Auditory evoked potential index does not correlate with observer assessment of alertness and sedation score during 0.5% bupivacaine spinal anesthesia with nitrous oxide sedation alone.

Purpose: The aim of this study was to evaluate the auditory evoked potential (AEP) index as a hypnosis monitor during nitrous oxide (N(2)O) sedation added to spinal analgesia.

Methods: Forty-five patients scheduled to undergo surgery under spinal anesthesia were recruited after giving informed consent. Adequate anesthesia levels were confirmed, and a disposable AEP index sensor (aepEX, Medical Device Management) was placed. A tight facemask was fitted, and a fresh gas flow of 100% oxygen 10 L/min was provided. AEP index monitoring was then initiated, and measurements and observer assessment of alertness/sedation (OAA/S) scores were recorded manually. N(2)O was administered in stepwise increases in the end-tidal concentration of 33%, 50%, and 67%. Paired AEP index and OAA/S scores were obtained immediately before each change in N(2)O concentration.

Results: Sixteen patients were excluded from final analysis because of nausea, vomiting, or abnormal excitatory behaviors. The increases in N(2)O concentration induced significant decreases in OAA/S scores and no substantial AEP index changes. Although OAA/S scores of 1 and 2 were observed in only two and five patients, respectively, a reduction in the OAA/S score from 5 to 1 was associated with a significant decrease in AEP index to the level indicative of moderate sedation.

Conclusion: The AEP index might not be a suitable indicator of light hypnosis as defined by an OAA/S score of ≥3 during sedation with N(2)O alone.