Dissimilar atrial rhythms in man and dog

Dissimilar atrial rhythms were demonstrated in 10 patients by recording right and left atrial activity simultaneously with intracavitary right atrial and esophageal electrodes. Eight patients had fibrillation recorded in one atrium and slower, more regular activity recorded in the other atrium; two patients had no recordable activity in one atrium and a tachyarrhythmia recorded in the other atrium. Right atrial pacing failed to terminate the slower, more regular right atrial rhythm when left atrial fibrillation was present. Discrete atrial activity reflecting depolarization from one atrium can be recorded in surface electrocardiograms during contralateral atrial fibrillation or in the absence of recordable activity in the opposite atrium. In an animal study, dissimilar atrial rhythms were induced in seven dogs in which vagotomy and administration of ouabain lengthened the atrial refractory period. Vagal stimulation restored similar rhythms by causing both atria to fibrillate rapidly. Administration of ouabain and vagotomy did not uniformly prolong the atrial refractory period, and it was possible to record with one electrode localized atrial activity that could not be recorded in closely adjacent electrodes placed in the same or the contralateral atrium. We conclude that during some forms of atrial fibrillation, particularly those called “coarse atrial fibrillation” or “flutter fibrillation,” more uniform depolarization may occur in one atrium or portion of one atrium and result in a rhythm that is slower and more regular than the rhythm occurring in the contralateral fibrillating atrium.