Paradoxical behavior of PR interval dynamics during exercise and recovery and its relationship to cardiac memory at the atrioventricular node

This preliminary study deals with the dynamics of the PB interval during ex ercise testing by using a Bruce protocol in 31 nonmedicated, normal patient s with ages of 33 +/- 14 years. The behavior of the PR interval permitted i ts categorization into 2 groups. In Group I (27 of 31, 87.1% of patients) t he PR interval showed the expected biphasic behavior with a gradual shorten ing 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 d ecrease in duration. Because of this temporal relation to the phases of exe rcise, this paradoxical response, in analogy to what happened to the QT int erval during exercise, could have been caused by a form of short-term memor y or to varying, but normal, intergroup differences in autonomic function. However, further studies involving a greater number of patients are require d to reach definite conclusions.