1091 Paradoxical Shortening of the Second PR Interval During 3:2 Atrioventricular Nodal Block SERGIO L. PTNSKI, M.D., PATRICK J. TCHOU, M.D., and RICHARD G. TROHMAN, M.D. From the Department of Cardiology. Cleveland Clinic Foundation, Cleveland. Ohio Paradoxical Shortening in Second-Degree AV Block. A patient with 3:2 second-degree AV block after acute inferior wall myocardial infarction showed consistent PR interval shorten- ing on the second conducted beat in eacb periodicity. Intracardiac electropbysiologic evaluation revealed tbat tbe site of block was nodal. A typical Wenckebacb pattern with prolongation of the AH interval was noted. The shorter PR resulted from a paradoxical sbortening of the HV inter- val in the second beat, most likely due to supernormal conduction in tbe setting of concomitant trifascicnlar disease. (J Cardiovasc Electrophysiol, Vol. 7, pp. iO9l-I(J94. November 1996) atrioventricular block, His-Purkinje system, atrioventricular node, acute myocardial infarction, electroca rdiog raphy Introduction The differentiation between second-degree type I and type II AV block can be made easily and re- liably from the surface ECG in most instances. This distinction is clinically useful. In the patient with acute myocardial infarction, type I AV block usually accompanies inferior infarction, is tran- sient, and usually does not require temporary pac- ing. Type II AV block occurs in the setting of an anterior infarction, is accompanied by intraven- tricular conduction defects, frequently requires tem- porary or pennanent pacing, and is associated with a high mortality rate due to pump failure.' There is general agreement that Mobitz type I block is present when there is progressive prolongation of the PR interval before a blocked beat,- but dis- crepancies regarding the exact definition of Mo- bitz. type II block remain. A strict definition re- quires that the PR of all beats, before and after a blocked P wave, not be measurably different.''* Other authors will consider Mobitz II block to be present if blocked P waves occur without an in- crement in tbe preceding PR interval.^ Address for correspondence: Sergio L. Pinski, M.D.. The Cleve- land Clinic Foundalion. Desk Fi5. 9500 Euclid Ave.. Cleveland. OH 44195. Fax: 216-444-0456. Manuscript received 10 July 19%: Accepted for public Julv 1996. lication 19 July 1996. An ECG pattem with shortening of the PR in- tervid of the second beat during 3:2 second-degree block defies classification. We report a patient who had second-degree AV block with this pattem af- ter an inferior wall mycKardial infuivtion. A His- bundle electrogriun located the site of block to the AV node. The PR interval shonening resulted fh)m a paradoxical, maiked shonening of the HV in- terval during the second beat, most likely due to supemonnal conduction in the setting of trifasci- cular disease. Case Presentation A 68-year-oId white man with a previous his- tory of hypertension, coronary artery disease, two prior coronary bypass surgeries, and carotid endarterectomy presented 3 days after an epistxle of prolonged resting chest pain with recurrent chest discomfort. Assessment suggested an acute Q wave inferior wall myocardial infarction. Myociuxliiil en- zymes were elevated. Tlie admission ECG showed sinus rhythm, first-degi-ee AV bltick (PR 419 msec), and right bundle branch block (RBBB) with a nor- mal axis. There were pathologic Q waves in leads II, III, aVF. and V|. There was ST segment ele- vation with negative T waves in leads in and aVF. An ECG obtained 1 year before was normal ex- cept for the presence of nonspecific T wave ab- normaiities. Coronary angiography performed