In spite of the available clinical and electrocardiographic criteria for the differential diagnosis of wide QRS complex tachycardias, distinguishing orthodromic supraventricular tachycardias is still a challenge. We present a case of a 63-year old patient admitted in our clinic after experiencing two episodes of syncope. Echocardiography showed left ventricular hypertrophy, grade 1 diastolic dysfunction and left atrial enlargement. A Holter monitoring revealed episodes of atrial fibrillation and paroxysmal narrow QRS tachycardia alternating with wide QRS tachycardia (170-180 bpm). During an electrophysiology study we induced self-limiting orthodromic supraventricular tachycardias with narrow and left bundle branch block patterns. Retrograde mapping near the mitral annulus identified a concealed accessory posteroseptal by-pass tract which was successfully ablated. After the procedure the patient developed atrial flutter and atrial fibrillation with rapid ventricular response (196 beats per minute) with a 3.9 s post-tachycardia pause. The patient underwent implantation of a cardiac pacemaker which allowed us to start antiarrhythmic treatment with amiodarone. This case shows that occult accessory posteroseptal by-pass tracts can have a late-onset presentation in a 63-year old male and explains why latent rhythm disturbances require a step-by-step medical approach.

Introduction: Cardiac arrhythmias caused by electrical injuries are rare among emergency