The influence of high spinal anesthesia on sensitivity to midazolam sedation.

We tested whether a high spinal anesthesia may alter the susceptibility to the soporific effects of sedatives. Twenty ASA grade I and II women undergoing elective abdominal hysterectomy were randomly allocated into two groups. Patients in Group I were given a subarachnoid injection of 12 mg hyperbaric tetracaine and those patients who after 10 min had a sensory level of T4-6 (10 patients) were included in the study. Ten additional patients (Group II) received no spinal injection. Induction of anesthesia was performed on all patients by injecting 1 mg of midazolam intravenously every 30 s until the patient failed to respond to three repeated commands to squeeze the anesthetist's hand. This was considered the induction dose or end-point for the purposes of the study. Patients were then given a neuromuscular blocker, ventilated with oxygen, nitrous oxide, and a volatile anesthetic, tracheally intubated, and maintained under general anesthesia for the remainder of the operation. The dose of midazolam administered to the point of patient failure to respond to command was 7.6 +/- 0.72 mg SEM for Group I and 14.7 +/- 1.16 mg SEM for Group II, (P < 0.0001). These results support the conclusion that patients having a high spinal anesthetic are more sensitive to the sedative effects of midazolam.