A review of clinical applications of ECG QT-interval´s beat-by-beat variability studied by spectral techniques

Reliable software access to beat-by-beat QT interval series, as derived from high resolution ECG facsimiles, opened a novel insight to the sympathetic control of ventricular myocardium, quite different from that allowed by the RR interval approach. While experimental work have shown that QT interval closely follows the force of ventricular contraction on a beat-by-beat basis, studies in healthy humans forwarded the low frequency, 0.1Hz power in the spectrum of QT variability as an emerging index for the adrenergic control of the ventricles during both physical and mental stress. On this background, clinical studies in moderate hypertension patients suggest that preservation of the psycho-neuro-myocardial adrenergic linkage under mental stress may add to the currently increased sympathetic drive to ventricles an arrhytmogenic extra-factor. In the same framework, studies of spectral QT variability in type I young diabetics with autonomic dysfunction in the earliest phase suggest a vagal down-regulation, while preservation of the sympathetic responsiveness of ventricles to stress may introduce an early arrhythmia risk.