[Pacemaker ECG quiz no. 22. A CRT non-responder with atrial fibrillation].

A patient with heart failure functional class NYHA III and permanent atrial fibrillation received a system for cardiac resynchronization therapy. Unfortunately, he reports at a follow-up visit about a completely unchanged poor exercise capacity with dyspnea during exercise at a very low level. The ECG reveals an unusual mode of stimulation.