What does low heart rate variability (HRV) mean?

What Low Heart Rate Variability Means

Low heart rate variability (HRV) indicates reduced fluctuation in time intervals between consecutive heartbeats, reflecting impaired autonomic nervous system function with diminished parasympathetic activity and represents a significant predictor of increased total mortality and adverse cardiovascular outcomes. 1, 2

Physiological Interpretation

Low HRV fundamentally reflects autonomic dysfunction, specifically reduced parasympathetic (vagal) tone and/or excessive sympathetic activity acting on the sinoatrial node. 3 The autonomic nervous system directly controls HRV through its input to the heart's pacemaker, with parasympathetic activity normally increasing variability while sympathetic activity reduces it. 3

• Studies using autonomic blockade have definitively demonstrated that HRV is almost completely due to autonomic input to the sinus node. 3
• This autonomic imbalance represents the body's reduced capacity to adapt to physiological and environmental stressors. 1

Clinical Significance and Prognostic Implications

Low HRV carries substantial prognostic weight across multiple cardiovascular conditions:

Mortality Risk

• Post-myocardial infarction patients with low HRV have a relative mortality risk of 3.2, independent of left ventricular ejection fraction and ventricular ectopy, as demonstrated in the landmark ATRAMI study. 4, 2, 3
• Meta-analysis data shows low HRV is associated with a pooled hazard ratio of 2.27 for all-cause death in patients with cardiovascular disease. 5
• The increased mortality risk extends beyond cardiac causes—low HRV predicts death from multiple causes and cannot be attributed to a specific etiology. 6

Important Distinction

Low HRV is generally a better predictor of total mortality than sudden cardiac death specifically. 4, 2 This is a critical clinical nuance:

• In the DINAMIT trial, using low HRV to select post-MI patients for ICD implantation did not improve survival because nonarrhythmic mortality increased in the ICD group. 4, 2
• Low HRV in these patients indicated more advanced hemodynamic disease rather than purely arrhythmic risk. 4, 3
• Patients with appropriate ICD shocks ultimately died of congestive heart failure, not arrhythmias. 4

Cardiovascular Disease Progression

• Low HRV independently predicts rapid progression of coronary atherosclerosis, with the lowest tertile of HRV showing mean decrease in minimum luminal diameter of -0.17 mm compared to +0.01 mm in the highest tertile. 7
• The association between low HRV and cardiovascular events shows a pooled hazard ratio of 1.41. 5

Common Causes of Low HRV

Cardiovascular Conditions

• Post-myocardial infarction is one of the most well-established causes. 3
• Congestive heart failure significantly reduces HRV, with reduction correlating with disease severity. 3
• Dilated cardiomyopathy (both ischemic and nonischemic) reduces HRV. 3
• Advanced hemodynamic disease manifests as low HRV. 3

Metabolic and Lifestyle Factors

• Low HRV is associated with metabolic syndrome prevalence and metabolic dysfunction. 2
• Physical deconditioning reduces HRV through diminished cardiovascular fitness. 3
• Obesity affects HRV through altered autonomic balance and increased sympathetic activity. 3
• Poor sleep patterns, smoking, and adverse eating habits all reduce HRV. 8

Psychological Factors

• Higher psychological distress is consistently associated with reduced HRV. 2

Clinical Application Caveats

When interpreting low HRV, recognize these important limitations:

• Short-term HRV has limited data linking it to sudden death, and its use for sudden cardiac death risk stratification is not currently recommended. 1, 2
• In nonischemic dilated cardiomyopathy, the Marburg Cardiomyopathy Study showed low HRV was not a multivariate predictor of transplant-free survival or arrhythmic events. 4, 3
• The utility of HRV for risk stratification in dilated cardiomyopathy remains unclear with conflicting evidence. 2

Risk Stratification Enhancement

Low HRV provides greatest value when combined with other risk markers:

• The combination of low HRV and depressed baroreflex sensitivity significantly increases risk, with 1-year mortality increasing from 1% when both markers are preserved to 15% when both are depressed. 2
• Association of left ventricular ejection fraction <35% with low HRV further increases cardiovascular risk. 2
• A composite autonomic index including baroreflex sensitivity and time-domain HRV measures increased relative risk to 8 in post-MI patients. 4

Measurement Considerations

• Both time-domain measures (SDNN <70 ms, rMSSD, pNN50) and frequency-domain measures provide valuable information about autonomic function. 1, 2
• Circadian rhythms and patient activity influence 24-hour HRV recordings, as autonomic control changes throughout the day. 3
• Uncontrolled breathing patterns during measurement can introduce artifacts. 3