This ECG (3 rhythm strips) initially shows a sinus rhythm with 1st degree AVB grade I and wide QRS complexes (presumably RBBB pattern). A PAC (P-wave premature, different form than in sinus rhythm) appears approximately in the middle of the ECG, this is not conducted . Due to the strong prematurity, this is not surprising. But why is the 2nd PAC also blocked? The answer can be found in the second picture.