This is a narrow QRS complex tachycardia with a cycle length of 300 ms (200 beats/min). The differential diagnosis of a narrow-QRS complex tachycardia is AV nodal re-entry tachycardia, (AVNRT), AV re-entry tachycardia (AVRT), atrial tachycardia and sinus tachycardia. Atrial flutter and atrial fibrillation are less likely given the frequency and regularity of this tachycardia. …