SENSORY CHANGES AND PAIN AFTER ABDOMINAL HYSTERECTOMY - A COMPARISON OF ANESTHETIC SUPPLEMENTATION WITH FENTANYL VERSUS MAGNESIUM OR KETAMINE

Drugs interacting with opioid or N-methyl-D-aspartate (NMDA) receptors may have differing effects on post surgical sensory changes, such as central inhibition or spinal excitation. We compared the effect of sup plementing isoflurane/N2O/O-2, anesthesia with an opioid agonist (fent anyl [n = 15]) or two drugs inhibiting the NMDA system differently (ma gnesium, ketamine [n = 15 in each group]) on sensory changes after abd ominal hysterectomy. Electric sensation, pain detection, and pain tole rance thresholds were determined (preoperatively and 1, 4, 24h, and 5 days postoperatively) in arm, thoracic, incision, and leg dermatomes t ogether with pain scores and cumulative morphine consumption. Threshol ds relative to the arm were derived to unmask segmental sensory change s hidden by generalized changes. Absolute thresholds were increased 1- 24 h, returning to baseline on Day 5, without overall differences amon g drugs. Fentanyl thresholds were lower Ih and higher 5 days postopera tively compared with magnesium and ketamine; thresholds were lower at 24 h for magnesium versus ketamine. Relative thresholds increased comp ared with baseline only with fentanyl (1-4 h); none decreased. Pain sc ores and morphine consumption were similar. Thus, all adjuvants suppre ssed spinal sensitization after surgery. Fentanyl showed the most, and magnesium the least, central sensory inhibition up to 5 days postoper atively, with different patterns of inhibition directly postsurgery ve rsus later. Differences in sensory processing were not reflected in cl inical measures. Implications: We studied the effects on postsurgical sensory processing of general anesthesia supplemented by drugs affecti ng opioid or N-methyl-D-aspartate receptors using sensory thresholds. Generalized central sensory inhibition, differently affected by the dr ugs, predominated after surgery. All drugs suppressed spinal excitatio n. Clinical pain measures did not reflect sensory change.