Measuring depth of sedation with auditory evoked potentials during controlled infusion of propofol and remifentanil in healthy volunteers

Avoiding excessively deep levels of sedation is a major problem in intensive care patients. We studied whether clinically relevant levels of sedation can be objectively assessed using long latency auditory evoked potentials. We measured the auditory evoked potentials at 100 ms after the stimulus (N100) in 10 healthy volunteers during stepwise increasing, clinically relevant levels of sedation (Ramsay score [RS] 2–4). The volunteers were studied on three separate occasions and received an infusion of either propofol or a combination of propofol and remifentanil. Effects of remifentanil infusion alone were tested during target controlled infusion (target plasma concentrations: 1, 2, and 3 ng/mL). Remifentanil did not affect evoked potential amplitudes and latencies. During both propofol-induced and propofol/remifentanil-induced sedation, the N100 amplitude decreased similarly without an effect on the latencies as the level of sedation increased from Ramsay score 2 to Ramsay score 4 (P < 0.01). At the same clinical level of sedation, propofol plasma concentrations were larger when sedation was achieved by propofol alone (propofol versus propofol/remifentanil, RS 3: 2.12 μg/mL ± 0.51 versus 1.32 ± 0.43, P < 0.01; RS 4: 3.37 ± 0.47 versus 1.86 ± 0.34, P < 0.01). Our results suggest that long latency auditory evoked potentials provide an objective electrophysiological analog to the clinical assessment of sedation independent of the sedation regime used.