A middle aged patient with history of hypertension presents with palpitation. This is a narrow QRS tachycardia with the rate of almost 150 bpm and there is also electrical alternans of QRS and T waves. Last 3 beats also show wide QRS complex probably related to bundle branch aberrancy. I think the only clue for the diagnosis of this though ECG is in V1! Please look at carefully to fig 2. Red arrows denote "P" waves. Is it atrial tachycardia? Click "see tweets" button to see the thread on twitter!