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Relationship of Heart Rate Variability to Sleepiness in Patients With Obstructive Sleep Apnea With and Without Heart Failure
Study Questions:
Do heart failure patients with obstructive sleep apnea (OSA) have different heart rate (HR) variability based on subjective sleepiness?
Methods:
Study subjects were referred to a single center for evaluation of OSA. None had been treated for OSA prior to enrollment. Patients with severe OSA were divided into those with and without excessive daytime sleepiness (EDS). Severe OSA was defined as apnea-hypopnea index (AHI) ≥30. The Epworth Sleepiness Scale (ESS) was used to assess degree of subjective daytime sleepiness. Subjects were divided into two groups: an ESS score ≥11 (EDS group) and ESS score <11 (non-EDS group). Heart failure was defined as those with a clinical history of failure and a measured ejection fraction <45%. Excluded were subjects with pacemakers, unstable cardiac symptoms within the prior 3 months, atrial fibrillation, or large number of ectopic heart beats during the overnight polysomnogram. HR signals were acquired in stage 2 sleep during periods of recurrent apneas and hypopneas and submitted to spectral analysis.
Results:
Ninety-one patients (71 men and 20 women) met criteria for this study; of these, 31 met criteria for EDS based on ESS score. There were 26 with and 65 without heart failure. No significant differences were observed in terms of age, body mass index, or medication use between groups. The non-EDS group had a higher AHI and a lower mean oxygen level during the sleep study than the EDS group. The impact of sympathetic nervous activity on HR variability occurs in a specific frequency range of spectral analysis, the very low frequency (VLF). Subjects without EDS had higher VLF (944 ± 839 vs. 447 ± 461 msec2, p = 0.003) than those with EDS, irrespective of the presence or absence of heart failure (1218 ± 944 vs. 426 ± 299 msec2, p = 0.043, and 1029 ± 873 vs. 503 ± 533 msec2, p = 0.003, respectively). ESS scores correlated inversely with VLF in all (r = -0.294, p = 0.005) and in heart failure subjects (r = -0.468, p = 0.016).
Conclusions:
The authors concluded that among the entire group, those without EDS had greater HR variability in the VLF range, than those with EDS. And among those with heart failure, ESS scores were inversely related to HR variability. Subjects with severe OSA, but without EDS have greater sympathetic modulation of HR than those with EDS, which could reflect elevated alertness mediated by adrenergic stimulation.
Perspective:
Clinicians have long noted the paradox among many with severe OSA who lack EDS. This article provides one possible explanation for the observation. The lack of subjective daytime sleepiness in many with OSA could be (in part) explained by increased sympathetic drive, which may counteract the sleepiness induced by sleep disruption common among patients with OSA. Resolution of sleepiness as an endpoint of treated OSA may not be as valuable of an outcome as other factors, such as cardiac autonomic function. However, more studies are necessary to test this possibility.
Keywords: Heart Diseases, Body Mass Index, Sleep Stages, Polysomnography, Sympathetic Nervous System, Heart Failure, Norepinephrine, Oxygen, Heart Rate, Autonomic Nervous System, Sleep Apnea, Obstructive
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