Not Premature to Quantitatively Define the Near-Field Bipolar Electrogram for Premature Ventricular Contraction Ablation

We are writing you to express our appreciation of the strong interest in our recent paper published in the Journal “Quantitative analyses of the distal bipolar electrogram for focal premature ventricular contractions [1]”, submitted by Sathian et al. [2, 3] in letters to the editor. We value their thoughtful opinions, many of which we have shared in the section of limitations of the study. However, some other critics are worthy of discussion.

Our study attempts to quantify the bipolar EGM numerically; undoubtedly, it is an overdue study moving forward from the long-held empirical approaches. Among the 5 parameters we have used, three (i.e., half time of the activation, t_½, slope factor obtained by fitting the Boltzmann equation, S, and time to the surface QRS, T_s) showed excellent performance in distinguishing the near-field bi-EGM from the far-field. These 3 parameters quantify the sharpness and the earliness of the bi-EGM—quantitative but consistent with the empirical approach we and other labs have been using in daily practice. Certainly, the performance of these parameters needs to be further validated with prospective multi-center studies. Thank you to the authors for supporting that idea.

When it comes to unipolar vs. bipolar mapping (uni- and bi-EGM), the experience of many years seems telling us that the uni-EGM helps us “get to the door, not into the house”. We certainly use uni-EGM embedded in commercial mapping software, as stated in the Methods section, to guide us for the initial localization of PVC foci. But uni-EGM appears to have a few intrinsic short comes to pin down the precise focus: (1) the “large bipolar nature” of the uni-EGM clearly falls short in distinguishing the difference of EGM at the millimeter scales; and (2) it is location-dependent, more useful at the sites where propagation of the activation wave front is rather uni-directional, such as bypass tract insertion site at the atrioventricular annuli and PVCs at the free wall (e.g., the free wall of the tricuspid/mitral annulus and the ventricular outflow tract), but not so at the sites where the propagation of the wave front is 4-directional, such as scar VT circuit or PVCs deeper embedded. The usefulness of the two has long been debated, that is beyond the scope of our current study. We consider that they are complementary, with the approach we proposed in Methods section [1].

For fractionation and successful ablation, it was a surprise to us that the fraction was not predictive for success in ablating focal PVCs as it has been long used in this setting. However, physiologically, it does make sense if the concept of that focal PVC is due to localized triggered activity or increased automaticity is correct [4]. Certainly, it needs further validation and more precise quantification such as frequency analyses. Additionally, our unpublished observation suggests that the timing of the fractionation may play a role in predicting success of ablation, that we are hoping to finish as a separate project.

For the lesion depth, thanks to the authors for citing the reference by Fu et al. [5], which was published in late 2024 and we have missed while finishing our manuscript. However, we still consider our assumption of a conduction velocity of 40 cm/s to be reasonable as the conduction of a PVC inside the myocardium to the endocardial surface must be mostly transverse given the orientation of the myocardial fibers (see the illustration in the graphic abstract [1]). The myocardium may consist of 3 layers: the inner layer is known to run mostly longitudinal along the long axis of the heart, the middle layer mostly circular around the short axis, and the epicardial layer obliquely slanted. Between the layers, there are cleavage planes that make the conduction non-isotropic; in other words, much slower transversely [6, 7]. Further, if a faster conduction velocity is assumed, eg., 60 cm/s, then the predicted near-field bi-EGM would reach 6 mm, therefore the lesion depth. It seems to be too deep for currently known lesions created by thermal ablation, even with irrigated catheters. After all, this depth is just an estimate, which may be affected by multiple factors such as contact force, the content of connective tissue and scar, and probably even the amount of blood flow and the type of irrigation fluid, and needs to be verified further.

Again, we are grateful for the opportunity of writing this letter and appreciate the interest in our study. We certainly hope our study will prompt more robust researches in quantitating the bi-EGM and defining their use in the ablation of PVCs.