Electroencephalographic changes in the late cardiopulmonary bypass period are not reflected in the bispectral index

Rhythms on electroencephalography (EEG) are known to slow with cardiopulmonary bypass (CPB), however electroencephalographic bicoherence analysis, one of the methods to examine EEG synchronization, which is necessary for the understanding of the network regulation involved, has been poorly examined in this period.

Subjects comprised 11 patients scheduled to undergo heart surgery under extracorporeal circulation. Anesthesia was maintained by intravenous administration of propofol combined with remifentanil, using a bispectral index (BIS) monitor to keep an identical BIS value (target BIS value, 40-45). Raw EEG signals were collected through the BIS monitor. Approximate entropy, bicoherence and power spectrum were analyzed before and after CPB, in addition to other EEG parameters.

In the late hypothermic CPB period, both 95% spectral edge frequency and approximate entropy decreased, notwithstanding a lack of significant difference in BIS index. Bicoherence growths in delta-theta and alpha areas decreased simultaneously.

In the late CPB period, although BIS value is unchanged, EEG often expresses a slow EEG rhythm accompanied by disproportional de-synchronous characteristics.

CPB caused the EEG de-synchronous features, which are not quantified by the BIS algorithm. It has the possibility to increase the vulnerability of nociceptive perception.