1Department of Medicine, University of British Columbia, Vancouver, BC, Canada, 2Department of Medicine, Division of Cardiology, Seattle Veterans Affairs Medical Center and the University of Washington, Seattle, Washington
Background: Normalized ratios of portions of the power frequency spectrum of heart rate (HR) are commonly used to gain insight into cardiac “sympathovagal balance.” Whether aging, which alters both sympathtetic and parasympathetic activities, infl uences these measures has not been well characterized.
Objectives: We examined the ability of normalized ratios of the power frequency spectrum of heart rate to describe autonomic activity at the sinus node in older and younger adults during conditions of sympathetic predominance.
Methods: 20 older (mean age 70.0 ± 1.5 years) and 24 younger (mean age 25.4 ± 0.9 years) normal adults were screened by history, physical examination, blood work (CBC, electrolytes, creatinine, liver function tests), ECG, exercise tolerance test, echocardiogram and myocardial perfusion scan (if 65 years old). A 2-channel Holter was used to monitor heart rate. Total (TP), low frequency (LF) and high frequency power (HF) were obtained by Fast Fourier Transform. Intravenous atropine (2 boluses of 0.01 mg/kg) was followed by isoproterenol infusions of 7 and 21 ng/kg/min to tilt the “sympathovagal balance” to the sympathetic nervous system side.
Results: Normalized HF power gave expected results in response to atropine only in younger subjects. Changes in normalized LF power had a much stronger correlation with changes in heart rate in older as opposed to younger subjects.
Conclusions: The response of normalized power ratios to atropine and isoproterenol varies between different age groups.
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