Anna M. Bianchi

Time-variant power spectrum analysis for the detection of transient episodes in HRV signal

A time-variant algorithm of autoregressive (AR) identification is introduced and applied to the heart rate variability (HRV) signal. The power spectrum is calculated from the AR coefficients derived from each single RR interval considered. Time-variant AR coefficients are determined through adaptive parametric identification with a forgetting factor which obtains weighted values on a running temporal window of 50 preceding measurements. Power spectrum density (PSD) is hence obtained at each cardiac cycle, making it possible to follow the dynamics of the spectral parameters on a beat-by-beat basis. These parameters are mainly the LF (low-frequency) and the HF (high-frequency) powers, and their ratio, LF/HF. These together account for the balanced sympatho-vagal control mechanism affecting the heart rate. This method is applied to subjects suffering from transient ischemic attacks. The time variant spectral parameters suggest an early activation of LF component in the HRV power spectrum. It precedes by approximately 1.5-2 min the tachycardia and the ST displacement, generally indicative of the onset of an ischemic episode.<<ETX>>

Assessment of Heart Rate Variability Changes During Dipyridamole Infusion and Dipyridamole-Induced Myocardial Ischemia: A Time Variant Spectral Approach

OBJECTIVESnWe sought to evaluate changes in RR interval variability during dipyridamole infusion and dipyridamole-induced myocardial ischemia.nnnBACKGROUNDnMyocardial ischemia and the autonomic nervous system can be mutually interdependent. Spectral analysis of RR interval variability is a useful tool in assessing autonomic tone.nnnMETHODSnWe used a time variant autoregressive spectral estimation algorithm that could extract spectral variables even in the presence of nonstationary signals. Two groups were considered: group A (patients with ischemia, n = 15) with effort or mixed angina, angiographically assessed coronary artery disease and positive exercise and dipyridamole echocardiographic test results, and group B (control subjects, n = 10) with normal exercise and dipyridamole echocardiographic test results. We investigated the following variables: RR interval mean and variance, low frequency (LF) and high frequency (HF) power in normalized units, LF ratio (LF/LFbasal power), HF ratio (HF/HFbasal power) and LF/HF ratio. For each test epoch, we calculated for group A and group B the mean value +/- SE of all indexes considered. Differences due to an effect either of group (ischemic vs. control) or of time (including both drug and ischemia effects) were analyzed by using analysis of variance for repeated measurements.nnnRESULTSnDipyridamole injection was characterized by a reduction of all spectral components in negative test. The LF ratio was the only variable able to discriminate patients with ischemia from control subjects (p < 0.05), whereas a time effect was evident for both mean RR interval and high frequency power in normalized units (p < 0.05). The LF ratio decreased in group B from 1 +/- 0.00 (basal) to 0.31 +/- 0.22 (peak), and increased in group A from 1 +/- 0.00 to 15.41 +/- 6.59, respectively. Results of an unpaired t test comparing the peak values of the two groups were also statistically significant (p < 0.01).nnnCONCLUSIONSnOur data show that time variant analysis of heart rate variability evidences an increase in the low frequency ratio that allows differentiation of positive from negative test results, suggesting that the electrocardiogram may contain ischemia information unrelated to ST-T variations, even if their enhancement requires a more complex data processing procedure.

Spectral Analysis of Cardiovascular Variability Signals

It is well known that the control mechanisms of heart rate and blood pressure, as well as of many other cardiovascular parameters, manifest themselves though beat-to-beat variations in the related signals1,2. The quantification of these variations, both in control conditions and after provocative tests, plays a fundamental role for a better comprehension of the pathophysiological properties of such mechanisms which act through neural, mechanical, vascular, humoral and other factors.

Time-variant heart-rate variability spectral parameters during dobutamine test

Time-variant spectral parameters are used to monitor the Autonomic Nervous System (ANS) status in normal and coronary artery disease (CAD) patients during pharmacological infusion of Dobutamine in Dobutamine Echocardiography Test (DET). During the test, the spectral parameters (LF and HF power, LF/HF ratio) are estimated on a beat-to-beat basis through the time-variant identification of an AutoRegressive (AR) model, using a Recursive Least Square (RLS) algorithm. Different LF responses have been found in the early stages of drug infusion (10-20 /spl gamma/) sequentially before the appearance of echo, ECG or clinical signs of ischemia. An early increase of LF/LF/sub basal/ value was found in 6 of 8 (75%) positive tests, and only in 2 of 9 (22%) negative ones, suggesting that the different neurovegetative response in the sub-maximal stages of drug infusion may be a marker of an actual or successive ischemia.

Single Sweep Analysis of Evoked and Event Related Potentials

Different approaches of single sweep analysis of evoked and event related potentials are presented. The autoregressive with eXogenous input (ARX) model is described with different applications in the study of dynamical changes of the brain responses and in artifact removal. A study of the modifications induced in the EEG by a sensory stimulation via ARX and AR models is also described. Finally, the wavelet transform is employed for the reconstruction of the single evoked response.