If a single premature atrial extrasystole arrives at the SA node and then conducts very slowly through it, it may emerge at a time when the atrium is recovered and reexcite the atrium. If this activity persists, it will produce an episode or paroxysm of supraventricular tachycardia. Reentrant activity can occur within the atrium itself. Single reentrant cycles will give rise to single atrial extrasystoles, while sustained atrial reentry at a frequency slightly above that of the sinus node will produce supraventricular tachycardia. As we mentioned before, it is impossible to determine the mechanism of supraventricular tachycardia from the ECG alone, and reentrant supraventricular tachycardias look the same on the ECG as those produced by an accelerated ectopic pacemaker.
Ventricular rate is very regular and exactly one-fourth that of the atrium.
Most atrial flutter is thought to be due to circus movement reentry. 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.