Nocturnal Arousal Phenomenon and the Autonomic Nervous System

Electroencephalographic (EEG) arousals are intrusions of wakefulness within sleep. These phasic events occur rarely as isolated phenomena, but appear organized in regular sequences known as the cyclic alternating pattern (CAP). Lack of EEG arousals is scored as non-CAP. The CAP metrics offer a more dynamic evaluation of polysomnographic features beyond the conventional sleep stages. The temporal relation between CAP and autonomic activation (phase A) and deactivation (phase B) provides a coupled view of what happens upstairs (brain, EEG) and downstairs (body, vegetative functions). In obstructive sleep apnea syndrome (OSAS), most respiratory events are followed by a dual response (A phase associated with a relevant pulse wave amplitude drop). The combined cortical and autonomic activation suggests a possible role of pulse wave amplitude as a marker of the EEG response to airflow reduction or increased upper airway resistance. Moreover, dual responses show a progressive increase from flow limitations to hypopneas and apneas indicating that the arousal response evolves into a more powerful and extensive activation as airway obstruction increases. Cardiopulmonary coupling (CPC) is a frequency-based method that captures the phase differences (or degree of coupling) between the electrocardiogram (ECG)-derived respiration and the R-R intervals. Analyzing ratios between CPC and the main heart rate frequency bands, the preponderance of power ratio in the low (sympathetic) frequency is correlated with CAP periods and can be an indication of periodic respiration during sleep-disordered breathing. In contrast, a preponderance of power in the high (parasympathetic) frequency is associated with deep sleep, physiologic respiratory sinus arrhythmia, and non-CAP periods.