Midlatency median nerve evoked responses during recovery from propofol/sufentanil total intravenous anaesthesia

Median nerve somatosensory evoked responses (MnSSER) are frequently used to monitor the integrity of the somatosensory pathway during surgery. We investigated MnSSER components during the wakeup phase from anaesthesia with propofol/sufentanil, because detailed information is lacking about the reversibility of anaesthetic induced changes of MnSSER. The aim of the study was to document precisely the MnSSER waves in relation to the clinical awakening. The hypothesis was that anaesthetic induced MnSSER changes are reversed when the patient becomes responsive after anaesthesia.

In 20 gynaecological patients anaesthesia was maintained with propofol 8 mg kg(-1) h(-1) supplemented by bolus injections of sufentanil. MnSSER were recorded at C4' (N20, P25, N35, P45, N50) following electrical median nerve stimulation on the day before surgery, after the end of surgery during anaesthesia and every 5 min during recovery, till the patients were responsive again and able to identify a shown object.

While the primary cortical MnSSER complex N20P25 regained baseline values, the cortical latencies > or =35 ms remained prolonged (P<0.001) and the amplitudes P45N50 were suppressed (P< or =0.013), when the patients were responsive after 26+/-7 min following anaesthesia. However, the amplitudes P25N35 exceeded their corresponding baseline value (P<0.01)

Persistent changes of MnSSER waves > or =35 ms reflect impaired signal processing along the somatosensory pathway following propofol/sufentanil anaesthesia when the patients are responsive again. Further studies combining MnSSER recording with distinct neuro-psychological tests are needed to define the clinical relevance of these findings.