Since the atrial rate in flutter is on the order of 200 to 400 bpm, the AV node will not allow all of these impulses through to the ventricles. Finally, there may be many small reentrant circuits in the atrium giving rise to multiple wavelets of activity. Again the AV node will not permit the passage of all of these impulses, so ventricular frequency will be high, but not as high as the atrial rate. The ECG will show rapid irregular QRS activity with a fine-grained noise due to the rapid irregular atrial activation replacing the P waves. QRS complexes are normal but ventricular rhythm is erratic due to random activation of the AV node by multiple wavelets of atrial fibrillation.
Reentrant activity involving the AV node can also produce paroxysmal supraventricular tachycardia.
It has been demonstrated that the AV node consists of at least two subdivisions, the alpha and beta pathways. An atrial extrasystole occurring early enough after the last normal beat may travel through the alpha pathway and block in the beta pathway. Activation of the ventricle then leads to retrograde AV transmission through the beta pathway. This reexcites the atrium and a reentrant cycle is complete. Sustained reentrant activity in this loop produces supraventricular tachycardia.