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

Single channel rhythm strip from a patient with the Wolff-Parkinson-White syndrome shows progressive shortening of the PR interval with the development of a widened QRS complex due to a delta wave. The P wave appears to merge with the QRS complex (arrows). The last two QRS complexes again show a normal PR interval and a narrow QRS complex.