Age difference in heart rate changes associated with micro-arousals in humans
Introduction
The American Sleep Disorders Association (ASDA) has defined a micro-arousal (MA) as an abrupt shift in electroencephalographic (EEG) frequency, which may include theta, alpha and/or frequencies greater than 16 Hz. This shift generally contrasts with the background EEG activity and must last between 3 and 10 s to be scored (ASDA, 1992). Usually, subjects are unaware of MA but when they occur frequently, MA can be a major cause of excessive daytime sleepiness (Bonnet and Arand, 1996).
Recently, Sforza et al. (2000) have studied heart rate (HR) changes associated with MA in 27 healthy adults (19–55 years). These authors have found that MA was associated with a tachycardia followed by a bradycardia. Detailed analyses of the HR changes showed that the tachycardia started before EEG signs of MA. Using spectral analysis, Bonnet and Arand (1997) also observed that the increase in HR generally begins before the onset of visually detected EEG arousal in young adults.
An age-related difference in these MA-related cardiac variations has been observed (Noda et al., 2000). In a study of EEG arousals at the termination of sleep apneas/hypopneas in 13 middle-aged (40–60 years) and 10 elderly (60–77 years) patients with obstructive apnea syndrome (OSAS), the authors observed a higher cardiac activation associated with arousals in middle-aged patients in comparison to elderly patients. However, sleep apnea is known to affect autonomic sympathetic and parasympathetic activities and to possibly increase risk for cardiovascular diseases (Narkiewicz and Somers, 2001, Leung and Bradley, 2001). Therefore, the reduction in the HR activation following the EEG arousals associated with sleep apnea/hypopnea in elderly patients could be due either to the effect of repetitive hypoxia in OSAS or to normal age-related changes in cardiac activation. The present study was aiming at evaluating the age-related differences in HR changes associated with MA in a population of subjects without sleep disorders.
Section snippets
Population
Sixteen young adults (mean age: 24.6, range: 20–35 years, 8 men and 8 women) and 16 middle-aged subjects (mean age: 54.8, range: 50–65 years) matched for gender were included in this study. None of the subjects were taking medication reported to influence cardiovascular functions or sleep architecture. In addition, they were all free of sleep complaints and cardiovascular diseases. All subjects signed a consent form and the study was approved by the hospital committee.
Polysomnographic recordings
All subjects underwent one
Results
As shown in Table 1, no between-group difference was found for MA indices. Young and middle-aged subjects showed similar MA indices for the entire night (10.25±3.92 and 11.69±4.46 respectively, ns) and for stage 2 sleep separately (11.44±5.3 and 9.64±4.77 respectively, ns). In young and in middle-aged subjects, MA were associated with significant HR changes (F(25, 375)=19.04, P<0.001, Fig. 1a and F(25, 375)=12.3, P<0.001, Fig. 1b, respectively). In young subjects, planned comparisons showed an
Discussion
Results of the present study showed that MA are associated with a tachycardia starting prior to the onset of MA and followed by a bradycardia; these changes lasting at least 20 R-R intervals. In young subjects, HR changes began 5 R-R intervals before EEG signs of MA. In middle-aged subjects, HR changes began only 1 R-R interval before MA. The early tachycardia observed in young subjects suggests that HR changes are not an autonomic response to MA but rather a primary event that may contribute
Acknowledgements
This research was supported by the Canadian Institutes of Health Research (grant to J.M. and studentship to M.M.) and by the National Sciences and Engineering Research Council (studentship to N.G.). The authors are grateful to Dominique Petit, PhD, for reviewing the manuscript. The authors also thanks Gaétan Poirier, MSc, and Jean Paquet, PhD, for helping with the analyses and statistics.
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