Cardiac Autonomic Function in Older Adults with Sleep-Disordered Breathing

We searched for possible associations between various measures of severity of sleep-disordered breathing (SDB) and indices of cardiac autonomic function in older subjects (>60 years). Twenty four overnight unattended home-based polysomnograms obtained from the Sleep Heart Health Study were analyzed using spectral analysis. For each subject, six autonomic indices reflecting heart rate variability were quantitatively determined during wakefulness, REM sleep and non-REM sleep. Each individual autonomic marker was regressed against each of 4 measures of SDB, including the respiratory disturbance index (RDI), respiratory oscillation index, cumulative oxygen desaturation, and arousal index. In general, we found no correlation between any of these measures of SDB severity and each of the autonomic indices. However, mean heart rate was found to decrease as RDI increased. As well, the ratio of low-frequency to high-frequency power (LHR) decreased with increasing RDI. Contrary to previous reports, our preliminary findings suggest that sympathetic activity decreases with increasing severity of SDB. This paradoxical association between SDB and cardiac autonomic function may be the result of natural compensatory mechanisms at work, allowing some subjects with SDB to be protected from systemic hypertension or other cardiovascular diseases.