Periodicity of a Noninvasive Measure of Cardiac Vagal Tone During Non-Rapid Eye Movement Sleep in Non-Sleep-Deprived and Sleep-Deprived Normal Subjects

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Abstract
Cyclic alternating pattern (CAP) of microarousals reported in EEG studies is now regarded as an integrating mechanism for the different parts of the central nervous system including the autonomic system. A recently developed continuous index of cardiac parasympathetic activity (CIPA) can be time locked to the EEG allowing the relationship between EEG and autonomic changes to be studied in sleep. Eleven normal subjects were studied for evidence of periodicity in CIPA in non-REM sleep, five of whom had been sleep deprived. Fast Fourier's transformation of the CIPA data confirmed its periodicity with four frequency peaks in the range of 0.015 to 0.3 Hz. The frequency peaks previously reported as caused by respiration, Mayer waves and vasomotor thermoregulator activity, were within what we called the alpha and beta rhythms of CIPA. There was an additional very slow peak not previously described and we called it the gamma rhythm. It covered the frequency range below 0.03 Hz. The gamma rhythm was the largest of all peaks in CIPA rhythms and its magnitude increased further in sleep-deprived subjects, but it invariably decreased during stage 4 sleep in both groups. Bursts of alpha waves in the EEG recorded concomitantly with CIPA in stage 1 sleep were associated with both peaks and troughs of the gamma rhythm. These results support previous proposals that cyclic alternating pattern in the EEG may be an integrating mechanism associated with functions of the central nervous system, and we have shown here its relationship with CIPA. Because cyclic alternating pattern can also be measured in CIPA, clinical exploitation of this phenomenon could include monitoring of epilepsy, studies of the effects of drug therapy, and assessment of other sleep disorders. All these are known to affect cyclic patterns of sleep EEG.