The combination of an unfortunately timed atrial ectopic and the existence of two strands of conducting tissue with opposite electrical properties in the junctional region of the heart can establish a rapidly discharging, self sustaining, re-entrant loop of depolarising current. This current will be discharged distally into the ventricles and also in a retrograde fashion into the atria. This is the mechanism underlying a common paroxysmal supraventricular tachycardia (SVT); AVNRT. We will explain the mechanism underlying AVNRT in this video. Understanding the underlying mechanism generating this tachycardia will help you identify this common arrhythmia on the ECG.