Spinal anaesthesia with midazolam in the rat

Purpose: This study examined in an animal model whether intrathecal midazolam, alone or with fentanyl, can achieve anaesthesia sufficient for laparotomy, comparable to lidocaine. Effects on consciousness and whether anaesthesia was segmental were also examined. The haemodynamic and respiratory changes were compared with those of intrathecal lidocaine or intrathecal fentanyl alone. Methods: Sixty Wistar strain rats, with nylon catheters chronically implanted in the lumbar subarachnoid theca, were divided into six groups. Group I (n = 12) received 75 mu L intrathecal lidocaine 2%. Group 2 (n = 12) received 75 mu L intrathecal midazolam 0.1%, Group 3 (n = 12) received intrathecal 37.5 mu L midazolam 0.1%, plus 37.5 mu L fentanyl 0.005%. Group 5 (n = 6) received 75 mu L midazolam 0.1% iv. Group 6 (n = 6) received halothane 0.6% in oxygen by inhalation. Results: Both groups that received intrathecal midazolam, alone or combined with fentanyl, developed effective segmental sensory and motor blockade of the hind limbs and abdominal wall, sufficient for a pain-free laparotomy procedure. Neither of these groups, unlike the group that received intrathecal lidocaine, developed a reduction in blood pressure or change in heart rate at the time of maximal sensory or motor blockade, nor were there changes in the arterial blood gases or respiratory rate. Conclusion: Midazolam, when injected intrathecally, produces reversible, segmental, spinally mediated antinociception, sufficient to provide balanced anaesthesia for abdominal surgery.