Intra-QT spectral coherence as a possible noninvasive marker of sustained ventricular tachycardia

Abstract

Sudden cardiac death is the main cause of mortality in patients affected by chronic heart failure (CHF) and with history of myocardial infarction. No study yet investigated the intra-QT phase spectral coherence as a possible tool in stratifying the arrhythmic susceptibility in patients at risk of sudden cardiac death (SCD). We, therefore, assessed possible difference in spectral coherence between the ECG segment extending from the q wave to the T wave peak (QTp) and the one from T wave peak to the T wave end (Te) between patients with and without Holter ECG-documented sustained ventricular tachycardia (VT). None of the QT variability indexes as well as most of the coherences and RR power spectral variables significantly differed between the two groups except for the QTp-Te spectral coherence. The latter was significantly lower in patients with sustained VT than in those without (0.508 ± 0.150 versus 0.607 ± 0.150, P < 0.05). Although the responsible mechanism remains conjectural, the QTp-Te spectral coherence holds promise as a noninvasive marker predicting malignant ventricular arrhythmias.

Figure 1

The upper left panel shows the four intervals studied: a = RR; b = QT_e; c = QT_p; d = T _e. The lower left panel shows a beat-to-beat ECG recording extended to include the first 20 beats with the relative RR intervals. The right panels give the RR (a), QT_e (b), QT_p (c), and T _e (d) intervals measured on the 20 beats shown in the lower left panel. The asterisks (∗) in this panel indicate the second, seventh, eleventh, and eighteenth beats, shown also in panels (c) and (d). In panels (c) and (d) the four lowest QT_p with an asterisk correspond to four interval peaks T _e. The daggers (†) show the fifth, ninth, and fifteenth beats that vary in line with the three highest QT_p values and correspond to the three lowest T _e values. This pattern indicates that the two ECG signals oscillate in a substantially coherent manner. Conversely, the RR intervals yield poor coherence (a) with the other intervals studied.

Figure 2

Median and variance in a short-term (5-minute) ECG recording of the studied variables, RR (a), QT_e (b), QT_p (c), and T _e (d).

Figure 3

Power spectral analysis of RR (a), QT_e (b), QT_p (c), and T _e (d) calculated on a short-term (5-minute) ECG recording from the same patient shown in Figure 2. All the power spectra contain a high-frequency power component synchronous with breathing (between 0.20 and 0.30 Hz) and low-frequency power around 0.10 Hz.

Figure 4

Spectral coherence between the tested variables in a patient with chronic heart failure: QT_e→RR (a), QT_p→RR (b), T _e→RR (c), and eQT_p→T _e (d). Spectral coherence reaches maximum in QT_p→T _e.