AV Re-entrant Tachycardia Patterns

There are two patterns of AV re-entrant tachycardias – orthodromic and antidromic. These terms refer to the direction of flow in the re-entry loop. Antidromic is much less common, in under 10% of WPW.

Orthodromic flow occurs when a premature atrial impulse is transmitted via the AV node antegradely to the ventricles. It then passes retrogradely back up the accessory pathway, depolarising the atria again, before passing back via the AV node and down the bundle of His antegradely to the ventricles, establishing a re-entry circuit.

Fig 1: Orthodromic re-entry tachycardia

P waves will therefore be present, but inverted and will follow the QRS, and the delta wave disappears.

Fig 2: Orthodromic re-entry tachycardia

Antidromic flow occurs when the accessory pathway conduction is antegrade, delivering the impulse to tissue in the right ventricle not specialised for conduction. Consequently, ventricular depolarisation spreads from this point leading to a widened and abnormal QRS.

Fig 3: Antidromic re-entry tachycardia

There is retrograde conduction through the AV node leads to atrial depolarisation, and consequently the P waves will be buried in the wide and abnormal QRS (see figure 4 below). This rhythm may be indistinguishable from ventricular tachycardia unless a previous ECG is available- one clue is that the re-entry tachycardia will be exactly regular.