What does a lower than normal coefficient of variability in Heart Rate Variability (HRV) indicate?

What Lower Than Normal HRV Coefficient of Variability Indicates

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. 1, 2

Physiological Interpretation

Low HRV fundamentally represents autonomic dysfunction with the following characteristics:

• Diminished parasympathetic (vagal) activity - The parasympathetic nervous system normally mediates respiratory sinus arrhythmia and influences overall heart rate variability, so reduced variability indicates impaired vagal tone 3, 4
• Sympathetic predominance - Low HRV reflects an imbalance where sympathetic activity is relatively enhanced compared to parasympathetic modulation 3, 5
• Loss of cardiac adaptability - The heart demonstrates reduced capacity to respond appropriately to internal and external stimuli, indicating compromised cardiovascular health 1

Clinical Significance and Risk Implications

Low HRV carries substantial prognostic implications across multiple domains:

Cardiovascular Risk

• Increased mortality risk - Patients in the lowest tertile for HRV have significantly increased risk of cardiovascular death 3
• Arrhythmic events - In heart failure patients, a standard deviation of R-R intervals <15 ms predicts arrhythmic mortality, with diminished low-frequency power during controlled breathing associated with a 5-fold increase in arrhythmic mortality 3
• Sudden cardiac death risk - The combination of impaired low-frequency power and >86 ventricular premature beats per hour confers a 23% sudden cardiac death risk compared to 3% in those with preserved HRV 3

Specific Clinical Contexts

• Post-myocardial infarction - Low HRV in post-MI patients serves as a risk factor for subsequent mortality 3
• Heart failure - Reduced HRV indicates poor prognosis and increased risk of cardiovascular disease progression 1
• Anxiety disorders - Low HRV in anxious patients reflects a specific deficit—the inability to disengage threat detection mechanisms, perpetuating physiological arousal even when no real threat exists 2, 6
• Diabetic patients - Low HRV suggests impaired autonomic function and early diabetic neuropathy 1

Autonomic Nervous System Dysfunction

The mechanism underlying low HRV involves disrupted autonomic balance:

• Cardiac arrhythmias are often initiated by or occur in patients with enhanced sympathetic and diminished parasympathetic tone 3
• HRV provides a surrogate for autonomic effects in the ventricle that are important in the pathogenesis of ventricular tachycardia and ventricular fibrillation 3
• The sympathetic division acts as a low-pass filter on heart rate variability, reducing overall variability during activation 4

Important Clinical Caveats

Several factors complicate the interpretation of low HRV:

• Poor reproducibility - Short-term HRV is less reproducible in patients with congestive heart failure compared to normal subjects 3
• Marked interindividual variation - There is substantial variation in the relationship of short-term HRV to parasympathetic effect, making identification of clear normal/abnormal limits difficult in individuals 3
• Heart rate confounding - HRV is significantly associated with average heart rate, so low HRV may partially reflect heart rate changes rather than pure autonomic dysfunction 7
• Limited sudden death prediction - Despite associations with mortality, 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 3, 1

Modifiable Contributing Factors

Low HRV is influenced by multiple lifestyle and pathological factors:

• Poor sleep patterns directly reduce HRV 1, 2
• Sedentary behavior and lack of regular aerobic exercise 1, 2
• Smoking, excessive alcohol consumption, and uncontrolled hypertension or diabetes 1
• Chronic stress and anxiety disorders 2, 6