Assessment of Atrio-ventricular Conductive Function in Horses by Atrial Pacing

Abstract

Six thoroughbred horses, in which electrocardiogram had been recorded for 24 hours, were electrophysiologically examined for atrio-ventricular conductive functions. The second degree atrioventricular block was detected in five cases. The test cases were divided into two groups; two sporadiccases with 0 and 32 episodes/ 24hr and four high-frequency cases with 216-2, 643 episodes/24hr. The beat dropping occurred in all cases after stimulation at an atrial pacing rate of 40 or 50/min. The mean P-R intervals extended with increase in atrial pacing rates from 40 to 100/min, being longer in the high-frequency group than the sporadic group at the same pacing rate. The mean R-R interval was constant irrespective of a pacing rate. The shortest R-R interval, that was measured by the atrio-ventricular conduction curve (the relationship of the minimum R-R interval and the pacing rate), in the sporadic and high-frequency groups were less than 700 msec and more than 750 msec, respectively. The results suggested that the conductive functions of the atrioventricular node electrophysiologically lowered in cases with high-frequency group.