Epidural anesthesia has been shown to have a direct sedative effect, and to markedly reduce the hypnotic requirement of intravenous anesthetics. The purpose of this study was to investigate whether the level of sensory block by epidural anesthesia is related with the degree of sedation, and to the hypnotic requirement of propofol. In addition, we tested whether bispectral index (BIS) monitoring can quantify the sedative effects of epidural anesthesia.

Thirty two patients scheduled for elective upper abdominal or lower extremity surgery were allocated into two groups. 2% lidocaine 15 ml was administered through an epidural catheter at the T₉ T₁₀ level (group T, n = 16) or at the L₃-L₄ level (group L, n = 16). Observer's assessment of alertness/sedation (OAA/S) and BIS were evaluated 20 min after the epidural injection. Hypnotic doses of propofol were then determined by evaluating the loss of open eyes in response to a verbal command as an end point while propofol was administered by target controlled infusion (target effect concentration 5 µg/ml, induction time 3 min). At the time of induction of hypnosis, the target effect concentration and BIS were recorded.

The average level of sensory block was up to T₃ in group T, and up to T₁₁ in group L. Compared with the group L, more decreases were noticed in group T in terms of the hypnotic dose of propofol (1.13±0.31 mg/kg vs. 1.67±0.36 mg/kg; P = 0.03). A decrease in the OAA/S scale from 5 to 4 was more frequently noticed in group T than in group L (93.7% vs. 12.5%, P < 0.05). BIS recorded at the time of induction of hypnosis was higher in group T than in group L (94.8±2.8 vs. 82.6±2.2; P < 0.001).

A higher sensory block observed in group T than in group L induced a more profound sedative effect and hypnotic induction with a lesser dose of propofol. These results support the condition that the level of sensory block by epidural anesthesia affects the degree of sedation. Moreover, BIS monitoring could not quantify the sedative effect induced by epidural anesthesia.