What is a normal Heart Rate Variability (HRV) in a healthy adult?

Normal Heart Rate Variability (HRV) in Healthy Adults

In healthy adults, normal HRV values show considerable variation by age and sex, but reference values from a large Brazilian study indicate that SDNN (standard deviation of all N-N intervals) averages approximately 43.7 ± 17.3 ms in men and 40.3 ± 15.8 ms in women aged 35-74 years, with values declining progressively with age. 1

Key Reference Values for Short-Term HRV

Time Domain Measures

• SDNN: Men show 43.7 ± 17.3 ms versus women at 40.3 ± 15.8 ms in healthy, non-medicated adults 1
• N-N variance: Men demonstrate 2,214 ± 1,890 ms² compared to women at 1,883 ± 1,635 ms² 1
• rMSSD (root-mean square of successive differences): Reflects parasympathetic activity and is one of the most clinically useful time domain parameters 2
• Standard deviation of R-R intervals ≥15 ms has been identified as a threshold in heart failure patients, with values below this associated with increased arrhythmic mortality 3

Frequency Domain Measures

• High-frequency (HF) component (0.15-0.45 Hz): Women show higher normalized HF at 50.4 ± 18.5 n.u. versus men at 39.4 ± 18.0 n.u., reflecting greater parasympathetic tone 1
• Low-frequency (LF) component (0.04-0.15 Hz): The LF/HF ratio is higher in men (2.30 ± 2.68) than women (1.33 ± 1.82) 1
• The high-frequency component is primarily mediated by respiratory sinus arrhythmia and parasympathetic tone 3

Age and Sex Differences

Age-Related Changes

• HRV decreases progressively with aging across nearly all indices, as demonstrated by percentile curves in the Brazilian study 1
• Heart rate and normalized HF are higher in younger versus older women 1
• In males, HRV-CV (coefficient of variation) increases after approximately 40 years of age 4
• In females, HRV-CV shows a U-shaped pattern, declining through midlife and rising after approximately 50 years 4

Sex Differences

• Men demonstrate lower resting heart rate (64 ± 8 bpm) compared to women (68 ± 8 bpm) 1
• Women show higher parasympathetic activity as reflected by higher normalized HF values 1
• Men exhibit higher overall variability (higher SDNN and N-N variance) but lower normalized HF 1

Clinical Context and Interpretation

What HRV Reflects

• HRV represents autonomic nervous system modulation of the sinus node, with contributions almost completely due to autonomic input 3
• High HRV is associated with healthy cardiovascular function, while low HRV correlates with pathological conditions 5
• The parasympathetic system affects overall variability (total power), while sympathetic activity acts like a low-pass filter, predominantly influencing the low-frequency band 3

Important Measurement Considerations

• At least 5 nights of nocturnal HRV data are required to reliably estimate 7-day HRV-CV (intraclass correlation coefficient ≥0.80) 4
• Short-term HRV has moderate reproducibility in normal subjects but is less reproducible in patients with heart failure 3
• There is marked interindividual variation in the relationship between short-term HRV and parasympathetic effect, making identification of clear normal/abnormal limits difficult in individuals 3

Prognostic Significance

• SDNN below 50 ms is associated with high risk of cardiac death in post-MI patients 2
• Decreased HRV is an independent risk factor for sudden cardiac death after myocardial infarction, comparable to ejection fraction 2
• In a 900-subject cohort, those in the lowest tertile for HRV from 2-minute ECG recordings had increased risk of cardiovascular death 3

Factors Influencing HRV

Lifestyle and Behavioral Factors

• Higher HRV-CV associates with greater alcohol consumption, lower physical activity, shorter and less consistent sleep, and greater behavioral variability 4
• Physical activity, eating habits, sleep patterns, and smoking all influence HRV 6
• Higher BMI is associated with increased HRV-CV in both sexes 4

Autonomic Balance

• The sympathetic nervous system contributes importantly to low-frequency HRV, while respiratory sinus arrhythmia (parasympathetic) determines the high-frequency component 3
• Low-frequency power in absolute values does not reflect sympathetic activity; only the relative proportion of low-to-high frequencies provides an approximate indication of sympathetic modulation 3
• With extreme sympathetic activation and parasympathetic withdrawal, overall variability becomes so small that the low-frequency component cannot be measured 3

Common Pitfalls

• Avoid interpreting low-frequency power in absolute terms as sympathetic activity—it must be expressed in relative terms (as percentage of global HRV) 3
• Do not assume that short-term HRV measurements are highly reproducible in all populations; reproducibility is lower in heart failure patients 3
• Recognize that marked interindividual variation limits the ability to define precise normal/abnormal cutoffs for individual patients 3
• Remember that HRV changes due to lifestyle factors precede the onset of cardiovascular disorders, making it useful for early risk assessment 6