Assessment of autonomic function by long-term heart rate variability: beyond the classical framework of LF and HF measurements

In the assessment of autonomic function by heart rate variability (HRV), the framework that the power of high-frequency component or its surrogate indices reflects parasympathetic activity, while the power of low-frequency component or LF/HF reflects sympathetic activity has been used as the theoretical basis for the interpretation of HRV. Although this classical framework has contributed greatly to the widespread use of HRV for the assessment of autonomic function, it was obtained from studies of short-term HRV (typically 5‑10 min) under tightly controlled conditions. If it is applied to long-term HRV (typically 24 h) under free-running conditions in daily life, erroneous conclusions could be drawn. Also, long-term HRV could contain untapped useful information that is not revealed in the classical framework. In this review, we discuss the limitations of the classical framework and present studies that extracted autonomic function indicators and other useful biomedical information from long-term HRV using novel approaches beyond the classical framework. Those methods include non-Gaussianity index, HRV sleep index, heart rate turbulence, and the frequency and amplitude of cyclic variation of heart rate.

HEART RATE TURBULENCE INDICATORS IN CLINICAL PRACTICE

Background. Heart disease is the leading cause of death in developed countries. Approximately half of these fatalities are due to sudden cardiac death. Electrocardiogram recording from the body surface allows stratification of patients according to the risk of cardiac arrest without the use of invasive methods. Arrhythmias, particularly ventricular extrasystole, can affect the sinus rhythm pattern. The change in sinus rhythm that occurs after an extrasystole is defined as heart rate turbulence. This phenomenon is not pathological, but some variants are associated with a risk of fatal arrhythmias. Aim. To analyze the indices and clinical significance of cardiac rhythm turbulence according to Holter monitoring in patients with various cardiological abnormalities at the hospital profile department. Material and methods. The study included 54 patients who were routinely treated in a cardiac hospital. Cardiac rhythm turbulence indices were obtained by Holter monitoring. Patients in severe and moderately severe clinical condition were not included in the study. Concomitant and past somatic diseases affecting the state of heart and cardiovascular system were considered. Results and discussion. Deviations in heart rhythm turbulence indices can be detected both in life- threatening arrhythmias and in benign extrasystoles. They are associated with the influence of the autonomic nervous system, but probably have different mechanisms. Deviations of heart rhythm turbulence indices were associated with left ventricular myocardial hypertrophy of concentric remodeling type and with an increased number of low-risk ventricular extrasystoles. To identify patients with arrhythmias of different risk, various threshold values of cardiac rhythm turbulence indices can be used. Conclusion. Factors likely to affect the indices of cardiac rhythm turbulence such as left ventricular myocardial remodeling and hypertrophy were revealed, as well as changes in autonomic nervous system regulatory function, including those associated with the constitutional features of the patient.

Heart rate turbulence as potential adjuvant index for optimal treatment guidance in severe pulmonary artery hypertension

Purpose Despite significant progress achieved in the management of pulmonary arterial hypertension (PAH), the prognosis remains poor. Sudden cardiac death is one main cause of death in advanced disease. It is already known that in pts with PAH there is an alteration in cardiac autonomic nervous activity. Heart rate turbulence (HRT) is an ECG–related autonomic marker, helpful in risk stratification of proarrhythmia and sudden cardiac death. Aim of the study was to examine HRT in patients with severe idiopathic PAH (IPAH) and its relation to disease's progress. Methods The study population consisted of 46 pts, (32 men and 14 women), mean age 50±9 years, Group A 20 pts: NYHA III, III–IV diagnosed as severe IPAH and Group B 26 pts: NYHA II, II–III diagnosed as moderately severe IPAH. They underwent 24h Holter monitoring-HRT analysis with calculation of 2 parameters, turbulence onset (TO) and turbulence slope (TS) (Abnormal values: TO>0, TS<2.5msec/RR). Pts were defined as HRT(+) when TO and/or TS were abnormal and as HRT(−) when both TO and TS were normal or when HRT could not be calculated because of none or too few suitable PVCs. Reassessment of HRT was undertaken in 6 mo and 1 year in case of change in clinical status. Six pts from group A and 7 pts from group B underwent HRT reassessment because of change in clinical status. Results Initially abnormal HRT was found in 12 out of 20 pts (n=12, 60%) in group A and in none out of 26 (n=0, 0%) in group B (p=0,001). In 5 pts out of 8 with clinical improvement and 4 pts with clinical deterioration who had HRT reassessed, significant positive correlation was found between TS and change of clinical status (r=0,64, p=0,04). There was a tendency to greater normal TS mean values with clinical improvement even in group where TS was already normal (6,95±4,5 vs 3±0,3 p=0.077). Conclusion In severe IPAH cardiac autonomic system impairment was demonstrated mainly as abnormal TS in HRT. Significant positive correlation between normalization of TS and clinical improvement was revealed and that raises promises for use of HRT optimization as adjuvant index to guide optimal IPAH therapy and efficacy. FUNDunding Acknowledgement Type of funding sources: None.

Heart rate variability and heart rate turbulence in patients with vasovagal syncope

Aim The autonomic nervous system plays an important role in the pathogenesis of vasovagal syncope, but studies on the effect of basal autonomic tone have found confusing results. The aim of this study was to investigate the effect of basal autonomic functions, as assessed by heart rate variability (HRV) and heart rate turbulence (HRT), in patients with vasovagal syncope.Material and methods Patients who underwent head-up tilt test (HUTT) due to unexplained syncope and who had a 24 hr Holter ECG recording in the same period were retrospectively analyzed. Patients with diabetes, a history of myocardial infarction, heart failure, orthostatic hypotension, atrial fibrillation, or use of vasoactive drugs, such as beta blockers, were excluded from the study. 161 patients who met these criteria were included in the study. Time domain HRV parameters from Holter ECG recordings and HRT parameters from patients with sufficient number of ventricular premature contractions were measured.Results The age of the patients varied from 16 to 75 yrs (mean: 44.8±18.5 yrs). HUTT results of 60 (37.2 %) patients were evaluated as positive. There were no significant differences in the basal demographic, clinical, or laboratory findings of the tilt-positive and tilt-negative patient groups. Likewise, there were no significant differences between the time domain HRV parameters and HRT parameters of both groups.Conclusion HRV and HRT parameters reflecting basal autonomic function were not different between HUTT positive and HUTT negative patient groups. These findings suggest that basal autonomic functions have no effect on vasovagal syncope pathogenesis.