Obstructive sleep apnea syndrome (OSAS) is characterized by multiple interruptions of airflow between periods of arousals. A key feature of OSAS is the 20- to 40-s cyclic pattern of electrophysiologic parameters. The periodicity of the OSAS-related phenomena is reminiscent of the natural electroencephalographic (EEG) arousal rhythm of non-rapid eye movement (NREM) sleep known as the cyclic alternating pattern (CAP). Morphologically, CAP consists of transient arousals (phase A) that periodically interrupt the tonic theta/delta activities of NREM sleep (phase B). Functionally, CAP translates a condition of sustained arousal instability oscillating between a greater arousal level (phase A) and a lesser arousal level (phase B). CAP is also related to the controls of the motor and autonomic mechanisms. On the basis of the information simultaneously derived from EEG activities, muscle tone, and neurovegetative responses, it is possible to distinguish three subtypes of A phases corresponding to different levels of arousal power: A1 (dominated by EEG synchronization and weak activation of polygraphic variables); A2 (mixture of EEG synchronization/desynchronization and intermediate activation of polygraphic variables); and A3 (dominated by EEG desynchronization and strong activation of polygraphic variables). Unlike standard criteria, CAP parameters offer a more suitable perspective for evaluating sleep pathologies in which brief and frequent arousals appear as a prominent feature. The present study aimed at (a) assessing CAP parameters in OSAS patients and (b) investigating the reciprocal interactions between CAP and the cyclic variations in respiratory rate. Twelve obese middle-aged OSAS subjects complaining of daytime sleepiness were polygraphically compared with age-matched and gender-matched volunteers in good health and with no complaints about sleep and wakefulness (controls). In OSAS patients, conventional parameters showed predictable decrements in total sleep time, slow wave sleep, and REM sleep and increases in stage 1 and nocturnal awakenings. Sleep fragmentation was associated with a significant enhancement of CAP and of the A phases with longer and more desynchronized EEG patterns (especially A3). The increase of A3 subtypes permitted scoring and detecting CAP also in REM sleep. The great majority of respiratory pauses (96\% in NREM and 80\% in REM sleep) were coupled with CAP. All CAP-related respiratory events rose in close temporal connection with a phase B, while effective breathing was always recovered during phase A (especially A2 and A3 subtypes). These data suggest that (a) phase B of CAP offers a vulnerable background for upper airway collapse and for attenuation of biochemical and neural mechanisms in the control of the ventilatory drive and (b) survival in OSAS patients is effected by the enhancement of the strongest components of the natural arousal rhythm at sleep quality's expense.
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