Novel mapping and ablation approaches within myocardial scar
Creators: name: Luther, Vishal
Publisher: name: Imperial College London
Year: 2017

description: The arrhythmogenic substrate within myocardial scar contains regions of slow conduction that support reentry, characterized by low amplitude and fractionated electrograms (EGMs). Ablation therapy to prevent arrhythmias associated with such scar needs both accurate identification and interpretation of these EGMs and effective delivery of radiofrequency energy. This thesis explored new clinical approaches to the mapping and ablation of these EGMs within scar based arrhythmia. Previous studies from our institution have shown how robotically controlled catheters offer increased stability to achieve deeper lesions. At the start of this thesis, we demonstrated the feasibility of using the Hansen Robotic System within ventricular scar during post-infarct Ventricular Tachycardia (VT) ablation to reduce future implantable cardiac defibrillator (ICD) therapies. This included patients with advanced substrates having already incurred multiple previous ICD therapies. Having proven the possibility of robotic VT ablation, we investigated, as a randomised study, whether earlier substrate ablation after the first ICD therapy may improve outcomes. Earlier ablation delayed the time to VT recurrence compared to a non-ablative approach, though this did not reach statistical significance. In patients with LV ejection fraction &gt;30%, ablation did significantly delay time to recurrence. In these studies, the substrate was broadly characterized by EGMs with bipolar voltage &lt;1.5mV. Inaccurate delineation of the arrhythmogenic substrate may have been responsible for future VT recurrence. We analysed the effectiveness of increasing EGM resolution within scar using the ultra-high density Rhythmia system. This was preliminarily tested during post-AF ablation Atrial Tachycardia (AT) procedures, a more common and stable reentrant model than VT, but also involving complex circuits between scar and conductive tissue. Improving EGM/mapping resolution using the Rhythmia system was not entirely useful, as we were mis-lead by small rotational activations observed on most of the maps, the majority of which were pseudo-reentrant, often related to mis-annotation of EGM local activation time (LAT) and inaccurate interpolation of activation within regions of low voltage. An alternative means of displaying activation within scar without the need to reduce EGMs to a single LAT was required. Ripple Mapping (RM) was previously developed in our institution to provide this alternative means, and displays an EGM in its entirety as a moving bar on the map. We prospectively tested the feasibility of Ripple mapping within scar; we first developed a method of using RM to differentiate regions of ablation related scar from low voltage but functional tissue in iatrogenic ATs, and observed very high ablation success. This was reproducible during multi-centre studies. Within post-infarct ventricular scar, mapping in sinus rhythm/controlled pacing, RM helped us to differentiate local from far-field activation, especially with small tip and narrowly spaced electrodes, and visualise channels of delayed conduction through scar that collocated with the diastolic pathway mapped in VT, ablation of which reduced future VT recurrence. However, ablation within these ventricular channels did not eliminate future VT recurrence. We considered whether the lines of block bordering these channels may be functional by comparing the scar locations under different cycle lengths and activation directions. Remapping post ablation atrial scars under different cycle lengths and activation directions using RM demonstrated a generally fixed scar distribution. Within ventricular scars, where the diastolic pathway could be mapped in VT, in certain areas, it was formed of functional block during tachycardia with preserved EGMs in sinus rhythm; further studies within ventricular scar under different pacing rates and activation directions may better our understanding of the ventricular substrate. In conclusion, this thesis proposes that RM be considered the gold standard approach to mapping EGMs to determine the arrhythmogenic substrate in post ablation ATs. In post-infarct VT, undertaking ablation using robotic navigation after the first ICD therapy, specifically in patients with EF &gt;30%, and using RM based approach to define this arrythmogenic substrate can help to improve outcomes.