Epidural morphine strongly depresses nociceptive flexion reflexes in patients with postoperative pain

The comparative effects of low doses (0.03-0.04 mg/kg) of epidural morphine on a nociceptive flexion reflex of the lower limb and on postoperative pain in volunteer patients were studied after orthopedic surgery on one knee. According to the stimulation parameters, it was found that 40-50 min after the injection, morphine produced an increase of 87% and 83% of the reflex threshold and of the threshold of maximal reflex response, respectively, as well as a 80-90% depression of the nociceptive responses when elicited by a constant level of stimulation. Onset of pain relief occurred by the 25th min and increased to a maximum stable level 40-50 min after the injection. These data support the hypothesis that the main site of the pain-relieving effect of epidural morphine is located directly at a spinal level.