Noninvasive clues for diagnosing ventricular tachycardia mechanism

Genetically Determined Serum Calcium Levels and Markers of Ventricular Repolarization: A Mendelian Randomization Study in the UK Biobank

BACKGROUND

ECG markers of ventricular depolarization and repolarization are associated with an increased risk of arrhythmia and sudden cardiac death. Our prior work indicated lower serum calcium concentrations are associated with longer QT and JT intervals in the general population. Here, we investigate whether serum calcium is a causal risk factor for changes in ECG measures using Mendelian randomization (MR).

METHODS

Independent lead variants from a newly performed genome-wide association study for serum calcium in >300 000 European-ancestry participants from UK Biobank were used as instrumental variables. Two-sample MR analyses were performed to approximate the causal effect of serum calcium on QT, JT, and QRS intervals using an inverse-weighted method in 76 226 participants not contributing to the serum calcium genome-wide association study. Sensitivity analyses including MR-Egger, weighted-median estimator, and MR pleiotropy residual sum and outlier were performed to test for the presence of horizontal pleiotropy.

RESULTS

Two hundred five independent lead calcium-associated variants were used as instrumental variables for MR. A decrease of 0.1 mmol/L serum calcium was associated with longer QT (3.01 ms [95% CI, 2.03 to 3.99]) and JT (2.89 ms [1.91 to 3.87]) intervals. A weak association was observed for QRS duration (secondary analyses only). Results were concordant in all sensitivity analyses.

CONCLUSIONS

These analyses support a causal effect of serum calcium levels on ventricular repolarization, in a middle-aged population of European-ancestry where serum calcium concentrations are likely stable and chronic. Modulation of calcium concentration may, therefore, directly influence cardiovascular disease risk.

Ventricular tachycardia as a consequence of triggered activity

One of the less frequent underlying mechanisms of ventricular tachycardia (VT) is triggered activity. Triggered activity refers to an extrasystole due to a premature depolarization that occurs when the amplitude of an early or delayed afterdepolarization brings the cardiac membrane to its threshold potential. Hydrochlorothiazide and hydroxyzine can prolong repolarization and QT interval and are associated with early afterdepolarizations. Cyclic AMP-mediated, delayed afterdepolarizations can occur as a result of catecholaminergic surge. Delayed afterdepolarization is classically associated with outflow tract (OT) tachycardia, a type of VT that is uniquely defined by its termination with adenosine. We present a case of triggered OT tachycardia for which intravenous amiodarone through its antiadrenergic effect may have been effective. Infusions of magnesium and a cardioselective, β-receptor antagonist that does not prolong repolarization may have been more appropriate given the concurrent, acquired prolonged QT syndrome. After initial stabilization, considering the underlying VT mechanism may prompt the clinician to select the most appropriate, further treatment.

Ventricular Arrhythmias

Ventricular tachycardia is commonly seen in medical practice. It may be completely benign or portend high risk for sudden cardiac death. Therefore, it is important that clinicians be familiar with and able to promptly recognize and manage ventricular tachycardia when confronted with it clinically. In many cases, curative therapy for a given ventricular arrhythmia may be provided after a thorough understanding of the underlying substrate and mechanism. In this article, the authors broadly review the current classification of the different ventricular arrhythmias encountered in medical practice, provide brief background regarding the different mechanisms, and discuss practical diagnosis and management scenarios.