From the Guidelines
Cyclic alteration of the QRS complex is a pattern where the morphology of the QRS complexes changes in a regular, repeating manner, often associated with alternating bundle branch block, as seen in Figure 1 of the study by 1. This phenomenon can be observed in different leads of the ECG or on successive ECG tracings, showing clear evidence of block in all three fascicles, such as right bundle branch block in one lead and left bundle branch block in another.
- The study by 1 highlights the importance of excluding and correcting reversible causes of AV block, such as electrolyte abnormalities, before making a decision to implant a pacemaker.
- The presence of alternating bundle branch block can be a sign of severe conduction system disease, as seen in the 79-year-old woman in the study by 1, who developed complete heart block and required implantation of a pacemaker.
- The clinical significance of cyclic alteration of the QRS complex is substantial, as it often indicates a potentially life-threatening condition, such as complete heart block, which requires immediate intervention.
- The physiological basis of cyclic alteration of the QRS complex involves changes in the ventricular depolarization pathways with each heartbeat, resulting in alternating QRS complexes with different morphologies.
- The study by 1 provides evidence that cyclic alteration of the QRS complex can be a sign of underlying cardiac disease, and highlights the importance of careful evaluation and management of patients with this condition.
From the Research
Cyclic Alteration of the QRS Complex
- Cyclic alteration of the QRS complex refers to the periodic change in the morphology of the QRS complex during a tachycardia episode.
- This phenomenon can be observed in various types of supraventricular tachycardias, including atrioventricular nodal reentrant tachycardia (AVNRT) and circus movement tachycardia with an accessory atrioventricular (AV) pathway 2.
- The presence of QRS alternation during sustained narrow QRS supraventricular tachycardia is highly indicative of a retrograde accessory AV pathway in the tachycardia circuit 2.
- QRS alternation can also occur due to alterations in the exit site and the zone of slow conduction adjacent to the exit site, resulting in a change in QRS morphology and tachycardia cycle length 3.
Mechanisms and Associations
- QRS alternation is often associated with circus movement tachycardias, which involve an accessory AV pathway 2.
- Intra-AV nodal reentrant tachycardia (AVNT) and atrial tachycardias can also exhibit QRS alternation, although it is less common in these types of tachycardias 2.
- The alteration in QRS morphology can be due to changes in the ventricular activation sequence, which can be influenced by the presence of an accessory AV pathway or other factors affecting ventricular conduction 3, 2.
Clinical Significance
- The presence of QRS alternation can be a useful diagnostic clue in determining the type of supraventricular tachycardia and the underlying mechanism 2.
- Understanding the mechanisms and associations of QRS alternation can help guide treatment decisions, including the use of catheter ablation or other therapeutic interventions 4, 5, 6.