Heart Rate Variability as a Clinical Tool

Established Clinical Applications

HRV testing is a simple and reliable test of cardiovagal function that detects diabetic polyneuropathy with nearly the same sensitivity as nerve conduction studies (Class II evidence) 3
The Toronto Diabetic Neuropathy Expert Group concluded that HRV is one of the five most sensitive and specific approaches currently available to evaluate cardiovascular autonomic neuropathy in clinical research 1, 2
The specificity of HRV testing is high at 97.5% for identifying parasympathetic deficits when age-controlled values are used 3, 4

The American Heart Association reports that low HRV predicts total mortality with relative risks of 2-3 5
In patients with chronic heart failure, diminished low-frequency power during controlled breathing was associated with a 5-fold increase in arrhythmic mortality 6
The combination of preserved low-frequency power and fewer than 86 ventricular premature beats per hour was associated with only 3% sudden cardiac death risk compared with 23% for the remainder of the population 6

HRV reflects sympathetic and parasympathetic efferences to the sinus node, providing a surrogate for autonomic effects in the ventricle that are important in the pathogenesis of ventricular arrhythmias 6
High-frequency components (0.15 to 0.45 Hz) primarily reflect parasympathetic activity mediated by respiratory sinus arrhythmia 6
Low-frequency components (0.04 to 0.15 Hz) involve sympathetic nervous activity contributions, though parasympathetic modulation also affects this range 7

The absolute power in the low-frequency region should NOT be used as an index of sympathetic activity, as parasympathetic nervous system also modulates HRV in this range 7
The relative proportion (not absolute power) in low frequencies provides a relative measure of sympathetic modulation, but must be interpreted cautiously if respiratory artifacts cannot be excluded 7
Optimal recording time is 4-5 minutes during well-controlled rest with controlled breathing at 15 breaths per minute to avoid hyperventilation artifacts 7

Misinterpretation of power spectrum due to irregular respiratory patterns and verbalization during breathing creates artifactual low frequencies and false "sympathetic overactivity" 7
Intrinsic cardiac disease can affect test results and must be considered in interpretation 3, 4
Very low HRV (2-4% of total variability found in healthy subjects) makes interpretation of spectral components unreliable due to non-autonomic components 7

The American College of Cardiology recommends determining if structural heart disease has been excluded, as the combination of reduced ejection fraction with low HRV significantly increases cardiovascular risk 5
Non-cardiac causes affecting HRV must be evaluated, including thyroid dysfunction, anemia, and other systemic conditions, before attributing findings solely to cardiac causes 5

Low HRV is associated with increased risk of total mortality, cardiovascular disease progression, and various pathological conditions including heart failure 8
Regular aerobic exercise has been shown to improve HRV parameters, with consistent physical activity helping to counteract negative effects of sedentary behavior on autonomic function 8
Stress reduction practices that activate the parasympathetic nervous system, such as mind-body interventions, can improve HRV 8

Short-term HRV has moderate reproducibility in normal subjects but is less reproducible in patients with congestive heart failure 6
There is marked interindividual variation in the relationship of short-term HRV to parasympathetic effect, making identification of clear normal/abnormal limits difficult in individuals 6
The American Heart Association does not currently recommend routine use of HRV for sudden cardiac death risk stratification despite its prognostic associations 6, 5