Effects of premature beats on repolarization of postextrasystolic beats

How to measure electrocardiographic QT interval in the anaesthetized rabbit

Many drugs prolong QT or QU intervals [QT(U)] in the electrocardiogram (ECG), and this may be associated with the generation of drug-induced torsades de pointes. Therefore, it is essential to assess the ability of the newly developed drugs to prolong QT(U) interval. For this purpose, both in vivo and in vitro rabbit models are frequently used. However, it is very difficult to locate the end of the QT(U) interval in most rabbit ECGs when repolarisation is delayed, as the shape of the T and U waves may be deformed. In addition, as the heart rate of the rabbit is very high, the T (or U) wave may overlap the P wave or even the QRS complex of the following sinoatrial beat. In these circumstances, application of the "extrapolation method" makes it possible to determine the length of the QT(U) interval. This article describes the extrapolation method, shows ECG examples of typical T and U waves in the anaesthetized rabbit, and makes an attempt to provide a useful guide for researchers to measure reliably and reproducibly the duration of the QT(U) interval in rabbit studies.

Exploiting rate-related hysteresis in repolarization alternans to improve risk stratification for ventricular tachycardia

OBJECTIVES

We sought to study the effect of heart rate acceleration and deceleration on the ability of repolarization alternans (RPA) to stratify ventricular tachycardia (VT) risk.

BACKGROUND

Heart rate fluctuations alter arrhythmic propensity, yet it is unclear whether fluctuations, as well as absolute rate, dynamically increase VT risk. We hypothesized that repolarization heterogeneity reflected by RPA would exhibit hysteresis during rising and falling heart rate, which may reflect arrhythmic propensity.

METHODS

The RPA magnitude (absolute voltage of alternation [V(alt)] and T-wave alternans ratio [TWAR]) and temporal distribution were determined from the electrocardiogram (ECG) in 60 patients during paced heart rate acceleration from 100 to 150 beats/min, then deceleration to 100 beats/min at electrophysiologic study (EPS). The V(alt) and TWAR thresholds were varied prospectively to generate receiver-operating characteristics (ROC) for the prediction of inducible VT at EPS.

RESULTS

Thirty-six patients were induced into VT and 24 were not. Hysteresis of RPA was seen. The V(alt) reached steady-state within 60 beats of each rate transition and was higher in deceleration than in acceleration at matched heart rates. In induced patients, V(alt) rose then fell with heart rate. In noninduced patients, V(alt) was insensitive to acceleration, but rose on initial deceleration. The RPA distributed later within repolarization in induced patients but, on deceleration, moved earlier in both groups. By ROC analysis, V(alt) = 2.6 microV in late repolarization at 120 beats/min provided optimal sensitivity and specificity for VT in acceleration (87.5% and 88.7%, respectively) versus deceleration (80% and 62.5%, respectively; p = 0.004, chi-square test).

CONCLUSIONS

1) Physiologic fluctuations in heart rate may affect the clinical utility of RPA for VT risk stratification; and 2) repolarization dispersion measured by RPA is more exaggerated during deceleration than acceleration at matched heart rates (rate hysteresis).

Spectral analysis of periodic fluctuations in electrocardiographic repolarization

Repolarization alternans (RPA) indicates alternate-beat fluctuations in the temporal or spatial characteristics of the echocardiogram (ECG) STU segment which may represent dispersion in repolarization. Spectral decomposition has revealed microvolt-level RPA which has been found to correlate with ventricular tachycardia (VT) and fibrillation, and is increasingly being used for clinical risk stratification. However, while interruptions in periodicity are known to affect spectral decomposition, their quantitative impact on RPA and its clinical utility have been poorly described. We therefore studied the effect of variable alignment, extrasystoles, dissimilar beats and beat exclusion on RPA magnitude in simulations and on the sensitivity and specificity of RPA for VT in a pilot clinical study. RPA magnitude was exquisitely sensitive to QRS alignment such that +/- 1 ms random beat misalignment reduced it by 68% in simulations. Correspondingly, suboptimal QRS alignment in clinical ECG's caused the sensitivity of RPA for inducible VT to fall from 93% to as low as 63%; while JT alignment was also less effective for RPA recovery. As an experiment in minimizing morphometric irregularities in clinical ECG's, we found that RPA magnitude actually fell when replacing either measurably dissimilar or ectopic beats with more representative beats. In addition, inserting or deleting beats also reduced RPA magnitude in clinical sequences and simulations. These statistical analyses suggest that the precision of beat alignment and interruptions to ECG periodicity, which may occur physiologically, may greatly reduce the clinical utility of RPA for VT. Dynamic alterations in RPA in response to sequence irregularities require further study before RPA may be optimally applied to screen for ventricular arrhythmias.

Changes in repolarization dynamicity and the assessment of the arrhythmic risk

At the present time, the assessment of the arrhythmic risk from surface ECG recordings is built on time-domain and frequent-domain analysis of high resolution ECG acquisition together with interlead variability of QT interval duration (QT dispersion). The corresponding raw ECG tracings are obtained in resting conditions. However, the dynamic aspects of the ECG signal is a rapidly evolving matter of interest. In addition to the beat-to-beat oscillations of the ventricular repolarization amplitude (QT alternans), there is growing evidence that the patterns of QT interval shortening with increasing heart rate are linked to susceptibility to ventricular arrhythmias. In this report, we will mainly address the association between QT dynamicity and the risk of developing torsades de pointes.

Postextrasystolic U wave augmentation, a new marker of increased arrhythmic risk in patients without the long QT syndrome

OBJECTIVES

We attempted to determine the correlation between the presence of postextrasystolic changes in the STU segment and a history of sustained ventricular arrhythmias.

BACKGROUND

Postextrasystolic U wave augmentation (a marked increment in U wave amplitude after premature ventricular complexes [PVCs]) is an adverse prognostic sign in the "pause-dependent long QT syndrome." However, the prevalence of postextrasystolic changes in patients without the long QT syndrome is unknown.

METHODS

We compared the configuration of the STU segment of the postextrasystolic beat (the sinus beat after a PVC) with the STU configuration during sinus rhythm in three patient groups: 1) 41 patients with spontaneous ventricular tachycardia/fibrillation (VT/VF) (VT/VF group), 2) 63 patients with heart disease and high grade ventricular arrhythmias (control group), and 3) 29 patients with high grade ventricular arrhythmias but no heart disease (reference group).

RESULTS

Postextrasystolic T wave changes did not correlate with a history of ventricular tachyarrhythmias. However, postextrasystolic U wave changes were more common among the patients with VT/VF than among control subjects (39% vs. 8.7%, p < 0.001). By logistic multiple regression analysis, a low left ventricular ejection fraction (p < 0.001) and postextrasystolic U wave changes (p < 0.005) were independent predictors of ventricular tachyarrhythmias.

CONCLUSIONS

Postextrasystolic T wave changes are common and lack predictive value. Postextrasystolic U wave changes may be a specific marker of a tendency to the development of spontaneous ventricular arrhythmias. Prospective studies should be performed to confirm this association.