Electrocardiogram (ECG) showing orthodromic atrioventricular reentrant tachycardia (AVRT)

Atrioventricular reentrant tachycardia (AVRT) is a supraventricular tachycardia associated with an accessory pathway connection between the atria and ventricles. The pathway may conduct antegrade only, retrograde only, or both antegrade and retrograde. AVRT is usually initiated by an atrial premature beat (arrow). The most common type of AVRT, termed orthodromic AVRT, uses the AV node and His Purkinje system (which has a relatively short refractory period) for antegrade conduction to the ventricles and the accessory AV pathway (which in these patients has a relatively long antegrade refractory period when compared to the node) for retrograde conduction. During orthodromic AVRT, QRS complexes are narrow. During sinus rhythm, however, wide and bizarre QRS complexes (indicative of the WPW pattern) are caused by conduction over both the accessory and normal pathways, resulting in a fusion beat. Since there is 1:1 retrograde activation of the atrium, a negative P wave may be present following the QRS complex.

The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat.