12-lead electrocardiogram (ECG) showing concertina effect in Wolff-Parkinson-White syndrome

The 12-lead ECG and rhythm strip during sinus rhythm show a short PR interval and QRS complex which is aberrated and has a delta wave. After the fifth complex on the rhythm strip, there is a sinus pause after which the QRS compex is still aberrated but the PR interval is somewhat longer. Subsequently, the PR interval becomes progressively shorter. This variability in the PR interval during sinus rhythm is known as the "concertina effect."