Significance of T-wave inversion triggered by spontaneous atrial premature beats in patients with long QT syndrome

Paediatric/young versus adult patients with long QT syndrome

Introduction

Long QT syndrome (LQTS) is a less prevalent cardiac ion channelopathy than Brugada syndrome in Asia. The present study compared the outcomes between paediatric/young and adult LQTS patients.

Methods

This was a population-based retrospective cohort study of consecutive patients diagnosed with LQTS attending public hospitals in Hong Kong. The primary outcome was spontaneous ventricular tachycardia/ventricular fibrillation (VT/VF).

Results

A total of 142 LQTS (mean onset age=27±23 years old) were included. Arrhythmias other than VT/VF (HR 4.67, 95% CI (1.53 to 14.3), p=0.007), initial VT/VF (HR=3.25 (95% CI 1.29 to 8.16), p=0.012) and Schwartz score (HR=1.90 (95% CI 1.11 to 3.26), p=0.020) were predictive of the primary outcome for the overall cohort, while arrhythmias other than VT/VF (HR=5.41 (95% CI 1.36 to 21.4), p=0.016) and Schwartz score (HR=4.67 (95% CI 1.48 to 14.7), p=0.009) were predictive for the adult subgroup (>25 years old; n=58). A random survival forest model identified initial VT/VF, Schwartz score, initial QTc interval, family history of LQTS, initially asymptomatic and arrhythmias other than VT/VF as the most important variables for risk prediction.

Conclusion

Clinical and ECG presentation varies between the paediatric/young and adult LQTS population. Machine learning models achieved more accurate VT/VF prediction.

Inhibition of Ca2+-dependent protein kinase C rescues high calcium-induced pro-arrhythmogenic cardiac alternans in rabbit hearts

Cardiac alternans closely linked to calcium dysregulation is a crucial risk factor for fatal arrhythmia causing especially sudden death. Calcium overload is well-known to activate Ca²⁺-dependent protein kinase C (PKC); however, the effects of PKC on arrhythmogenic cardiac alternans have not yet been investigated. This study aimed to determine the contributions of PKC activities in cardiac alternans associated with calcium cycling disturbances. In the present study, action potential duration alternans (APD-ALT) induced by high free intracellular calcium ([Ca²⁺]_i) exerted not only in a calcium concentration-dependent manner but also in a frequency-dependent manner. High [Ca²⁺]_i-induced APD-ALT was suppressed by not only BAPTA-AM but also nifedipine. On the other hand, PKC inhibitors BIM and Gö 6976 eliminated high [Ca²⁺]_i-induced APD-ALT, and PKC activator PMA was found to induce APD-ALT at normal [Ca²⁺]_i condition. Furthermore, BIM effectively prevented calcium transient alternans (CaT-ALT) and even CaT disorders caused by calcium overload. Moreover, BIM not only eliminated electrocardiographic T-wave alternans (TWA) caused by calcium dysregulation, but also lowered the incidence of ventricular arrhythmias in isolated hearts. What's more, BIM prevented the expression of PKC α upregulated by calcium overload in high calcium-perfused hearts. We firstly found that pharmacologically inhibiting Ca²⁺-dependent PKC over-activation suppressed high [Ca²⁺]_i-induced cardiac alternans. This recognition indicates that inhibition of PKC activities may become a therapeutic target for the prevention of pro-arrhythmogenic cardiac alternans associated with calcium dysregulation.

Critical repolarization gradients determine the induction of reentry-based torsades de pointes arrhythmia in models of long QT syndrome

BACKGROUND

Torsades de pointes arrhythmia is a potentially lethal polymorphic ventricular tachyarrhythmia (pVT) in the setting of long QT syndrome. Arrhythmia susceptibility is influenced by risk factors modifying repolarization.

OBJECTIVE

The purpose of this article was to characterize repolarization duration and heterogeneity in relation to pVT inducibility and maintenance.

METHODS

Sotalol was infused regionally or globally in isolated Langendorff blood-perfused pig hearts (N = 7) to create repolarization time (RT) heterogeneities. Programmed stimulation and epicardial activation and repolarization mapping were performed. The role of RT (heterogeneities) was studied in more detail using a computer model of the human heart.

RESULTS

pVTs (n = 11) were inducible at a critical combination of RT and RT heterogeneities. The pVT cycle lengths were similar in the short and long RT regions. Short-lasting pVTs were maintained by focal activity while longer-lasting pVTs by reentry wandering along the interface between the 2 regions. Local restitution curves from the long and short RT regions crossed. This was associated with T-wave inversion at coupling intervals at either side of the crossing point. These experimental observations were confirmed by the computer simulations.

CONCLUSION

pVTs are inducible within a critical range of RT and RT heterogeneities and are maintained by reentry wandering along the repolarization gradient. Double potentials localize at the core of the reentrant circuit and reflect phase singularities. RT gradient and T waves invert with short-coupled premature beats in the long RT region as a result of the crossing of the restitution curves allowing reentry initiation.

Importance of over-reading ambulatory ECG-based microvolt T-wave alternans to eliminate three main sources of measurement error

BACKGROUND

Ambulatory electrocardiogram (ECG)-based microvolt T-wave alternans values measured by the modified moving average method (MMA-TWA) can be disrupted by T-wave changes that mimic true repolarization alternans.

METHODS

We investigated potential sources of measurement error by studying 19 healthy subjects (12 men; median age, 25) free of known heart disease with 36-month follow-up to establish freedom from significant arrhythmia or syncope. All participants underwent 24-hr continuous 12-lead ECG monitoring. Causes of automated MMA-TWA ≥42 µV episodes were classified based on visual inspection.

RESULTS

A total of 2,189 episodes of automated MMA-TWA episodes ≥42 µV were observed in all subjects (peak MMA-TWA: median, 94 μV; interquartile range, 81-112 μV). All episodes included one or more beats with T-wave deformation which lacked "repeating ABAB pattern" and therefore were identified as TWA measurement error. Causes of such error were categorized as: (a) artifact [72.6% (1,589/2,189), observed in 19 (100%) subjects], more frequently in limb than precordial leads; (b) T-wave changes due to changes in heart/body position [25.5% (559/2,189), observed in 14 (73.7%) subjects], frequently observed in leads V1-2; and (c) postextrasystolic T-wave changes [1.9% (41/2,189), observed in 2 (10.5%) subjects].

CONCLUSIONS

Relying only on automated MMA-TWA values obtained during ambulatory ECG monitoring can lead to incorrect measurement of TWA. Our findings offer the potential to reduce false-positive TWA results and to achieve more accurate detection of true repolarization alternans.

Effect of beta-blockade on quantitative microvolt T-wave alternans in 24-hour continuous 12-lead ECG recordings in patients with long QT syndrome

BACKGROUND

The aim of study was to investigate effects of beta-blockade on microvolt T-wave alternans (TWA), a precursor of lethal arrhythmia, in patients with long QT syndrome (LQTS).

METHODS

Eleven consecutive LQTS patients, types 1 (n = 6), 3 (n = 2), and "non-1, non-2, non-3" (n = 3) were enrolled. All patients underwent 24-hr continuous 12-lead ECG monitoring before and after initiation of beta-blockade therapy. TWA was measured using the modified moving average method.

RESULTS

Seven (63.6%) of the 11 patients studied were symptomatic, with history of cardiac arrest or documented Torsade de Pointes (TdP) in 4 and syncope in three patients. After a median follow-up of 34 months, beta-blockade reduced the number of symptomatic patients to 1 with TdP (p < 0.02), in whom TdP frequency decreased from 25 events/60 months (0.42 event/month) to seven events/69 months (0.1 event/month). In association with this reduction in symptoms, peak TWA decreased by 47% in the cohort after a median of eight months of beta-blockade therapy [from 95 (74-130) to 50 (39.5-64.5) µV, p = 0.01]. All patients exhibited TWA ≥42 µV before beta-blockade therapy, which eliminated these episodes in four patients. Daily frequency of TWA ≥42 µV episodes decreased by 87% [from 15 (6-26) to 2 (0-5) episodes/day, p = 0.009].

CONCLUSIONS

This study is limited by the small sample size and is mainly hypothesis generating. TWA monitoring deserves further evaluation as a risk marker and a guide to therapy in LQTS patients in future large-scale studies.