Bispectral index is related to the spread of spinal sensory block in patients with combined spinal and general anaesthesia.

BACKGROUND A relationship between the depth of sedation as measured by the bispectral index (BIS) and spinal sensory block height in patients with light to no additional sedation has been described previously. The present study was designed to investigate the hypothesis that BIS values closely correlate with the spread of spinal sensory block in patients deeply sedated with an i.v. target-controlled infusion of propofol. METHODS Subjects comprised 100 patients aged 20-64 yr and undergoing arthroscopic knee surgery. Patients were given spinal anaesthesia with bupivacaine 0.5% (3 ml). Propofol was administered to achieve a target effect-site concentration of 3.0 μg ml⁻¹. The relationship between the spinal sensory level at 15 min after spinal anaesthesia and BIS values during 1-5, 6-10, 11-15, and 16-20 min time intervals after the estimated effect-site concentration reached 3.0 μg ml⁻¹ was evaluated. RESULTS The sensory level of spinal analgesia significantly and strongly correlated with BIS values during each time period after the estimated effect-site concentration remained at 3.0 μg ml⁻¹ (P<0.0001). The correlation coefficient values were 0.8 during 1-5 min, 0.844 during 6-10 min, 0.801 during 11-15 min, and 0.804 during 16-20 min time periods. CONCLUSIONS We demonstrated that BIS values significantly correlate with the level of spinal sensory block under deep sedation with propofol. The depth of sedation induced by spinal anaesthesia depends on the spread of spinal sensory block.