Heart Rate Variability

Introduction to HRV

• Heart rate variability (HRV) serves as an indicator of autonomic nervous system function, reflecting the balance between sympathetic and parasympathetic influences on the heart, according to the American College of Cardiology 1

HRV Measurement and Interpretation

• HRV can be measured using time-domain and frequency-domain analyses, including standard deviation of normal-to-normal (NN) intervals (SDNN), standard deviation of 5-minute averages of NN intervals (SDANN), root mean square of successive differences between adjacent NN intervals (rMSSD), and percentage of adjacent NN intervals that differ by more than 50 ms (pNN50), as defined by the Diabetes/Metabolism Research and Reviews 2
• The low-frequency (LF) to high-frequency (HF) ratio is used as a relative indicator of sympathetic modulation, as reported by the Diabetes/Metabolism Research and Reviews 2
• Fractal scaling measures the complexity of heart rate fluctuations, as studied by the Journal of the American College of Cardiology 3
• Accurate R-wave peak identification and filtering of motion artifacts and ectopic beats are crucial for reliable HRV measurements, as stated by the American College of Cardiology 3
• A minimum of 5-minute segments is recommended for reliable frequency domain analysis, and consideration of circadian rhythms and patient activity is necessary when interpreting 24-hour recordings, according to the American College of Cardiology 3, 4
• The following table summarizes the frequency components of heart rate variability:

• The high-frequency component (0.15-0.45 Hz) of HRV primarily reflects parasympathetic activity and is synchronized with respiration, as stated by the American Autonomic Society and the American Heart Association 2, 5
• The low-frequency component (0.04-0.15 Hz) of HRV reflects a mix of sympathetic and parasympathetic influences, and the ratio of low-frequency to high-frequency power (LF/HF ratio) is used as a relative indicator of sympathetic modulation, according to the American Heart Association 2

Clinical Significance of HRV

• Reduced HRV is associated with increased all-cause mortality risk, as found by the American College of Cardiology 3
• Low HRV has been linked to increased risk of cardiac events and sudden cardiac death, according to the American College of Cardiology 3
• The ATRAMI study showed that post-MI patients with low HRV had a relative mortality risk of 3.2, accounting for left ventricular ejection fraction and ventricular ectopy, as reported by the American College of Cardiology 3
• HRV may be a better marker of non-arrhythmic rather than arrhythmic mortality, as suggested by the American College of Cardiology 3
• Day-to-day reproducibility of HRV can be variable, especially in patients with heart conditions, as found by the American College of Cardiology 3
• HRV during sleep can be used in cardiovascular risk assessment, especially in older adults, and may help identify patients at higher risk of atrial fibrillation or other cardiovascular conditions, according to the American College of Cardiology 3
• Breathing influences heart rate through respiratory sinus arrhythmia, where heart rate increases during inspiration and decreases during expiration, primarily mediated by parasympathetic nervous system activity, as explained by the American College of Cardiology 2, 6
• In healthy individuals over 50 years, heart rate variability during deep breathing (expiratory/inspiratory index) should be ≥15 beats per minute, according to the American Heart Association guidelines 6

HRV and Autonomic Function

• The neurovisceral integration model indicates that HRV reflects prefrontal inhibitory capacity and adaptability to environmental changes, as suggested by the American College of Cardiology 1
• The system of alertness in the human body functions through a complex interaction between the central and autonomic nervous systems, as explained by the American Academy of Sleep Medicine and the National Institute of Neurological Disorders and Stroke 1
• There are two types of alertness: tonic alertness, which maintains the state of vigilance over minutes, and phasic alertness, which is a moment-to-moment response to specific stimuli, as defined by the Society for Neuroscience and the American Neurological Association 1, 7
• Efficient self-regulation of alertness decreases with age, exacerbating normal declines in cognitive performance, and alertness training can significantly improve executive functions in older adults, as stated by the National Institute on Aging 1