QRS complex change during carotid sinus massage in ventricular pre-excitation

A 27-year-old man with a 4-year history of palpitations was seen in the outpatient clinic. His 12-lead electrocardiogram showed ventricular preexcitation during sinus rhythm. Of interest, and shown in Figure  1, are the changes in the QRS configuration during carotid sinus massage.
Source: Heart Rhythm - Category: Cardiology Authors: Tags: Josephson and Wellens ECG Lessons: A Monthly Visit to the 12 Lead ECG Source Type: research