Tachycardia is an arrhythmia characterized by heart rate > 100 / minute. According to the width of the QRS complex it can be divided into narrow QRS (< 120 ms) and wide QRS tachycardia (> 120 ms). Narrow QRS tachycardia is always supraventricular which means that its source is proximal to the bundle of His, while wide QRS tachycardia can be ventricular (source is in the ventricle, distal to the bundle of His) or supraventricular. The strategies for treating these two conditions are different so the correct diagnosis is prerequisite for optimal therapy. We present this case because the differential diagnosis of wide QRS tachycardia and therefore the treatment planning was particularly difficult due to concurrently present atrial fibrillation with hemodynamic compromise and an acute threat to the life of the patient.