Objectives: 1) To determine whether preanesthetic intrathecal administration of 0.5 mg morphine reduces isoflurane requirements for anesthetic maintenance. 2) To assess the duration of postoperative analgesia and the type and frequency of complications attributable to the procedure.
Patients and methods: A series of 45 adults were distributed into 3 groups of 15 patients each based on site of surgery and site of preanesthetic (30 min) injection of 0.5 mg pure morphine. Control group (C0) patients underwent lumbar surgery and received subcutaneous morphine. Group C0.5 patients also underwent lumbar surgery but received intrathecal morphine. Group A0.5 patients underwent long-duration high abdominal surgery and received intrathecal morphine. Anesthesia was maintained with nitrous oxide (60%) in oxygen (40%) and a variable concentration of isoflurane. Isoflurane needs were assessed by averaging six consecutive measurements of end-tidal isoflurane pressure (M30FETiso) taken at intervals of 5 min. Postoperative analgesia was evaluated by means of a visual analog scale that was converted to numerical units (VASn).
Results: M30FETiso in group C0 (0.8%) was always higher (p < 0.01) than in the other two groups. M30FETiso in group A0.5 was higher (p < 0.01) than in group C0.5 during the first 150 min of surgery. After 180 min, there were no differences in M30FETiso (0.10-0.16%) between the two groups receiving intrathecal morphine. VASn results (mean +/- SD) in the first 4 hours were higher in group C0 (7.33 +/- 0.6) than in group C0.5 (1.13 +/- 0.35) and group A0.5 (1.07 +/- 0.26). The time of morphine-dependent analgesia was shorter (p < 0.01) in group C0 (0.62 +/- 0.38 hours) than in groups C0.5 (30.4 +/- 5.11 hours) and A0.5 (28 +/- 4.34 hours). There were no significant differences between the two groups receiving intrathecal morphine.
Conclusions: Preanesthetic subarachnoid lumbar injection of 0.5 mg of pure morphine reduced early requirements for isoflurane in lumbar surgery (0.14% after 60 min). This reduction was initially less in patients undergoing abdominal surgery (0.44% at 60 min) but was the same after 150 min. Postoperative analgesia was long-term and independent of type or duration of surgery. There was no respiratory depression after surgery and the incidence of postoperative complications was similar in the two groups that received subarachnoid morphine.