Idioventricular tachycardia and unstable ventricular escape rhythm in a canine patient with third-degree atrioventricular block.

A 10-year-old female Labrador retriever was presented for evaluation and management of syncope. On presentation, idioventricular tachycardia (IT) was electrocardiographically evident but neither ventricular capture nor fusion complexes were recorded. Spontaneous termination of the IT resulted in prolonged ventricular asystole and revealed the underlying rhythm of third-degree atrioventricular block. The serum concentration of troponin-I was elevated, suggesting myocarditis, but this was not diagnostically confirmed. It is likely that overdrive suppression of subsidiary idioventricular pacemakers, and perhaps also disease of subsidiary ventricular pacemaker fibers, importantly contributed to the duration of asystole.