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Volume 10 No. 05
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Accepted Papers

Scientific Investigations

Hypercapnia is a Key Correlate of EEG Activation and Daytime Sleepiness in Hypercapnic Sleep Disordered Breathing Patients

David Wang, Ph.D.1,2; Amanda J. Piper, Ph.D.1,2; Brendon J. Yee, Ph.D.1,2,3; Keith K. Wong, Ph.D.1,2,3; Jong-Won Kim, Ph.D.3,4; Angela D'Rozario, B.Sc.(Hons)2,3; Luke Rowsell, B.Tech.2; Derk-Jan Dijk, Ph.D.5; Ronald R. Grunstein, Ph.D.1,2,3
1Department of Respiratory and Sleep Medicine, Royal Prince Alfred Hospital, Sydney, Australia (work performed); 2Woolcock Institute of Medical Research, University of Sydney, Sydney, Australia; 3Centre for Integrated Research and Understanding of Sleep (CIRUS), University of Sydney, Sydney, Australia; 4School of Physics, University of Sydney, Sydney, Australia; 5Surrey Sleep Research Centre, University of Surrey, UK


The key determinants of daytime drowsiness in sleep disordered breathing (SDB) are unclear. Hypercapnia has not been examined as a potential contributor due to the lack of reliable measurement during sleep. To overcome this limitation, we studied predominantly hypercapnic SDB patients to investigate the role of hypercapnia on EEG activation and daytime sleepiness.


We measured overnight polysomnography (PSG), arterial blood gases, and Epworth Sleepiness Scale in 55 severe SDB patients with obesity hypoventilation syndrome or overlap syndrome (COPD+ obstructive sleep apnea) before and ∼3 months after positive airway pressure (PAP) treatment. Quantitative EEG analyses were performed, and the Delta/ Alpha ratio was used as an indicator of EEG activation.


After the PAP treatment, these patients showed a significant decrease in their waking pCO2, daytime sleepiness, as well as all key breathing/oxygenation parameters during sleep. Overnight Delta/Alpha ratio of EEG was significantly reduced. There is a significant cross-correlation between a reduced wake pCO2, a faster (more activated) sleep EEG (reduced Delta/Alpha ratio) and reduced daytime sleepiness (all p < 0.05) with PAP treatment. Multiple regression analyses showed the degree of change in hypercapnia to be the only significant predictor for both ESS and Delta/ Alpha ratio.


Hypercapnia is a key correlate of EEG activation and daytime sleepiness in hypercapnic SDB patients. The relationship between hypercapnia and sleepiness may be mediated by reduced neuro-electrical brain activity.


A commentary on this article appears in this issue on page 523.


Wang D, Piper AJ, Yee BJ, Wong KK, Kim JW, D'Rozario A, Rowsell L, Dijk DJ, Grunstein RR. Hypercapnia is a key correlate of EEG activation and daytime sleepiness in hypercapnic sleep disordered breathing patients. J Clin Sleep Med 2014;10(5):517-522.

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