Microvolt Activation Alternans Identifies Conduction Velocity Heterogeneity and the Entrance Site of Ischemic Ventricular Tachycardia.

Background: Body surface microvolt QRS alternans has been shown to predict ventricular tachyarrhythmias in patients with cardiomyopathy; however, the mechanism is unclear.

Objectives: The aim of this study was to determine the spatiotemporal relationship of beat-to-beat bipolar electrogram (EGM) activation alternans (AA) to the ventricular tachycardia (VT) circuit and to surface QRS alternans.

Methods: Intraoperative mapping during ventricular pacing and induced VT was performed in 9 patients with ischemic cardiomyopathy using a 112-bipole endocardial array. AA was quantified at EGM sites during pacing, timed to the surface QRS complex (intra vs post), correlated to local conduction velocity (CV), the location of the VT circuit, and surface QRS alternans. In silico modeling was performed to investigate the relationship between conduction disturbances and AA.

Results: Intra-QRS and post-QRS AA was present in 8 of 9 patients with similar frequency (4.6% [Q1-Q3: 2.8%-8.3%] vs 3.5% [Q1-Q3: 1.0%-4.7%]; P = 0.77) and magnitude (192 μV [Q1-Q3: 72-246 μV] vs 470 μV [Q1-Q3: 270-601 μV]; P = 0.158). The presence of intra-QRS AA strongly correlated with the entrance of the VT circuit compared with remote regions (OR: 6.8; 95% CI: 3.7-12.5; P < 0.001). CV heterogeneity was the primary determinant of intra-QRS AA magnitude, in which each 0.1 increase in heterogeneity correlated with a 31-μV (95% CI: 3-59 μV) magnitude increase (P = 0.032). Intra-QRS AA magnitude strongly correlated with surface QRS alternans magnitude (r = 0.92, P = 0.001). Computer modeling supported AA sites representing <1-mm channels with 2:1 conduction block.

Conclusions: Microvolt intra-QRS AA in the bipolar EGM is a marker of CV heterogeneity that associates with the VT entrance and may guide VT ablation. Surface QRS alternans is a manifestation of intra-QRS AA and may inform risk stratification.