HRV IN HEART FAILURE – HOW TO INTERPRET DATA
Authors: Costel Matei
Heart rate variability (HRV), the variation over time of the period between consecutive heartbeats, is predominantly dependent on the extrinsic regulation of the heart rate (HR). HRV is thought to reflect the heart’s ability to adapt to changing circumstances by detecting and quickly responding to unpredictable stimuli.
The normal variability in HR is due to autonomic neural regulation of the heart and the circulatory system. The balancing action of the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) branches of the ANS controls the HR.
HRV analysis demonstrates to be an important tool for prognostic stratification in patients with cardiovascular disease with high mortality, i.e. post myocardial infarction, heart failure (HF). Studies coming from 1990 investigated this analysis in different conditions and now it is a common sense to consider decreased HRV as predictor for bad prognosis: high mortality rate, high incidence of malignant ventricular arrhythmias.
Potential confunding factors can influence the interpretation and the significance of data in specific population. HRV can independently be modified by volume overload, age, exercise training, drugs as common conditions in HF. Also co-morbidities associated to heart failure can modify HRV data (depression, sleep disorders, thyroid dysfunction).
Volume overload. Some small studies showed there is a statistically significant improvement in HRV with diuresis, which closely correlated with volume loss. This is the reason it is better to avoid HRV evaluation during acute decompensation of chronic HF as it can give false information if you think HRV is a predictor of survival.
Age. The influence of age on HRV was analyzed in healthy subjects showing a shift towards parasympathetic dominance with aging that leads to conclusion that healthy longevity depends on preservation of autonomic function, in particular, HRV–parasympathetic function. Other studies showed in older adults there is an increasing prevalence of sinus arrhythmia (“erratic rhythm”) that is not of respiratory origin nor of autonomic influences. Thus, increased values of beat-to-beat HRV measures are completely confounded in the elderly and cannot be taken as measures of parasympathetic dominance without verifying this is truly due to respiratory sinus arrhythmia rather than erratic rhythm.
Exercise. It is very well known that regular exercise training produces a shift of autonomic balance toward higher parasympathetic activity which is consistent with improved cardiac health. This finding could explain effects of exercise training in patients with HF and the potential benefits of regular training programs in this category of patients. Also, the effort intensity produces different responses on HRV. Moderate intensity effort seems not to produce a significant change in HRV, but severe intensity effort showed an increase of sympathetic tone both immediately and after 24 hours after exercise cessation.
Drugs. HRV can be influenced by various groups of drugs, so the influence of medication should be considered, while interpreting HRV. Beta-blockers and ACE-I are commonly used in HF, so their influence is important. Also, HRV can be used to quantify the efficacy of these drugs in HF.
Depression. Depression is frequently found in HF patients. Depression is associated with changes in autonomic activity and this imbalance seems to correlate with increased morbidity and mortality in chronic HF patients, independent of disease severity. The decreased HRV in patients with major depression could be a potential explanation for the association of a worse outcome in patients with major depression.
Thyroid dysfunction. Overt or subclinically hyperthyroidism can be associated with heart failure, especially in elderly or in patients with arrhythmogenic cardiomyopathy. There were some studies that have examined cardiac autonomic function in untreated and treated hyperthyroidism. Overall measures of HRV and those specific for cardiac vagal modulation are attenuated in patients with overt hyperthyroidism compared with normal subjects; measurements of HRV remained low in those with low levels of serum thyrotropin but returned to normal in patients with biochemical euthyroidism.
Also studies in patients with hypothyroidism demonstrated that hypothyroidism is associated with a decreased sympatho-vagal modulation of the heart rate and with an increased in-homogeneity of ventricular recovery times. Sleep deprivation and sleep disorders. A number of HRV studies showed the heart rate and HRV progressively decrease during non-REM sleep and increase during REM sleep. Results are consistent with increased vagal control of HRV in non-REM sleep and increased SNS control during REM sleep.
HRV can be influenced by various sleep disorders.
There were few studies comparing HRV between insomniacs and good sleepers. A reduced HRV in insomniacs compared with control was observed across all sleep stages (REM or non-REM). Different categories of subjects were assessed in several studies (college students, drivers, nurses night shift vs. day-shift) and there was a significant increase of sympathetic vs. parasympathetic tonus in all categories mentioned.
Obstructive sleep apnea (OSA) influences also HRV. Both normalized LF and HF are significantly higher at the end of respiratory events during sleep, compared with baseline sleep, even there was no change in heart rate. HRV results are consistent with SNS enhancement during sleep due to sleep apneas and may help explain increased cardiac risk. Use of effective CPAP in patients with OSA is consistently associated with changes in both night-time and day-time HRV. CPAP-treatment in patients with dilated cardiomyopathy (ischemic or non-ischemic, LVEF <0.45) had significantly increased HF and decreased LF/HF ratio compared to non-CPAP patients. CPAP also improved LVEF by a significant 8%. This suggested CPAP improves PNS modulation of HR in CHF patients with OSA, improved LV function and reduced SNS activation. Further studies of HRV and CPAP in HF are needed.
Conclusion: HRV is an important tool for prognostic evaluation in patients with heart failure. Both time-domain and frequency-domain measurements are useful, with a slight advantage of frequency-domain analysis. Particular conditions in which this evaluation is performed should be taken into account when interpreting the results.