Paradoxical Behavior of PR Interval Dynamics During Exercise and Recovery and its Relationship to Cardiac Memory at the Atrioventricular Node Federico Moleiro, MD, Francesca Misticchio, MD, Ivan Mendoza, MD, Ana Rodriguez, MD, Agustin Castellanos, MD, and Robert J. Myerburg, MD Abstract: This preliminary study deals with the dynamics of the PR interval during exercise testing by using a Bruce protocol in 31 nonmedicated, normal patients with ages of 33  14 years. The behavior of the PR interval permitted its categorization into 2 groups. In Group I (27 of 31, 87.1% of patients) the PR interval showed the expected biphasic behavior with a gradual shortening with increasing exercise and a gradual lengthening during recovery. In contrast, the PR interval in Group II (4 of 31, 12.9% of patients) showed a triphasic behavior since, toward the end of recovery, there was a second decrease in duration. Because of this temporal relation to the phases of exercise, this paradoxical response, in analogy to what happened to the QT interval during exercise, could have been caused by a form of short-term memory or to varying, but normal, intergroup differences in autonomic function. However, further studies involving a greater number of patients are required to reach definite conclusions. Key words: PR interval, cardiac memory, action potential duration, exercise testing, atrioventricular node. It is well known that the nonpaced PR interval is a function of heart rate, because it shortens with increasing rates (or decreasing cycle lengths) and lengthens with decreasing rates (or increasing cycle lengths) (1–5). As far as back as 1951, Lepeschkin (1) reviewed the already available mathematical formulas used to establish the relationship between PR intervals and RR intervals (both expressed in hundreds of a second.) Although variations in the PR interval have been studied under physiological ranges of rate and pre- sented in tables in several books dealing with elec- trocardiography, they have been less analyzed dur- ing exercise (1– 4). In 1990, Luceri et al. (5) published an interesting article on this topic. They analyzed the behavior of the PR interval during exercise to determine if there could be implications for implantation of dual chamber pacemakers (5). The purpose of this communication was to eval- From the Universidad Central de Venezuela, Caracas, Venezuela; and the University of Miami School of Medicine, Miami, FL. R.J.M. is supported in part by a grant from the American Heart Association Chair in Cardiovascular Research at the University of Miami. Reprint requests: Federico Moleiro, MD, 9741 Southwest 20 th St, Miami, FL 33165; e-mail: acastell@med.miami.ed. Copyright © 2001 by Churchill Livingstone ® 0022-0736/01/3401-0004$35.00/0 doi:10.1054/jelc.2001.22027 Journal of Electrocardiology Vol. 34 No. 1 2001 31