Increased Total Heart Rate Variability and Enhanced Cardiac Vagal Autonomic Activity in Healthy Humans with Sinus Bradycardia (Clinical Report‪)‬

Sinus bradycardia can be defined as a sinus rhythm with a resting heart rate (HR) of 60 beats per minute (bpm) or less. However, few patients actually become symptomatic until their heart rate drops to 50 bpm (1). That is, even while resting HR tends to diminish with age (2), physiological regulatory mechanisms maintain normal cardiac output by increasing stroke output both at rest and during exercise (2). Sinus bradycardia is not commonly included among relevant biomarkers precipitating overt cardiovascular disease (1); a low resting sinus rate even appears to be a protecting factor against heart failure, but a high sinus rate emerged as an independent predictor of mortality in prospective studies carried out in the general population (3) and in selected groups of patients with hypertension (4) or myocardial infarction (5).