INTRODUCTION

Catheter ablation is an important therapeutic strategy for ventricular tachycardia (VT). It can reduce the burden of defibrillator shocks, improve quality of life, reduce VT recurrences, and can be life-saving in VT storm. As such, understanding of the surface 12-lead electrocardiogram (ECG) is key for appropriate procedural planning. Despite technical and anatomical limitations, the analysis of the QRS morphologic patterns and vectors can discern the site of myocardial activation, which can be variable depending on the type of VT; the 12-lead ECG identifies the specific site of origin in idiopathic VT (IVT), while in structural heart disease, it identifies the exit from the circuit (Figure 19.1). Herein, we describe typical ECG characteristics for epicardial IVT localization.

VENTRICULAR TACHYCARDIA LOCALIZING CLUES

Several general features in localizing the VT using the ECG include (1) bundle branch block pattern, (2) frontal axis, (3) concordance, and (4) QRS width. The bundle branch block pattern is related to the sequence of right and left ventricular activation. Left bundle branch block (LBBB) morphologies are present in virtually all right ventricular (RV) tachycardias but also may be noted in tachycardias arising from the left ventricular (LV) side of the septum. Instead, right bundle branch block (RBBB) typically suggest LV free wall sites.¹

The frontal axis is related more to the wave of activation (inferior versus superior) of the muscle. Thus, inferior-axis VTs arise high in the basal areas of the heart, RV outflow tract, high LV septum, or high lateral LV, while superior-axis VTs arise from posterior areas of the heart (i.e., cardiac crux).¹

Figure 19.1 Four-step algorithm for identifying epicardial origin from basal superior and lateral LV in the setting of NICM. The three top steps have a high specificity, and the last step is the most accurate. The total sensitivity and specificity of the algorithm in the study population for pace map localization reach 96% and 93%, respectively. Adapted with permission from Valles et al., Circ Arrhythm Electrophysiol. 2010;3:63–71.

The precordial transition gives guidance in whether the origin is basal versus apical. RBBB with V4- to V6-predominant R waves suggests basal sites. (In this case, LV cardiomyopathy should be considered. In RBBB with V4- to V6-predominant S or QS, consider apical sites, and in LBBB, consider the apical septum). In the latter, when transition is at V4 or later, and Q in lead I, consider RV cardiomyopathy. In terms of width, septal VTs generally are narrower than VTs originating on the free wall.1

Several ECG characteristics have been suggested to recognize an epicardial origin of VT.^2,3 Some of the reported ECG features include (1) time to earliest rapid deflection in precordial leads (pseudodelta wave) 34 ms; (2) interval to peak of R wave in lead V2 (intrinsicoid deflection time) 85 ms; and (3) time to earliest QRS nadir in precordial leads (shortest RS complex) 121 ms.⁴ Additionally, a delayed precordial activation measured by a maximum deflection index of 0.55 identified an epicardial LV IVT origin primarily located in proximity to the aortic root and left anterior descending coronary artery, and to the posterior LV septum.⁵ Valles et al.⁶ identified four criteria (q waves in inferior leads, pseudodelta wave 75 ms, maximum deflection index 0.59, and q waves in lead I) having 95% specificity and 20% sensitivity to identify basal-superior/lateral epicardial VTs in nonischemic cardiomyopathy (Figure 19.1). This four-step algorithm identified the origin in 109 of 115 pace maps (95%), 21 of 24 VTs (88%) in the study population, and 19 of 21 VTs (90%) in a validation cohort.

Subsequently, Bazan et al.²

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