What is heart rate variability (HRV)?

From the Guidelines

Heart rate variability (HRV) is the physiological variation in the time interval between heartbeats, reflecting the autonomic nervous system's activity and balance between sympathetic and parasympathetic influences. This measurement serves as a crucial indicator of autonomic nervous system function, overall health, and cardiovascular fitness. A higher HRV is generally associated with better health outcomes, including improved resilience to stress, enhanced recovery capacity, and reduced risk of cardiovascular disease and mortality 1.

Key Aspects of HRV

• HRV naturally fluctuates throughout the day, increasing during rest and decreasing during stress or physical exertion, due to the body's response to various physiological processes such as breathing, hormones, and emotions.
• The measurement of HRV can be achieved through specialized devices like chest straps, smartwatches, or medical-grade ECG monitors, allowing for the tracking of improvements in fitness, detection of overtraining, and assessment of stress management effectiveness.
• The physiological basis for HRV lies in the autonomic nervous system's dual control of heart function, with the parasympathetic system (via the vagus nerve) slowing heart rate and the sympathetic system accelerating it 1.

Clinical Significance of HRV

• Low HRV has been linked to increased mortality and cardiovascular disease, particularly in patients with underlying cardiac conditions such as heart failure or those who have experienced a myocardial infarction.
• Studies have demonstrated that HRV can predict arrhythmic, cardiac, or total mortality, with a relative risk ranging from 2 to 3 in most studies, and up to 7.9 in certain populations 1.
• The combination of low HRV and other markers of autonomic dysfunction, such as baroreflex sensitivity (BRS), can further increase the risk of cardiac mortality, highlighting the importance of considering multiple factors in risk assessment 1.

Measurement and Interpretation of HRV

• HRV can be analyzed using time- and frequency-domain methods, as well as nonlinear techniques, each providing unique insights into autonomic function and cardiovascular health.
• The choice of analysis method depends on the specific research question or clinical application, with time- and frequency-domain measures being more established and widely used 1.
• The interpretation of HRV results should consider individual factors, such as age, physical condition, and underlying health status, to provide a comprehensive understanding of autonomic function and cardiovascular risk.

From the Research

Definition of Heart Rate Variability (HRV)

• Heart rate variability (HRV) is defined as the time variation between two consecutive heartbeats 2, 3, 4.
• It is a noninvasive, practical, and reproducible measure of autonomic nervous system function 2, 5.
• HRV is believed to correspond to the balance between the sympathetic and parasympathetic influences on the intrinsic rhythm of the sinoatrial node 2.

Factors Influencing HRV

• HRV is influenced by lifestyle factors, including physical activity, eating habits, sleep pattern, and smoking 2.
• Reduced HRV is associated with a higher risk of cardiac events, including death, and is a predictor of hypertension 2.
• HRV may also have predictive value for life expectancy and health 2.

Clinical Applications of HRV

• HRV has been considered an indirect biomarker of cardiac autonomic control and widely exploited for the assessment of autonomic function in many pathological subjects 4.
• HRV analysis has been proven to detect early autonomic involvement in several neurological disorders, including diabetes, stroke, multiple sclerosis, muscular dystrophies, Parkinson's disease, and epilepsy 6.
• Exercise therapy may improve HRV in myocardial infarction, chronic heart failure, and revascularization patients by increasing vagal tone and decreasing sympathetic activity 5.

Analysis and Measurement of HRV

• HRV can be measured using various methods, including Electrocardiogram (ECG), Electrodermal activity (EDA), photoplethysmography (PPG), and respiration (RESP) analysis 3.
• Effective analysis of HRV during ambulatory and motion situations, such as exercise, video gaming, and driving, could have a significant impact on improving social well-being 3.
• The clinical use of HRV should be further investigated, and its implementation in diagnostic and prognostic processes in routine clinical practice remains limited due to the lack of reliable reference values and low specificity of HRV indices 4.