In AVNRT, anterograde conduction occurs over the slow pathway to the ventricle, whereas near-simultaneous atrial activation occurs over the fast pathway of the AV node. These events lead to the parallel electrical activation of the atria and ventricles. Canon A waves which occur with AV dissociation result from simultaneous contractions of the atria and ventricles against closed mitral and tricuspid valves, causing reflux of blood into the neck veins. The characteristic flapping or bulging appearance of the neck veins is also described as the “Frog sign.” “Frog sign” during narrow QRS complex tachycardia has been considered to be particularly helpful in making the diagnosis of typical AVNRT. An atrioventricular mechanical dissociation may also occur in the case of ventricular extrasystoles. In this case, however, only one or few pulses are felt in the neck, and the rhythm is more irregular. Knowledge of these classic signs is often helpful in determining the underlying condition.