Myocardial structural associations with local electrograms: a study of postinfarct ventricular tachycardia pathophysiology and magnetic resonance-based noninvasive mapping

Abstract

Background: The association of scar on late gadolinium enhancement cardiac magnetic resonance (LGE-CMR) with local electrograms on electroanatomic mapping has been investigated. We aimed to quantify these associations to gain insights regarding LGE-CMR image characteristics of tissues and critical sites that support postinfarct ventricular tachycardia (VT).

Methods and results: LGE-CMR was performed in 23 patients with ischemic cardiomyopathy before VT ablation. Left ventricular wall thickness and postinfarct scar thickness were measured in each of 20 sectors per LGE-CMR short-axis plane. Electroanatomic mapping points were retrospectively registered to the corresponding LGE-CMR images. Multivariable regression analysis, clustered by patient, revealed significant associations among left ventricular wall thickness, postinfarct scar thickness, and intramural scar location on LGE-CMR, and local endocardial electrogram bipolar/unipolar voltage, duration, and deflections on electroanatomic mapping. Anteroposterior and septal/lateral scar localization was also associated with bipolar and unipolar voltage. Antiarrhythmic drug use was associated with electrogram duration. Critical sites of postinfarct VT were associated with >25% scar transmurality, and slow conduction sites with >40 ms stimulus-QRS time were associated with >75% scar transmurality.

Conclusions: Critical sites for maintenance of postinfarct VT are confined to areas with >25% scar transmurality. Our data provide insights into the structural substrates for delayed conduction and VT and may reduce procedural time devoted to substrate mapping, overcome limitations of invasive mapping because of sampling density, and enhance magnetic resonance-based ablation by feature extraction from complex images.

Figure 1

(A) Scar was identified by LGE-CMR. PI-ST and LV-WT were determined by both LGE-CMR and Cine-CMR. (B) Mapping points on EAM were registered to the corresponding region on short-axis planes of LGE-CMR. (C) Scar on LGE-CMR was divided into 8 types by scar transmurality (PI-ST/LV-WT = 0–25, 26–50, 51–75, or 76–100%) and intramural scar location (normal, endocardial, mid wall, transmural). (D) EGM characteristics on EAM were defined as EGM parameters (bipolar and unipolar EGM voltages, duration, deflections) and EGM types (normal, fractionated EGM, isolated potential, abnormal EGM).

Figure 2

The association of EGM parameters or EGM types with scar transmurality. (A) Bipolar and unipolar EGM voltage amplitudes were negatively associated with endocardial (P<0.0001) and mid wall scar transmurality (P<0.0001). (B) EGM duration and deflections were positively associated with endocardial (P<0.0001) and mid wall scar transmurality (P<0.0001). The Box plots show the interquartile range (grey bar) and the median (black triangle). (C) Fractionated and isolated potential were more frequently observed in scar regions with greater scar transmurality (P<0.0001).

Figure 3

The association of ablation sites and critical VT sites with scar transmurality. (A) Ablation sites were observed in regions with scar and normal myocardium adjacent to the scar. (B) Critical sites were identified only in scar region with >25% scar transmurality. (C) Significant associations among EGM parameters and EGM types with scar transmurality were observed (except EGM deflections) within critical sites. (D) S-QRS (R=0.768, P<0.0001) and %S-QRS/VT-CL (R=0.690, P<0.0001) were significantly associated with scar transmurality. Critical sites with ≥40 msec of S-QRS and ≥30% of %S-QRS/VT-CL were identified by >75% scar transmurality.

Figure 4

Qualitative comparison of the invasive versus non-invasive substrate map created using LV-WT and post-infarct scar thickness (PI-ST) on LGE-CMR, and a single iteration of the leave one cross-validation methodology. Upper panel: Leave one out cross-validation substrate map results for bipolar and unipolar EGM voltage, duration and deflections. Middle panel: Higher resolution qualitative comparison of the non-invasive map with invasive maps in image subsets. The resolution of the non-invasive maps (1.5×1.2×8 mm) was higher than that of invasive maps obtained by point-by-point mapping; therefore, minute differences in image characteristics of the predicted map versus the invasive map may be due to extrapolation of each “real” electrogram to nearby areas on the invasive map. Lower panel: Invasive EAMs of bipolar and unipolar voltage, duration and deflection and ablation points (red points). The EAM system does not provide duration and deflection maps routinely, the invasive duration and deflection measures were manually entered for each 3-dimensional location for color display and comparison to the predicted maps.