From the Guidelines
Abnormal heart rate variability (HRV) is a significant predictor of cardiac mortality, with reduced HRV indicating increased risk.
Significance of HRV
The significance of HRV lies in its ability to provide information about autonomic balance, with reduced vagal activity to the heart increasing the risk of cardiac mortality 1. HRV is a measure of the variation in time between each heartbeat, and it is influenced by the autonomic nervous system, which regulates the heart rate.
Constituents of Abnormal HRV
Abnormal HRV is characterized by a reduced standard deviation of normal-to-normal intervals (SDNN) of less than 70 ms, as demonstrated in the ATRAMI study, which found that patients with low HRV had a relative mortality risk of 3.2 1. Other studies have also shown that low HRV is associated with increased mortality, with a relative risk of 2 to 3 in patients with angina or heart failure, and those who had experienced a myocardial infarction (MI) 1.
Predictive Value of HRV
The predictive value of HRV for cardiac mortality has been extensively studied, with most studies showing that low HRV is associated with increased mortality. The ability of HRV to predict arrhythmic, cardiac, or total mortality has been studied in various populations, including patients with MI, heart failure, and angina 1.
- Time-domain measures: SDNN, which measures the standard deviation of normal-to-normal intervals, is a commonly used time-domain measure of HRV.
- Frequency-domain measures: Power spectral analysis, which measures the power of different frequency bands, is a commonly used frequency-domain measure of HRV.
- Nonlinear methods: Nonlinear methods, such as the power-law relationship, have also been used to examine HRV, and have been shown to be predictive of mortality 1.
Clinical Implications
The clinical implications of abnormal HRV are significant, as it can be used to identify patients at high risk of cardiac mortality. The ATRAMI study demonstrated that the combination of low HRV and reduced left ventricular ejection fraction (LVEF) identifies a group of post-MI patients at high risk for sudden and non-sudden cardiac death 1.
Limitations
While HRV is a significant predictor of cardiac mortality, its limitations include the influence of circadian rhythms and patient activity on long-term HRV measurements, and the debate about which factors alter HRV in each frequency band 1.
From the Research
Significance of Heart Rate Variability (HRV)
- HRV is a noninvasive, practical, and reproducible measure of autonomic nervous system function 2
- It reflects the balance between the sympathetic and parasympathic influences on the intrinsic rhythm of the sinoatrial node 2
- HRV is influenced by lifestyle factors, including physical activity, eating habits, sleep pattern, and smoking 2
- Decreased HRV is associated with a higher risk of cardiac events, including death, and is a predictor of hypertension 2
Constituents of Abnormal Heart Rate Variability
- Abnormal HRV is characterized by decreased variability, which is associated with a higher risk of cardiac events 2, 3
- Reduced Standard Deviation of NN intervals (SDNN) is correlated with morbidity and mortality of patients after an acute coronary event 3
- Lower SDNN levels are associated with ventricular arrhythmia in pediatric patients with acute myocarditis 3
- Higher non-linear indices after an acute coronary event have been related to all-cause mortality 3
Measurement and Analysis of HRV
- HRV can be measured using time-domain, frequency-domain, and non-linear dynamic analysis techniques 4, 5
- 24-hour recording is the gold standard for HRV assessment, but short and ultra-short recordings are also accurate and practical 3
- HRV metrics include time-domain indices, frequency-domain measurements, and non-linear analysis 3, 5
- Computer-based analytical tools are useful for in-depth study of HRV data and can be used for diagnostics 5