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Tissue characterisation for the management of cardiac arrhythmia

Project ID: 2015_XXY

Student: Karina Lopez

1st supervisor: Rene Botnar, King’s College London
2nd supervisor: Reza Razavi, King’s College London
Industry supervisor: Radhouene Neji, Siemens

The aim of this project is to develop novel quantitative MR techniques to characterize cardiac tissue for the management of cardiac arrhythmias. This project has a particular focus on driving the innovation that underpins an interventional MRI electrophysiology program, and these techniques can be divided into three main areas of arrhythmia management. Firstly, techniques will be developed for the assessment of the arrhythmia substrate within the atria and ventricles, guiding risk stratification and intervention planning. This will include substrate analysis for ventricular tachycardia (VT) and atrial fibrosis assessment, including fibrosis/scar quantification and wall thickness. Secondly, techniques will be developed for the live guidance of ablation procedures. Hyperacute (during radiofrequency (RF) energy delivery) and acute (within 60 min of RF delivery) techniques will include T1 mapping (3D and 2D), diffusion weighted imaging, T2 weighted imaging and gadolinium enhanced imaging (early and late). Thirdly, imaging techniques of the effect and quantification of ablation will be optimised, building upon a 3D LGE technique.

Imaging techniques play an important role in the management of arrhythmias, from characterisation of the baseline arrhythmia substrate, to guidance of interventional procedures and finally to assessment of the effects of interventions. This project will focus on two main, clinically important, arrhythmic pathologies: atrial fibrillation and ventricular tachycardia. MR imaging techniques will be developed and tailored for each pathology. In particular, MR imaging techniques will be developed to assist in the guidance and assessment of novel MR-guided ablation techniques.

The arrhythmias under investigation are:
Atrial fibrillation (AF) is a common cardiac arrhythmia characterized by rapid, uncoordinated atrial activation resulting in a reduced atrial mechanical function. In the UK, up to one million people are diagnosed with AF, with 50,000 new cases and 500,000 hospital admissions each year. Treatment strategies include rate control with drugs, direct current cardioversion or radiofrequency catheter ablation (RFA) to achieve pulmonary vein electrical isolation (PVI) and atrial electrical modification.  Catheter ablation treatment of the early form of AF (‘paroxysmal AF’) is effective in up to 90% of patients, while for those with the longer lasting ‘persistent’ form, the overall success rate is lower (60 – 70%) and up to half of patients require multiple procedures. The ablation procedure would highly profit from a more detailed knowledge of the atrial tissue, targets for ablation and assessment of lesion adequacy.

Ventricular tachycardia (VT), often degenerating into ventricular fibrillation, is responsible for 75–80% of the 70,000 annual sudden cardiac deaths (SCD) in England and Wales. Prevention and treatment are currently imperfect and new management strategies are required. Pharmacological treatment has variable effectiveness so the most reliable treatment strategy remains the implantation of internal cardioverter-defibrillators (ICDs). These are used in patients with previous episodes of sustained VT or ventricular fibrillation (‘secondary prevention’) or in patients deemed at high risk, including those with heart failure and reduced systolic function (‘primary prevention’). However, patients with ICDs continue to suffer from episodes of VT: these episodes and associated ICD shocks decrease quality of life (Irvine, Am Heart J, 2002) and increase mortality (Poole, NEJM, 2008).  Radiofrequency ablation (RFA) remains the only curative treatment strategy, but is limited by relatively low long-term success rates with up to 50% late recurrence post ablation (Ghanbari, CircEP, 2014). The pathophysiology of VT is complex and is most commonly associated with abnormal myocardial substrate, with ischaemic or structural heart disease causing scaring of the myocardium. The scarring affects the normal conduction of electrical signals, producing abnormal conducting channels that lead to re-entry circuits sustaining VT. The recent VTACH and SMASH-VT trials show the promise of prophylactic VT ablation in reducing the ICD shock burden. However, a substantial improvement in the success rate of VT ablation is required to obviate the need for ICD implantation.

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