Multiple ICD shocks in a patient with dilated cardiomyopathy: What is the mechanism?

A 45 year-old man with non-ischaemic cardiomyopathy and a single-chamber ICD (with a lead implanted in the right ventricular apex, RVA) for primary prevention was referred for urgent ablation due to multiple shocks (up to 40) secondary to sustained and incessant regular tachycardias despite amiodarone infusion and deep sedation. A previous cardiac magnetic resonance revealed a severely dilated left ventricle, with a left ventricular ejection fraction of 15% and absence of gadolinium late enhancement.