Accessory Tract Rhythms: AVRT and Wolff-Parkinson-White Syndrome

This section will be focused on rhythms related to the presence of accessory tracts (or accessory pathways), which are anatomical fibres present in some patients that allow an alternative pathway for signals to go from the atria to the ventricles and vice-versa, separate from the AV node. The most common of such rhythms are concealed-tract atrioventricular reentry tachycardia (AVRT) and Wolff-Parkinson-White syndrome.

Topics included will be:

• An introductory discussion on accessory tracts. We will discuss the difference between concealed and manifest tracts, which are named thusly depending on whether or not they exhibit a phenomenon known as "preexcitation", which manifests on the ECG via "delta waves". 

• Concealed-tract rhythms, a subset of rhythms without ECG evidence of the presence of an accessory tract at baseline. We will discuss:

  • Concealed-tract orthodromic AVRT, a reentrant rhythm related to anterograde conduction through the AV node and retrograde through the accessory tract. We will cover both the typical and atypical variants of this condition. We will also cover AVRT with aberrancy (i.e. bundle branch blocks) and the phenomenon of pseudoelectrical alternans, whereby rate-related conduction abnormalities cause a beat-to-beat alternation of the QRS axis.

• Wolff-Parkinson-White syndrome (WPWS), which is a type of "preexcitation syndrome". This is a condition whereby there is electrocardiographic evidence of the existence of accessory pathways, known as "delta waves". There are many unique ECG manifestations of WPWS covered in this section, such as:

  • Delta waves during sinus rhythm, with a discussion on localization of accessory tract (i.e. Type A, Type B, etc.). We will also cover how to identify WPWS in the context of a pre-existing bundle branch block.

  • Inapparent preexcitation, a variant in which the delta waves are not regularly apparent in normal sinus rhythm (because of enhanced AV nodal conduction), but where they can be induced by slowing down the AV node (i.e. with beta blockers).

  • Intermittent preexcitation, a condition in which delta waves occur with only a subset of all sinus beats. We will also discuss preexcitation alternans, a specific type of intermittent preexcitation   which is where there is an alternating pattern of preexcited beat with normal sinus beat.

  • Orthodromic AVRT, a narrow-complex tachycardia in which the delta waves are not apparent until after conversion to sinus rhythm.

  • Antidromic AVRT, a variant of AVRT only possible with Wolff-Parkinson-White syndrome, where the circuit exhibits anterograde conduction through the accessory pathway and retrograde conduction through the AV node. This creates a wide-complex tachyarrhythmia that is indistinguishable from ventricular tachycardia. 

  • Preexcited atrial arrhythmias, such as preexcited atrial flutter and fibrillation. Here, we will discuss the risk of 1:1 conduction, especially with AV nodal blockers, as well as specific features like the concertina effect.

  • Pre-excitation with concomitant AV block, a rare manifestation in which only the accessory tract is capable of transmitting signals from the atria to the ventricles.

  • Fasciculoventricular accessory pathways. These are part of a rarer and more benign subtype of accessory pathways wherein the atrioventricular bypass occurs below the level of the AV node. The heart maintains its protection from the AV node's decremental conduction, so there isn't the same risk of 1:1 conduction as with other accessory pathways.

  • We will also discuss the controversial eponyms of "Kent", "Mahaim", and "Lown-Ganong-Levine" fibres.