Heart Rate Variability and Autonomic Nervous System Function

Physiological Interpretation

A lower than normal coefficient of variability in HRV indicates impaired autonomic nervous system function, specifically reflecting reduced parasympathetic tone and elevated sympathetic activity, which is associated with increased cardiovascular risk and mortality, according to the American College of Cardiology 1, 2
Diminished parasympathetic (vagal) activity is a characteristic of low HRV, indicating impaired vagal tone, as stated by the Journal of the American College of Cardiology 3, 4
Sympathetic predominance is reflected in low HRV, representing an imbalance where sympathetic activity is relatively enhanced compared to parasympathetic modulation, as reported in Circulation 5

The American College of Cardiology recommends considering low HRV as a risk factor for increased mortality, with patients in the lowest tertile for HRV having significantly increased risk of cardiovascular death 3
Low HRV is associated with arrhythmic events, including a 5-fold increase in arrhythmic mortality in heart failure patients with diminished low-frequency power during controlled breathing, as stated by the American College of Cardiology and Circulation 3, 5
The combination of impaired low-frequency power and high ventricular premature beats per hour confers a significant sudden cardiac death risk, according to the American College of Cardiology 3

Cardiac arrhythmias are often initiated by or occur in patients with enhanced sympathetic and diminished parasympathetic tone, as reported by Circulation and the Journal of the American College of Cardiology 3, 5
HRV provides a surrogate for autonomic effects in the ventricle that are important in the pathogenesis of ventricular tachycardia and ventricular fibrillation, as stated by Circulation 5

The American Heart Association and American College of Cardiology state that short-term HRV use for sudden cardiac death risk stratification is not currently recommended due to limited data, highlighting the need for caution in interpreting low HRV 1, 3
Poor reproducibility and marked interindividual variation complicate the interpretation of low HRV, making identification of clear normal/abnormal limits difficult in individuals, as reported by Circulation and the Journal of the American College of Cardiology 3, 5

Poor sleep patterns, sedentary behavior, and lack of regular aerobic exercise are modifiable factors that can influence low HRV, according to the American College of Cardiology and Praxis Medical Insights 1, 2
Smoking, excessive alcohol consumption, and uncontrolled hypertension or diabetes are also contributing factors to low HRV, as stated by Praxis Medical Insights 1