Comparing the effect of hypercapnia and hypoxia on the electroencephalogram during wakefulness

Objective: Hypoxia has been postulated as a key mechanism for neurocognitive impairment in sleep-disordered breathing. However, the effect of hypoxia on the electroencephalogram (EEG) is not clear. Methods: We examined quantitative EEG recordings from 20 normal volunteers under three 5-min ventilatory control protocols: progressive hypercapnia with iso-hyperoxia (pO₂=150mmHg) (Protocol 1), progressive hypercapnia with iso-hypoxia (pO₂=50mmHg) (Protocol 2), and progressive hypoxia with a CO₂ scrubber in the circuit (Protocol 3). Each protocol started with a 5-min session of breathing room air as baseline. Results: In Protocol 1, compared to its baseline, iso-hyperoxia hypercapnia led to a lower Alpha% and higher Delta/Alpha (D/A) ratio. Similarly, in Protocol 2, the iso-hypoxia hypercapnia induced a higher Delta%, a lower Alpha% and higher D/A ratio. No difference was found in any EEG spectral band including the D/A ratio when Protocols 1 & 2 were compared. In Protocol 3, the Delta%, Alpha% and D/A ratio recorded during hypoxia were not significantly different from baseline. Conclusions: We found that hypercapnia, but not hypoxia, may play a key role in slowing of the EEG in healthy humans. Significance: Hypercapnia may be a greater influence than hypoxia on brain neuroelectrical activities.