Causes of Low Heart Rate Variability
Low HRV fundamentally reflects impaired autonomic nervous system function, specifically diminished parasympathetic (vagal) tone and/or excessive sympathetic activity modulating the cardiac sinus node. 1, 2
Primary Pathophysiological Mechanism
The autonomic nervous system directly controls HRV through its input to the sinoatrial node, with parasympathetic activity increasing overall variability while sympathetic activity acts as a low-pass filter, reducing variability. 1, 3 Studies using autonomic blockade have definitively demonstrated that HRV is almost completely due to autonomic input to the sinus node. 1
Cardiovascular Disease States
Post-myocardial infarction is one of the most well-established causes of reduced HRV, with the ATRAMI study demonstrating that post-MI patients with low HRV had a relative mortality risk of 3.2, independent of left ventricular ejection fraction and ventricular ectopy. 1
Congestive heart failure significantly reduces HRV, with the degree of reduction correlating with disease severity and left ventricular dysfunction. 1 Heart failure is associated with increased circulating catecholamines, neurohormonal activation (elevated norepinephrine, renin, and natriuretic peptides), and electrolyte imbalances—all of which contribute to autonomic dysfunction. 1
Advanced hemodynamic disease manifests as low HRV, as demonstrated in the DINAMIT trial where low HRV in post-MI patients with decreased LVEF indicated more advanced disease rather than purely arrhythmic risk. 1
Dilated cardiomyopathy (both ischemic and nonischemic) reduces HRV, though the Marburg Cardiomyopathy Study showed that in nonischemic dilated cardiomyopathy, low HRV was not a multivariate predictor of transplant-free survival. 1
Metabolic and Endocrine Disorders
Diabetes mellitus causes decreased HRV through autonomic neuropathy, making HRV assessment useful for detecting early phases of diabetic autonomic dysfunction. 4 Low HRV is associated with metabolic syndrome prevalence and metabolic dysfunction. 3
Hyperthyroidism can alter metabolic rate and autonomic balance, contributing to abnormal HRV patterns. 1
Lifestyle and Behavioral Factors
Physical deconditioning reduces HRV through diminished cardiovascular fitness and altered autonomic tone. 1 The combination of low peak VO2 with normal peak heart rate suggests deconditioning as a contributor to reduced HRV. 1
Obesity affects HRV through multiple mechanisms including altered autonomic balance, increased sympathetic activity, and metabolic dysfunction. 1, 3
Poor sleep patterns and sleep disorders are associated with reduced HRV through disruption of normal autonomic function. 5
Smoking directly impairs autonomic function and reduces HRV. 5, 6
Excessive alcohol consumption negatively impacts autonomic nervous system balance. 5
Psychological stress and distress consistently correlate with reduced HRV through chronic sympathetic activation and parasympathetic withdrawal. 3, 7
Measurement-Related Factors
Circadian rhythms and patient activity influence long-term HRV assessment from 24-hour recordings, as autonomic control of heart rate changes throughout the day, making high- and low-frequency power components non-stationary. 1
Uncontrolled breathing patterns during measurement can introduce artifacts, as respiratory sinus arrhythmia mediated by parasympathetic tone is a major determinant of high-frequency HRV components. 1
Clinical Interpretation Caveat
A critical distinction: While low HRV predicts total mortality with relative risks of 2-3 across multiple studies, it is a better marker of nonarrhythmic mortality rather than sudden cardiac death specifically. 1 Multiple trials (DINAMIT, Camm et al.) showed that low HRV increased all-cause mortality but did not predict arrhythmic mortality. 1 This suggests low HRV reflects overall disease severity and hemodynamic compromise rather than purely electrical instability. 1