Hyperventilation-induced hypocapnia changes the pattern of electroencephalographic bicoherence growth during sevoflurane anaesthesia

Background. Hyperventilation, with the resulting hypocapnia, reduces cerebral blood flow and causes slowing of the EEG activity. However, neuronal oscillating properties including the thalamocortical network during hyperventilation have not been elucidated. To assess these features provoked by hyperventilation, the present study examined quadratic phase coupling features by means of bicoherence analysis. Methods. Twenty-two patients were anaesthetized using sevoflurane 1.5% combined with remifentanil or epidural anaesthesia. After a stable normocapnic period, hypocapnia was induced by hyperventilation, and the raw EEG signals were collected. Bispectral analysis (bicoherence) and power spectrum analysis were performed before and after hypocapnia. Results. Mean (SD) peak bicoherence in the delta-theta area increased from 35.6 (10.9)% during normocapnia to 43.8 (10.9)% during hypocapnia (P < 0.05), whereas mean (SD) peak bicoherence in the alpha area decreased from 42.8 (14.4)% during normocapnia to 37.5 (12.3)% during hypocapnia (P < 0.05). Normalized power in the delta-theta frequencies on the power spectrum increased from 60.2 (13.1)% to 72.5 (12.7)% (P < 0.05). Bispectral index and spectral edge frequency changed from 45.9 (7.0) to 40.1 (5.6) (P < 0.05) and from 15.0 (2.3) to 14.0 (2.5) Hz (P < 0.05), respectively. No significant differences in these values were observed between the two types of anaesthesia. Conclusions. Hypocapnia enlarged bicoherence growth in the delta-theta frequency range, suggesting the contribution of subcortical oscillating mechanisms in regulating EEG during hyperventilation.