(E-165) Effects of Combined Biventricular Pacing Locations on Anti-Tachycardia Pacing Success in a Patient-Specific Model

Introduction:: Ventricular tachycardia (VT) affects over 300,000 Americans annually and is the leading cause of sudden cardiac death. Patients with drug-refractory VT often undergo implantation of a cardiac defibrillator (ICD), which can automatically deliver shocks during a VT episode. While life-saving, shock from an ICD can be traumatic. To combat the need for shock, ICDs come equipped with low-energy pacing protocols to help reduce the number of shock instances. These anti-tachycardia pacing (ATP) methods are conventionally delivered from a lead inserted at the apex of the right ventricle (RV) but has limited success. Recent in silico studies have shown the promise of biventricular leads placed in the left ventricle for ATP delivery. These studies have neither demonstrated efficacy in human heart models nor investigated simultaneous pacing protocols. Furthermore, ICD implantation is often done without the knowledge of scar distribution and VT exit sites. Therefore, finding the critical position at which a single-point stimulus terminates VT would be difficult without information from a computational model. Thus, a generalizable and clinically-feasible approach to biventricular ICD ATP delivery is needed. In this study, we hypothesized that stimulating ATP from multiple points simultaneously may allow for greater termination rates than the conventional RV apex approach by increasing the likelihood of stimulating from a site critical to VT termination. To address this hypothesis, we examined the effect of clinically relevant biventricular pacing locations on the ability to deliver ATP and terminate VT in a patient-specific computational model of the ventricles.


Materials and Methods:: Patient Late-Gadolinium Enhanced Magnetic Resonance Images were collected from a single patient at the University of Utah health systems and used to segment both ventricles. A volumetric mesh was then generated using TetGen. The resulting mesh contained over 1 million nodes and an average edge length of 0.928mm. A rule-based algorithm was used to assign cardiac fiber angle throughout the myocardium. The open-source finite element electrophysiology (EP) modeling software OpenCARP was used for all EP simulations. The ion exchange at the cell membrane was modeled according to Ten Tusscher and Panfilov. Membrane kinetics in areas of fibrosis were altered based on data from previous literature to replicate heterogeneity of diseased tissue. The patient-specific mesh underwent VT induction protocols where a train of eight S1 pulses was applied at a pacing interval of 600ms followed by a single S2 pulse with intervals ranging from 280-350ms. Simulations that initiated a fast VT circuit (< 320ms) continued until a stable reentry site was found. A single site at the RV apex and three sites along the lateral LV free wall (FW) in the apicobasal direction were defined as possible ATP delivery sites, representing feasible BiVP ICD lead implantation sites (RV, LowFW, MidFW, UpFW). Combinations of these ATP delivery sites are tabulated in Table 1. A single ATP train of 8 pulses at 88% of the VT cycle length was applied from each stimulus site combination. The absence of any active mesh elements ( >-40mV) following ATP delivery determined the successful termination of VT.


Results, Conclusions, and Discussions:: Termination of VT varied depending on the stimulus sites utilized for delivery. When the LowFW site was included, all ATP protocols resulted in the termination of the VT circuit. All locations and combinations on the LV free wall except for one did not have altered termination when combined with RV pacing. Previous research demonstrated the ability of proximal ATP locations to terminate fast VTs. The most proximal location to our exit site (LowFW) successfully terminated VT in all cases where that site was included for ATP pacing. Interestingly, the second most proximal site (RV apex) does not follow this trend. This suggests that the critical distance for a single-point stimulation to terminate VT may be highly sensitive and directional.Here we demonstrated that combining stimulation sites does not alter termination success when the critical point is included. Combining the RV apical stimulus site with any combination of LV sites did not affect ATP success in all but one case. Thus, including the RV as a stimulus site may allow the ATP delivery method to be directionally independent without affecting the efficacy of other stimulation sites. Combining sites may increase the likelihood of including a critical stimulus site when such information cannot be ascertained. Further studies will be needed to assess the robustness and feasibility of such an approach. We suspect these observations largely depend on the patient-specific infarct, its location, and the VT beat analyzed. Clinically, these factors are rarely known at the time of ICD implantation. To generalize ATP delivery methods, comparing against other patient-specific models is critical. In subsequent studies, we will add additional patient-specific models and VT beats of varying cycle length to this analysis and assess if the trends we observe in this case persist.


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