathecal diamorphine: a dose-response study.

A randomised double-blind study compared the dose-response relationship of intrathecal diamorphine (0, 0.25, 0.75, 1.5, and 2.5 mg) for postoperative pain relief, in 35 subjects who underwent total knee replacement surgery. Assessments commenced 2 h after the opioid injection and continued for 20 h. Pain, analgesic effect, supplementary analgesic requirements and adverse effects were noted. Intrathecal diamorphine was unable to delay the initial perception of discomfort. It was, however, capable of postponing the onset of severe pain requiring analgesic supplementation (control 5.25 h vs approximately 8 h: P less than 0.05). There was no significant difference in the quality of analgesia between the groups. Pruritus was the only undesirable feature unique to intrathecal diamorphine administration. Intrathecal diamorphine was safe and was not associated with clinically apparent respiratory depression. Its effects were inconsistent and its use was associated with irritating side effects. Possible explanations for the erratic behaviour of the diamorphine are discussed.