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TECHNOLOGY, COMPUTING, AND SIMULATION

The Association Between Propofol-Induced Loss of Consciousness and the SNAP^TM Index

Department of Anesthesiology, Northwestern University Feinberg School of Medicine, Chicago, Illinois

Address correspondence to Cynthia A. Wong, MD, Department of Anesthesiology, Northwestern University Feinberg School of Medicine, 251 E. Huron St., F 5-704, Chicago, IL 60611. Address e-mail to c-wong2{at}northwestern.edu Reprints will not be available from the authors.

The SNAP^TM is a processed electroencephalogram monitor that uses an algorithm based on low- and high-frequency spectral components to derive a SNAP^TM index. In this study we sought to determine the relationship of the SNAP^TM 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^TM 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^TM 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^TM index decreased from baseline in the subjects who dropped the syringe to a median of 76 (range, 57–94) at doses 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^TM index 95% effective dose for loss of consciousness of 71 (95% CI, 63–74) and 19 (95% CI, 16–22) for changes in SNAP^TM index from baseline. The areas under the receiver operator characteristic curves for these models were 0.837 and 0.864. The SNAP^TM 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.

IMPLICATIONS: The SNAP^TM is a processed electroencephalogram monitor that uses an algorithm based on low- and high-frequency spectral components to derive the SNAP^TM index. The SNAP^TM index correlated with propofol-induced loss of consciousness.

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