Isorhythmic Atrioventricular Dissociation in Labrador Retrievers

Background: Isorhythmic atrioventricular dissociation (IAVD) is a rhythm disturbance in which atria and ventricles are driven by independent pacemakers at equal or nearly equal rates. Objectives: To describe electrocardiographic and electrophysiologic features of IAVD in a group of 11 Labrador Retrievers and its possible correlation with focal junctional tachycardia (FJT). Animals: Between December 2004 and October 2010, medical records of 11 Labrador Retrievers with surface electrocardiographic findings compatible with IAVD were retrospectively analyzed. Methods: Twelve-lead surface electrocardiograms, thoracic radiographs, and echocardiographic findings of each dog and electrophysiologic mapping results of 3 dogs were retrospectively analyzed. Results: In 10 of 11 dogs, the ECG pattern revealed the presence of IAVD with type I synchronization. In 5 of 10 dogs, IAVD with type I synchronization was interrupted by periods of junctional tachycardia with 1 : 1 ventriculo-atrial conduction. One of 11 dogs presented IAVD with type II synchronization. The ECG diagnosis of IAVD with type I and type II synchronization, and junctional rhythm with 1 : 1 ventriculo-atrial conduction was confirmed in 3 of 11 dogs with endocardial mapping in which the diagnosis of focal junctional tachycardia was made. Conclusion and Clinical Importance: IAVD with type I synchronization is more common than IAVD with type II synchronization in Labrador Retrievers, and a correlation between IAVD and FJT can be hypothesized.