Evokde potentials and consciousness: a study in volunteers at sub-MAC concentrations of isoflurane / 中华麻醉学杂志

ABSTRACT

Objective: In order to search the credible parameters of evoked potentials (EP) for the depth of anesthesia. Method: The end-expiratory concentration of isoflurane was increased gradually from 0.2MAC by step of 0.1MAC in each of ten volunteers. The change of EP and the consciousness were observed at every end expiratory anesthetic concentrations. The explicit and implicit memories about intra-experiment events were investigated an hour after stopping inhalation of isoflurane. Result: The explicit memory disappeared at 0.2 and 0.3MAC of isoflurane without the stimulation of pain; The response to command disappeared at 0.3 and 0.4MAC; 0.4MAC of isoflurane could not vanish implicit memory in all subjects. The latencies of waves Pa and Nb of the MLAEP and P_(25) of the SEP prolonged gradually with increased concentrations of isoflurane in a linear relationship, and varied with the changes of response to command. The latency corresponding to the partial response increased significantly compared with that to the full response, but without difference between the partial and no response. Meanwhile the latencies of waves Pa, Nb and P_(25) increased significantly with the disappearances of explicit and implicit memory, without differences between the conscious and unconscious awareness. The critical values of latencies about those waves which could be applied to assess the disappearance of explicit memory, were defined. Conclusion: The disappearance of consciousness is not all-ornone but gradual. The sequence of disappearance about consciousness is explicit memory, response to command, then implicit memory. There is no explicit memory about stimulation of pain after response to command has vanished, but implicit memory may still exit. The waves Pa and Nb of MLAEP and the wave P_(25) of SEP may be regarded as important parameters to evaluate the depth of anesthesia about consciousness.