AUTONOMIC IMBALANCE IN HYPERTENSION. A MEASURE OF CARDIAC NEUROVEGETATIVE DYSAUTONOMIA
Hypertension represents worldwide one of the the most important condition for morbidity and mortality. Nowadays heart rate is evaluated by developed software's and offers fascinating data about the complexity of the autonomous cardiovascular control. Disturbances in the activity of the autonomic nervous system (ANS) significantly influence the outcome of patients with hypertension. The role of the ANS in the mechanism of hypertension is well known. The challenge is to evaluate the arrhythmic risk of hypertensive patients using methods that are well consecrated and to propose a risk score for the hypertensive patients.
High sympathetic activity is a potential cause of increased arrhythmic risk in hypertension. Alteration of the autonomic control, assessed by heart rate variability (HRV) and baroreflex sensitivity (BRS) are common in patients with hypertension. A subject open to debate is to compare HRV and BRS in hypertensive subjects vs. normotensive subjects complicated or not with ventricular arrhythmias. It seems that a new method for this analysis is represented by dynamic analysis techniques that may quantify abnormalities in heart rate variability (HRV) based on nonlinear and fractal analysis (chaos theory). Approximate entropy (ApEn) is applied to quantify the regularity and complexity of time series, as well as unpredictability of fluctuations in time series. The aim of this presentation is to evaluate the level of the autonomic dysfunction in hypertension. Heart rate variability, expressed in time domain as standard deviation of all RR intervals (SDNN) and in frequency domain as very low frequency (VLF, 0.01-0.04 Hz), low frequency (LF, 0.05 -0.15 Hz) and high frequency (HF, 0.15-0.50 Hz), and baroreflex sensitivity and respectively ventricular repolarization dynamics were studied in both groups. Hypertensive patients have a high sympathetic tone, expressed as the power spectral density (PSD, s2/Hz, ms2) of heart rate variability parameters and the LF/HF ratio. The DFA parameters showed higher values in the hypertensive group compared with the control group. It was found that the short-term fractal scaling exponent alpha (1) is significantly lower in arrhythmic hypertensive patients (0.75 vs 0.84; p < 0.03) compared with hypertensive patients. ApEn was non-significant in the two groups. Left ventricular ejection fraction (LVEF %), SDNN (ms), LF/HF, and the baroreflex sensitivity (BRS) parameters had been proved to be independent risk factors for ventricular arrhythmia. BRS is correlate with QT/RR ratio (r: 0.48) and with DFAalfa1 (r: 0.40).
The nonlinear dynamic methods could have clinical and prognostic applicability also in short-time ECG series. Dynamic analysis based on chaos theory point out the multifractal time series in hypertensive patients who loss normal fractal characteristics and regularity in HRV. Nonlinear analysis technique may complement traditional ECG analysis.
We can conclude that hypertensive patients have an autonomic imbalance comparable with those with depressed left ventricular performance (like heart failure) and it is necessary to improve ventricular arrhythmia prophylaxis in patients with hypertension.