Atrioventricular (AV) block. A, prolonged PR interval (first-degree AV block). B, loss of the QRS complex preceded by progressive prolongation of the PR interval (Mobitz type I [Wenckebach] second-degree AV block). C, independent rhythm of P waves and QRS complexes (third-degree AV block, AV junction escape rhythm). The PP intervals separated by the QRS complex are shorter than the PP intervals not separated by the QRS complex (arrows indicate P waves).