**8. Conclusion**

connected to the navigation system. In (b) and (c) on the right side can be seen the ultrastructural substrate which corresponds to normal zone with mitochondria, myofibrils, and a Purkinje cell of normal characteristics. In contrast, in the peripheral zone of substrate (e and f) note that when approaching to pathological area, cytoplasmic vacuolization, myofibrillar and mitochondrial disorganization with myofibrillar residue can be observed. The (h) and (i) corresponds to the central zone, which depicts strong vacuolization and cell destruction with intense cytoplasmic disorganization and myofibrillar residue. In the **Figure 8B**, the patient N° 11 in (a), (d) and (g) shows the voltage and electro-anatomic map (functional substrate) with central

side can be seen the ultrastructural substrate which corresponds to normal zone with normal characteristics of mitochondria, myofibrils, and a Purkinje cell. The approaching to pathological area in the peripheral zone of substrate (e and f), myofibrillar disorganization, citoplasmic vacuolization, swelling and disappearance of mitochondrial crests, myofibrillar rests and remains of erythrocytes were observed. The (h) and (i) corresponds to the central zone showing strong vacuolization and cell destruction and myofibrillar residue. Fat replacement, lymphocytic infiltration, Chagasic myocarditis, collagen tissue or apoptotic bodies were not observed.

It is important to note that when we approach to pathological areas progressive cell damage was observed. In the central zone of substrate low voltage systolic EGMs coincided with strong cell destruction and cytoplasmic disorganization. The peripheral zone of substrate with cell damage, mitochondrial swelling and myofibrillar residue without apoptotic bodies coincided with late potentials, diastolic and/or presystolic activity (**Figures 2** and **3**). These findings support mitochondrial energy loss as possible non-apoptotic progressive tissue damage and death cell. Our results suggest the interesting possibility that substrate could be generated by an abnormal expression of neural crest cells localized in RVOT during cardiac development. Because an epicardial and endocardial substrate was demonstrated, our findings together

Two theories suggest that an abnormal repolarization (local re-excitation by phase 2 reentry in the epicardium) or a defect of the depolarization (disturbances in depolarization of RVOT can cause conduction delay) may be responsible of phenotype and VF in BrS [40–43]. However, in BrS the arrhythmias are usually polymorphic VT or VF, and these cannot be supported by macro-reentry mechanisms. VF depends of a firing focus initiated by early or delayed afterdepolarization or a micro-re-entry [44]. Surviving cells surrounded by fibrosis has demonstrated to be responsible of slow conduction and reentry in inhomogeneous scars [45]. The residual electrical activity within scar was reported as delayed or isolated EGMs, late potentials or diastolic EGMs and their elimination during sinus rhythm was effective to prevent VT/VF [45]. In peripheral zone of substrate when a sufficient degree of cell damage was reached such as we found with TEM and resting potentials are reduced the polymorphic VT/VF may occur. This event could be originated through a firing focus or by multiple wavelets from a reentrant microcircuit, and would explain the "diastolic electrical activity" observed in our patients.

with those of other researchers support the probability of a transmural substrate.

located in the top-septal zone of the RVOT. In (b) and (c) on the right

area of substrate of 8mm2

136 Cardiac Arrhythmias

**7. Mechanisms of arrhythmias**

In patients with BrS of high risk the substrate RFA may be a potential option of treatment. We successfully ablated the substrate of BrS from the endocardium based on the electrophysiological and ultrastructural findings. Our data together with the observations of other researchers suggest a transmural substrate, contributing to future definition of the arrhythmogenic substrate in BrS. As many phenotypes are involved in BrS, it is not unthinkable that different substrates may exist in BrS. ECG analysis during administration of a sodium channel blocker allows approximately determined the size and location of substrate. Careful endocardial mapping allows identify late potentials, presystolic and diastolic EGMs as a new risk marker to guide an endocardial substrate RFA, probably with the same results what a more complex epicardial ablation. A comparative study between endocardial and epicardial RFA should be performed.
