INTRODUCTION

Amid increasing sophistication in cardiac imaging and other noninvasive tools, the baseline 12-lead ECG should not be overlooked as an important tool in noninvasively detecting epicardial ventricular tachycardia (VT) substrate. Among patients with nonischemic cardiomyopathy (NICM), arrhythmogenic substrate most commonly involves the basal-lateral left ventricle (LV) and often includes the epicardium.^1,2 In many cases, this scar then can produce changes on the sinus rhythm ECG that somewhat mimics what might be observed on an ECG obtained from a patient with prior posterior myocardial infarction (MI): greater R-wave amplitude in V1 and diminished R wave transition in the lateral precordial leads (Figure 22.1).

In a series of NICM patients without bundle branch block or ventricular pacing at the University of Pennsylvania, this hypothesis was tested by comparing 12-lead QRS morphologies among patients with VT and lateral perimitral scar, and those from NICM patients without VT.³ ECGs from 25 eligible patients who underwent electrophysiological study and ablation (more than half with epicardial substrate) were compared with ECGs from 18 reference patients with NICM and no VT, in the first of a two-phase analysis.³ R-wave amplitude (0.25 mV vs. 0.09 mV, P = 0.001) and corresponding R:S ratio (0.3 vs. 0.1, P = 0.001) in lead V1, as well as V6 S-wave amplitude (0.23 mV vs. 0.09 mV, P < 0.001) and S:R ratio (0.5 vs. 0.1, P = 0.001), were significantly greater than those with NICM but no VT or scar. Additionally, in multivariable analyses adjusting for LV ejection fraction and multicollinearity, R-wave amplitude ≥ 0.15 mV in lead V1, as well as S amplitude ≥ 0.15 mV and S:R ratio ≥ 0.2 in lead V6, distinguished those with NICM and VT with basal-lateral scar from reference patients with NICM but no VT. When applying these criteria prospectively to a validation cohort comprised of 15 NICM patients without bundle branch block or V-pacing, 7 of whom had VT with basal-lateral scar involving the epicardium, a combination of V1 R ≥ 0.15 mV and V6 S ≥ 0.15 mV provided sensitivity and specificity of 86% and 88%, respectively (Table 22.1).³

A post-infarct scar in a posterior distribution may also mimic the above findings. As the mapping/ablation approach (including the need for epicardial access) and outcomes may differ based on the underlying etiology, additional criteria to differentiate ischemic versus nonischemic basal-lateral scar have been developed.⁴ These clues may be particularly useful, as the phenomenon of mixed cardiomyopathy, that is, prior coronary artery disease but nonischemic VT substrate, is increasingly recognized.⁵ Sinus rhythm or atrial-paced ECGs without bundle branch block or ventricular pacing among patients presenting to the University of Pennsylvania for VT ablation were compared, 30 from those with ischemic cardiomyopathy (ICM) due to prior inferior or inferolateral MI (confirmed by presence of right coronary artery or left circumflex arterial occlusions on angiography) and 25 from those with NICM and basallateral scar. Features of QRS fragmentation in the lateral leads (Figure 22.2), absence of inferior Q waves, and lead V6 S/R ratio were independently associated with NICM on multivariable analysis. In adjusted forward stepwise regression analysis, a V6 S/R ratio of ≥ 0.25 was found to produce the best discriminating capability between the substrates, with 100% sensitivity, 77% specificity, 100% negative predictive value, and 78% positive predictive value (Figure 22.3).⁴

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