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Teaching Points:

  • Everything you need to know about AVRT and AVNRT
  • What is Wolff-Parkinson-White/ WPW syndrome?
  • What is a slow and what is a fast pathway?


The next two arrhythmias, AVRT and AVNRT, are also supraventricular tachycardias. The basic prerequisite for both is the involvement of an additional congenital pathway connecting the atria with the ventricles. AVRT stands for atrioventricular reentry tachycardia, also known as Wolff-Parkinson-White-syndrome or short: WPW syndrome. AVNRT stands for AV node reentry tachycardia.

AV node reentry tachycardia (AVNRT)

The AV node is normally the only excitable connection between the atria and the ventricles. In some individuals, there is a second connection pathway at the AV node, which has different conduction velocities and refractory times.

Usually, one of the two pathways is fast and has a long refractory period, while the second is slow and has a shorter refractory period.

As a rule, excitation originating from the atrium reaches the His-Purkinje system via the faster pathway. Initially, the slower pathway is also excited. However, when the excitation going via the slower pathway reaches the site that combines both paths in front of the bundle of His, this site has already been activated by the faster pathway. The excitation via the slow pathway thus terminates here without reaching the ventricles.

Since the slower pathway has a shorter refractory period, the excitation of an atrial premature beat can reach the excitable tissue of this slow pathway while the fast pathway is still refractory. In this case, the excitation is conducted via the slow pathway to the bundle of His, where it meets the once more excitable fast path.

The excitation is then passed on to the ventricles via the Bundle of His and at the same time back to the atria via the fast pathway. Thus, a circuit of excitation begins in the area of the AV node, which causes conduction into the ventricles after each cycle.

Slow-fast AVNRT = typical AVNRT

This form of AVNRT is also called "slow-fast AVNRT" because of the order in which the anatomical structures are affected, and accounts for about 95% of all AV node reentry tachycardias.

Rarely, it may commence with a premature ventricular beat, which sends a retrograde signal to the atria via the slow pathway. This then excites the fast pathway, leading to the usually shorter-lasting “fast-slow AVNRT”.

In both cases, we see a narrow complex tachycardia with heart rates of between 130 and 220 per minute in the ECG. Because the atria are excited after the ventricles, the P waves are no longer visible before the QRS complex. They either disappear within the QRS complex or they emerge shortly after it. If P-Waves are visible, they are negative in leads II, III, and aVF because excitation spreads in the opposite direction as in sinus rhythm. Now, you understand the AVNRT.

AVRT, Wolff-Parkinson-White-Syndrome or WPW-Syndrome

AVRT, also known as Wolff-Parkinson-White (or WPW-Syndrome), also involves an additional pathway. However, unlike in AVNRT, this so-called accessory pathway is not in the area of the AV node. Instead, it connects the atria and ventricles at some other point on the ventricular level, meaning in the right or left heart.

So how does a tachycardia occur?

In principle, this is quite similar to what happens in AVNRT. Usually, AV nodes and the accessory pathway are excited almost simultaneously. Retrograde conduction is not possible due to refractory cells. As in AVNRT, tachycardia is triggered by premature atrial or ventricular beats, which meet excitable cells of the accessory pathway while the regular pathway, that is the AV-node, is still refractory. This phenomenon is called "unidirectional block" and is a prerequisite for all macro-reentry tachycardias.

Orthodromic AVRT

Two different AVRTs are differentiated, depending on the direction of the spread of excitation.

One AVRT takes the normal route, which means that the excitation spreads along the physiological pathway from the atria via the AV node to the ventricles, and then back to the atria via the accessory pathway. The other goes in the opposite direction, meaning that the excitation spreads along the accessory pathway from the atria to the ventricles, and then back to the atria via the AV node. Depending on the direction, we refer to an orthodromic or an antidromic AVRT. In the orthodromic AVRT, we see a narrow-complex tachycardia in the ECG, since as with all narrow-complex tachycardias, the excitation is directed via the His-Purkinje system. It is significantly more common than antidromic AVRT.

Antidromic AVRT

The rarer antidromic AVRT presents in the ECG as a regular wide complex tachycardia. The morphology of the QRS complex is dependent on the location of the accessory pathway. If the pathway is to the right, we see a left bundle branch block-like morphology. If it is to the left, the morphology is more like that of a right bundle branch block. The morphology of wide complex tachycardias is discussed in greater detail in the chapter on ventricular tachycardias.

In both forms of AVRT, P waves may be seen, but compared with the AVNRT, they appear much later after the QRS complex.

Delta wave

You may have heard the term “Delta-Wave” before in connection with the AVRT or WPW syndrome. So what is a Delta wave and how does it originate?

In the case of a patient with an accessory pathway and normal sinus rhythm, once the atria are excited, excitation is conducted via the AV node to the bundle of His, just as in any other individual.

Depending on the anatomical position and conduction speed of the accessory pathway, premature ventricular excitation may be triggered by the accessory pathway.

This means that some part of the ventricle is excited earlier than the rest, which is still activated via the AV node and the His-Purkinje system. In the 12-lead ECG, we recognize this premature excitation as a so-called “delta wave" and a shortened PR-interval.

Excitation can only run in one direction. Therefore, in the case of an orthodromic tachycardia, where the accessory pathway excites the atria retrogradely, no premature antegrade excitation of the ventricles occurs via this pathway. The delta wave disappears during an AVRT.

Masked WPW-Syndrome

The delta wave may also be absent at rest. If the duration of excitation via the AV node is shorter than the conduction time via the accessory pathway, the delta wave is completely missing. In the case of pathways on the left-lateral wall of the heart, excitation from the sinus node to the pathway must travel a greater distance to the accessory pathway than to the AV node.

Since the conduction time of the AV node varies depending on the status of the vegetative nervous system, the presence of a delta wave may fluctuate. If delta waves are absent, the condition is referred to as a masked WPW syndrome.