Heart repolarization – QT interval

Cardiac repolarization is measured by the QT interval on the surface electrocardiogram (ECG). Its prolongation is associated with a higher risk of cardiac death and arrhythmias. The QT interval is highly dependent on heart rate (RR intervals) and many correction methods have been suggested to compute corrected QT (QTc). The QTc means the QT interval value that corresponds to 60 beats per minute (bpm). QTc is used as the basic parameter in patient diagnosis and drug testing, but its contribution is limited. Limitations may be given by the correction method used and that QTc is a static parameter. Dynamic parameters of QT/RR coupling are mostly neglected, despite the fact that Torsades de Pointes are typically initiated by a variable sudden change in heart rate.
MediSig focus on repolarization analysis began in 2006. The recommended Bazett or Fridericia heart rate QT corrections provide debatable results especially during ECG measurements with different heart rate excitation. This problem is solved by the dynamic model of QT/RR coupling. The dynamic model is based on an optimized ARMA model, without any preliminary assumption about dynamic dependency of QT/RR coupling. The optimized QT step responses fully agree with physiological QT step response. All static and dynamic QT properties (QTc, QT memory, QT restitution and time constant of QT adaptation) may be analyzed. The dynamic solution was used in the following studies: QT hysteresis elimination, irregularities of QT/RR coupling, dependency of repolarization parameters on sex and type of heart rate stress, QT parameters in long QT subjects, coupling between QT/RR slope and QTc, linearity/nonlinearity of QT/RR coupling.
Various aspects of QT/RR relationship have been investigated by MediSig. For an analysis of the whole set of statistical and dynamic parameters of QT and QT/RR relationship, the original and patented method (WO/2008/146168, PCT/IB2008/002198) was developed. The patented description of QT/RR relationship contains five parameters: QTc corrected to the dynamic characteristics of QT and hysteresis; QT/RR gain for slow RR variability (GainS) - the parameter is equivalent to “QT/RR slope” but does not require a long term stabilized measurement; QT/RR gain for fast variability (GainF) - the parameter describes immediate change of QT during a change of RR; time constant of QT adaptation to a change of RR (τ) and QT variability independent of RR (QTvar). There is no other analysis that can provide this complete set of parameters.
The most important contribution of our method is the implication of dynamic parameters during analysis of QT/RR relationship. Two parameters (GainF and τ) describe newly analyzed physiological characteristics of QT interval.
MediSig provided complete experimental study management, data recording, processing and analysis. We developed custom software for robust detection of RR and QT intervals during exercise allowing the application of the theory of QT/RR dynamic coupling.