13th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 1998

January 24-31, 1998
Marilleva, Trento, Italy

RT-173

Relationship between RR and variability. A measure of dispersion of ventricular repolarisation

Federico Lombardi, Andrea Colombo, Alberto Porta*, Giuseppe Baselli*, Sergio Cerutti*.
Division of Cardiology, Institute of Biomedical Sciences, S. Paolo Hospital, University of Milan, *Department of Bioengineering, Polytechnic of Milan, Italy

Introduction

The existence of a relationship between duration of ventricular repolarisation measured as QT interval and heart period has been known for long¹. However, two kinds of responses of QT interval to sudden sustained changes in heart period have been described^2,3: a fast adaptation, revealing a tight dependence of QT interval from the preceding cardiac cycles and a slower one, lasting for a few minutes and interpreted as an indirect evidence of a memory mechanism involved in the regulation of ventricular repolarisation duration. More recently, the coupling between RR and QT interval has been confirmed by analysing their variabilities over short term recordings^4-6. In normal subjects, beat to beat variabilities of RR and RT-apex interval (a measure of the temporal distance between R- and T-wave apex that appears more suitable for an automatic analysis of ventricular repolarisation duration) are characterised by similar rhythmical oscillations with the same period but different amplitude. Thus, also in RT-apex variability a low (LC:\WWW 0.04-0.15 Hz) and a high (HC:\WWW 0.15-0.40 Hz) have been identified^4-6.

It is however evident that, in addition to heart period, several factors may determine changes in QT interval even during resting controlled conditions. Direct neural influences, primary or drug-induced alteration on electrophysiologic mechanisms controlling ventricular repolarisation, cardiac axis movements synchronous with respiration are only a few of the possible factors capable of altering QT duration and its variability.

In the clinical setting, the possibility of quantifying the direct variations of QT interval and of separating them from the concomitant modification mediated by changes in QT/RR relationship has stimulated the development of new approaches aimed not only to provide new insight to the understanding of the mechanisms regulating ventricular repolarisation but also to obtain indices of its abnormality.

The aim of the present study is to propose a dynamic parametric model⁷ of the RR/RT-apex variability interaction whose parameters can be directly estimated from the beat to beat series RR and RT-apex intervals. The model is designed to separate the fraction of RT-apex variability driven by RR changes from that independent of RR variations and to quantify the gain and phase of the relationship between RR and RT-apex intervals.