FEATURED ARRHYTHMIA Two-to-one atrioventricular block: What is the mechanism? Ilknur Can, MD,* Amrit Guptan, MD,* Jian-Ming Li, MD, PhD, FHRS, † Venkatakrishna N. Tholakanahalli, MD, FHRS † From the *Cardiac Arrhythmia Center, Cardiovascular Division, Department of Medicine, University of Minnesota Medical School, and † Veterans Affairs Medical Center, Division of Cardiac Electrophysiology, Department of Medicine, University of Minnesota, Minneapolis, Minnesota. Case report A 62-year-old-male without structural heart disease pre- sented with fatigue during the last 2 weeks. Baseline elec- trocardiogram (ECG) showed intermittent 2:1 atrioventric- ular (AV) block with narrow QRS and a PR interval of 200 ms (Figure 1). He underwent electrophysiologic study to delineate the site of the block. Figures 2 and 3 show the intracardiac recordings. Where is the mechanism of block? Commentary In 2:1 AV block, the site of block cannot be reliably deter- mined by the surface ECG, but the PR interval of the con- ducted P wave and QRS width may give clues about the site of the block. A PR interval 0.3 seconds suggests block in the AV node, whereas a PR interval 0.16 seconds suggests block in the His-Purkinje system or His bundle. A normal QRS morphology suggests that the site of block is in the AV node or His bundle. 1 The ECG showed that the QRS duration and PR interval were within normal range, suggesting block within the AV node or His bundle (Figure 1). Electrophysiologic study demonstrated intermittent 2:1 conduction with evidence of a split His potential in the conducted beats and the site of block in the His bundle ( Figure 2). The AV nodal conduction (AH = 74 ms) and infra-His conduction times (H= V = 35 ms) were within normal range, whereas conduction was delayed in the His region (HH= = 35 ms). 1 Thus, there was second-degree intra-His block, which is defined as intermittent conduction between the proximal and distal His bundle potentials. An intra-Hisian site of block should be suspected in the presence of a narrow QRS, failure of block to improve with isoproterenol or atropine, and paradoxical improvement in conduction with carotid pressure. 1 In Figure 3, the surface ECG shows second-degree type I AV block. The intracardiac recordings interestingly demon- strated consecutive His bundle extrasystoles, which appeared at varied times in the cardiac cycle, between the ventricular KEYWORDS His extrasystole; Atrioventricular block ABBREVIATIONS AVN = atrioventricular nodal (Heart Rhythm 2009;6: 1526 –1527) Address reprint requests and correspondence: Venkat Tholakanahalli, M.D., Assistant Professor of Medicine, Division of Cardiology, University of Minnesota, One Veterans Drive, VAMC, Minneapolis, Minnesota 55417. E-mail: thola001@umn.edu. (Received October 31, 2008.) Figure 1 Baseline electrocardiogram showing 2:1 AV block. The PR interval is 200 ms, and the QRS width is within the normal range. 1547-5271/$ -see front matter © 2009 Heart Rhythm Society. All rights reserved. doi:10.1016/j.hrthm.2008.11.004