Spinal anesthesia potentiates sedative drug effects. We speculated that an acute decrease in tonic afferent input by spinal anesthesia would decrease the level of consciousness and thereby increase susceptibility to sedative drugs. The experiments were approached with the assumption that the amount of sedative would reduce in the following order: first, the group without spinal anesthesia, second, the group with unilateral spinal anesthesia, and third, the group with conventional spinal anesthesia.

Patients in group 1 (n = 20) were given midazolam without spinal anesthesia and patients in group 2 (n = 20) received unilateral spinal anesthesia. Those patients who after 20 minutes had a sensory level of T9-11 and a motor block of 3 in dependent side, no sensory block, and a motor block of 0 in the non-dependent side were included. In group 3 (n = 20), patients received conventional spinal anesthesia, and those patients who after 20 minutes had a sensory level of T9-11 and a motor block of 3 were included. The amount of midazolam administered at 33µg/kg/min to a Bispectral Index score (BIS) of 80 was measured.

The dose of midazolam administered to a BIS of 80 was significantly lower in group 3 than in group 1 or group 2.

The sensitivity of sedatives was in the following order: the group with conventional spinal anesthesia, the group with unilateral spinal anesthesia, and the group not administered spinal anesthesia.