BACKGROUND: It has been reported that spinal anesthesia has a sedative effect and so this decreases the hypnotic requirement of intravenous anesthetic. Therefore, we have conducted a prospective randomized study to investigate the effect of the spinal anesthesia level on the hypnotic requirements for conscious sedation.

METHODS: Forty adult patients were scheduled to undergo spinal anesthesia, and they were randomly allocated to one of the two groups. After subarachnoid injection of 0.5% hyperbaric bupivacaine 16 mg, the patients in group 1 and group 2 were maintained in a reversed Trendelenburg position and a Trendelenburg position, respectively. After fifteen minutes, the target controlled infusion of propofol was started for achieving a target concentration of 1 µg/ml, and the mean BIS for 1 min was checked after an effect site concentration (Ce) of 1 µg/ml was reached. The target controlled infusion of propofol was restarted at a target concentration (Tc) of 1.5 µg/ml, and the mean BIS for 1 min was checked after the Ce level of 1.5 µg/ml was reached.

RESULTS: The mean BIS at 1 µg/ml Ce was 90.0 ± 8.5 and 77.8 ± 10.3 in group 1 and group 2, respectively. The mean BIS at 1.5 g/ml Ce was 73.6 ± 19.4 and 60.0 ± 13.1, respectively.

CONCLUSIONS: There was a significant difference in the requirements of propofol for conscious sedation between the below T12 block group and the above T4 block group.