The association between propofol-induced loss of consciousness and the SNAP index.

The SNAP is a processed electroencephalogram monitor that uses an algorithm based on low- and high-frequency spectral components to derive a SNAP index. In this study we sought to determine the relationship of the SNAP index with loss of consciousness in subjects receiving a bolus of propofol. Unpremedicated subjects were randomized to receive 1 of 11 doses of IV propofol (0, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, or 2.4 mg/kg; n=20 per group). The SNAP index was recorded when the subject became unconscious (end-point) or at 160 s after the injection. Sixty-five percent of subjects achieved the end-point (defined as the time at which the subject dropped a weighted syringe). The 50% effective dose for propofol was 0.97 mg/kg (95% confidence interval [CI], 0.86-1.07 mg/kg). The median awake SNAP index was 92 (range 78-99) and did not differ between subjects who reached the end-point and those who did not. The end-point SNAP index decreased from baseline in the subjects who dropped the syringe to a median of 76 (range, 57-94) at doses > or =1.0 mg/kg but was not different among doses. The index was not different from baseline at 160 s in subjects who did not reach the end-point. Binary logistic regression models predicted a SNAP index 95% effective dose for loss of consciousness of 71 (95% CI, 63-74) and 19 (95% CI, 16-22) for changes in SNAP index from baseline. The areas under the receiver operator characteristic curves for these models were 0.837 and 0.864. The SNAP index correlated with propofol-induced loss of consciousness. It appears to be a useful indicator of loss of consciousness and should be further investigated as a monitor of anesthesia depth.