Background Ventricular tachycardia with structural heart disease is dependent on re-entry within scar regions. We set out to assess the VT circuit in greater detail than has hitherto been possible, using ultra highdensity mapping. Methods All ultra high-density mapping guided ventricular tachycardia ablation cases from six high-volume European centres were assessed. Maps were analysed offline to generate activation maps of tachycardia circuits. Topography, conduction velocity and voltage of the VT circuit were analyzed in complete maps. Results Thirty-six tachycardias in 31 patients were identified, 29 male and 27 ischaemic. VT circuits and isthmuses were complex, eleven were single-loop and 25 double-loop; three had two entrances, five had two exits and 15 had dead ends of activation. Isthmuses were defined by barriers which included anatomical obstacles, lines of complete block and slow conduction (in 27/36 isthmuses). Median conduction velocity was 0.08m/s in entrance zones, 0.29m/s in isthmus regions (p<0.001), and 0.11m/s in exit regions (p=0.002). Median local voltage in the isthmus was 0.12mV during tachycardia and 0.06mV in paced/sinus rhythm. Two circuits were identifiable in five patients. The median timing of activation was 16% of diastole in entrances, 47% in the mid isthmus, and 77% in exits. Conclusions VT circuits identified were complex, some of them having multiple entrances, exits and dead ends. The barriers to conduction in the isthmus appear to be partly functional in 75% of circuits. 3 Conduction velocity in the VT isthmus slowed at isthmus entrances and exits, when compared with the mid isthmus. Isthmus voltage is often higher in VT than in sinus or paced rhythms.

This study shows that localized structural alterations underlie a significant subset of previously unexplained sudden cardiac death. In the other subset, Purkinje electrical pathology seems as a dominant mechanism.

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Mark Potse, Corentin Dallet, et al.. Effect of the torso conductivity heterogeneities on the ECGI inverse problem solution.Abstract-The effect of torso conductivity heterogeneities on the electrocardiographic imaging (ECGI) inverse problem solution is still subject of debate. In this study we present a method to assess the effect of these heterogeneities. We use an anatomical model containing the heart the lungs the bones and the torso surfaces. We use the bidomain model and we solve it using finite element methods in order to generate in silico data taking into account the torso heterogeneities. We add different noise levels on the body surface potentials and we solve the inverse problem for both homogenous and heterogeneous torso conductivities. We analyse the reconstructed solution using the relative error and the correlation coefficient.

Background: Catheter ablation of ventricular tachycardia (VT) in structural heart disease is challenging because of noninducibility or hemodynamic compromise. Ablation often depends on elimination of local abnormal ventricular activities (LAVAs) but which may be hidden in far-field signal. We investigated whether altering activation wavefront affects activation timing and LAVA characterization and allows a better understanding of isthmus anatomy. Methods: Patients with ischemic cardiomyopathy underwent mapping using the ultra-high density Rhythmia system (Boston Scientific). Maps were generated for all stable VTs and with pacing from the atrium, right ventricular apex, and an left ventricular branch of the coronary sinus. Results: Fifty-six paced maps and 23 VT circuits were mapped in 22 patients. In 79% of activation maps, there was ≥1 line of block in the paced conduction wavefront, with 93% having fixed block and 32% showing functional partial block. Bipolar scar was larger with atrial than right ventricular (31.7±18.5 versus 27.6±16.3 cm 2 , P =0.003) or left ventricular pacing (31.7±18.5 versus 27.0±19.2 cm 2 , P =0.009); LAVA areas were smaller with atrial than right ventricular (12.3±10.5 versus 18.4±11.0 cm 2 , P <0.001) or left ventricular pacing (12.3±10.5 versus 17.1±10.7 cm 2 , P <0.001). LAVA areas were larger with wavefront propagation perpendicular versus parallel to the line of block along isthmus boundaries (19.3±7.1 versus 13.6±7.4 cm 2 , P =0.01). All patients had successful VT isthmus ablation. In 11±8 months follow-up, 2 patients had a recurrence. Conclusions: Wavefronts of conduction slowing/block may aid identification of critical isthmuses in unmappable VTs. Altering the activation wavefront leads to significant differences in conduction properties of myocardial tissue, along with scar and LAVA characterization. In patients where few LAVAs are identified during substrate mapping, using an alternate activation wavefront running perpendicular to the VT isthmus may increase sensitivity to detect arrhythmogenic substrate and critical sites for reentry.

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Ventricular (VT) and atrial (AT) tachycardias are some of the most common clinical cardiac arrhythmias. For ablation of tachycardia substrates, two clinical diagnosis methods are used: invasive electro-anatomical mapping for an accurate diagnosis using electrograms (EGMs) acquired with intra-cardiac catheters, and localized on the surface mesh of the studied cavities; and non-invasive electrocardiographic imaging (ECGi) for a global view of the arrhythmia, with EGMs mathematically reconstructed from body surface electrocardiograms using 3-D cardio-thoracic surface meshes obtained from CT-scans. In clinics, VT and AT are diagnosed by studying activation time maps that depict the propagation of the activation wavefront on the cardiac mesh. Nevertheless, slow conduction areas - a well-known pro-arrhythmic feature for tachycardias - and tachycardia specific propagation patterns are not easily identifiable with these maps. Therefore, local characterization of the activation wavefront propagation can be helpful for improving VT and AT diagnoses. The purpose of this study is to develop a method to locally characterize the activation wavefront propagation for clinical data. For this, a conduction velocity (CV) vector field is estimated and analyzed using divergence and curl mathematical operators. The workflow was first validated on a simulated database from computer models, and then applied to a clinical database obtained from ECGi to improve AT diagnosis. The results show the relevancy and the efficacy of the proposed method to guide ablation of tachyarrhythmias.

Meshing the heart and measurement surfaces can be time consuming, especially when dealing with complicated geometries or cardiac motion. To overcome this, a meshless method based on the method of fundamental solutions (MFS) has been adapted to non-invasive electrocardiographic imaging (ECGI). In the MFS, potentials are expressed as a summation over a discrete set of virtual point sources placed outside of the domain of interest (named 'pseudo-boundary'). It is well-known that optimal placement of the pseudoboundary can improve the efficacy of the MFS. Despite this, there have been no attempts to optimize their placement in the ECGI problem as far as we are aware. In the standard MFS, the sources are placed in two pseudo-boundaries constructed by inflating and deflating the heart and torso surfaces with respect to the geometric center of the heart. However, for some heart-torso geometries, this geometric center is a poor reference. We here show that adaptive placement of the pseudoboundaries (depending on the distance between the torso electrodes and the nearest heart locations) improves the conditioning of the inverse problem, making it less sensitive to the regularization process.