The term tachycardiomyopathy refers to a specific form of tachycardia-related cardiomyopathy caused by supraventricular or ventricular tachyarrhytmias. We present a case of 72 years old man with persistent atrial fibrillation. In the past he received three electrical atrial cardioversions and one radiofrequency ablation. At the time of relapses no signs of left ventricular contractile dysfunction. In february 2015 he experienced dyspnoea and palpitations. The ECG showed atrial fibrillation alternating with strips of tachycardia with a wide QRS morphology doubtful for preexcitation or ectopia. These strips were iterative with variable heart rate and were not showing critical coupling and appeared after lengthening of the cycle. The echocardiogram showed severe left ventricular contractile dysfunction. No coronary artery disease and the electrophysiological study confirmed the presence of posteroseptal accessory pathway with decremental and calcium dependent properties. This case report confirms the existence of a cause-effect relationship between irregular heart rhythm and cardiomyopathy. Specifically, the alternation of ventricular activation through nodo- Hisian and accessory pathway can generate dyssynchrony and contractile dysfunction which are reversible after blocking the conduction through accessory pathway.