The concertina effect (or accordion effect) in Wolff-Parkinson-White (WPW) syndrome refers to the dynamic, beat-to-beat variation in the degree of ventricular pre-excitation visible on a single ECG rhythm strip. On an ECG, this manifests as a QRS complex that sequentially widens and narrows, accompanied by an inversely fluctuating PR interval, resembling the expansion and compression of a concertina.
Electrophysiological Mechanism
The degree of pre-excitation in WPW depends on a “race” between two parallel electrical routes: the normal AV node-His-Purkinje system and the accessory pathway (Bundle of Kent). The concertina effect occurs due to shifting autonomic tone continuously altering the conduction velocity and refractoriness of the AV node.
- Increased Vagal Tone: Conduction through the AV node slows down. Because the accessory pathway typically does not possess decremental conduction properties (it isn’t slowed by vagal tone), a larger proportion of the ventricular myocardium is activated via the bypass tract.
- ECG Result: The PR interval shortens, the delta wave becomes more prominent, and the QRS complex broadens.
- Increased Sympathetic Tone: Conduction through the AV node accelerates. The normal pathway “wins” the race to a larger portion of the ventricles.
- ECG Result: The PR interval lengthens (approaching normal), the delta wave shrinks, and the QRS complex narrows.
Normal respiratory sinus arrhythmia is a common trigger for this effect, causing cyclic changes in vagal tone that perfectly correlate with the beat-to-beat expansion and narrowing of the QRS.
Clinical Significance
While the concertina effect itself is a benign physiological phenomenon reflecting normal autonomic fluctuations, its primary clinical importance lies in its ability to mimic other arrhythmias.
It is crucial not to misdiagnose the varying QRS morphology as:
- Frequent multiform premature ventricular complexes (PVCs): PVCs will typically lack a P wave and have a compensatory pause, whereas the concertina effect maintains a 1:1 AV association with fluctuating PR intervals.
- Electrical alternans: Alternans affects the amplitude of the QRS due to shifting anatomical axes (like in a massive pericardial effusion), not the width of the QRS or the PR interval.
- Intermittent Bundle Branch Block (BBB) or aberrancy: Rate-related BBBs will abruptly widen the QRS when a critical cycle length is reached, rather than showing a sliding scale of morphological changes.
In terms of risk stratification, a dynamic degree of pre-excitation (especially if it normalizes completely at higher sinus rates or during exercise) often suggests a relatively long refractory period of the accessory pathway. This is generally considered a lower-risk feature for rapid conduction during atrial fibrillation, though formal electrophysiology study (EPS) remains the gold standard for definitive risk assessment.