TITLE: The use of non-depolarizing muscle relaxants during lumbar laminectomy decreases postoperative morphine requirements

Introduction Information on the influence of non-depolarizing muscle relaxants during surgery on postoperative pain is not available. We tested the hypothesis that use of non-depolarizing muscle relaxants during lumbar laminectomy surgery reduces pain relief requirements postoperatively. Methods After Institutional Review Board approval, 42 adult ASA physical status I and II patients were enrolled in this study. All patients were scheduled to undergo a first time lumbar laminectomy and excision of herniated intervertébral disc. Exclusion criteria were recurrent disc disease after a previous surgical correction, chronic pain states other than back pain, chronic intake of narcotic pain relief medications other than codeine, and any lumbar spine surgery six months prior to the present surgery. None of the surgeries of this study involved fusions, hardware placements, or bony graft harvesting. In all patients, anesthesia was induced with sodium thiopental and maintained with nitrous oxide, oxygen and isoflurane. Intubation was performed after administration of mivacurium chloride (0.5 mg/kg, i.v. over 1 min). After placing the patients in the prone position, they were randomly assigned into one of two groups: 1) to receive muscle relaxants during surgery or 2) no muscle relaxants during surgery. In patients of both groups, complete spontaneous recovery of all muscle twitches from the intubating dose of mivacurium occurred before surgical incision as evidenced by monitoring the train of four and post tetanic stimulation with a peripheral nerve stimulator. Patients who were randomized to receive muscle relaxants during surgery received vecuronium bromide (VEC) to provide muscle relaxation during the entire duration of surgery. Patients who were not to receive muscle relaxants during surgery received only the intubating dose of mivacurium chloride without any further muscle relaxants. Fentanyl (2 ug/kg, i.v.) was administered at the conclusion of surgery to all patients. No other narcotics were administered during surgery. Muscle relaxation was reversed with neostigmine and glycopyrolate in patients who received VEC. In the recovery room, morphine was administered i.v. for pain relief as needed. In the ward, all patients had access to morphine via a PCA device with standardized administration. The duration of the study was 24 hours or until the PCA was discontinued upon the patient's requests based on absence of need for further analgesia. No other analgesics were used during the study period. An observer who was unaware of the nature of the anesthetic recorded the amount of morphine administered during the study period. All participating patients were also blinded to whether non-depolarizing muscle relaxants were used or not during surgery. Data were analyzed with unpaired t test. P<0.05 was considered significant. Results Results are expressed as mean ±SD. There were no significant differences in demographic variables between patients who received muscle relaxants and those who didn't (age 53+15 vs. 49+16 years; weight 84±26 vs. 81 ±21 kg; P>0.05). The median number of surgical intervertébral spaces was similar in both groups 1.0 vs. 1.0). Morphine intake was significantly less in the patients that received VEC during surgery (39±17 vs. 53±19, p=0.017). Discussion The major finding of this study is that patients who received VEC during lumbar laminectomy surgery consumed 36% less of morphine sulfate vs. patients who did not receive VEC. This implies that the use of muscle relaxants reduced postoperative pain in this population of patients. The mechanisms of action of VEC in reducing postoperative pain are not clear. We postulate that lumbar muscles of patients who received non-depolarizing muscle relaxants are subjected to less bruising during spreading with metallic retractors to expose the operative lumbar laminae.