Hemangiosarcoma involving the interventricular and interatrial septum, atrioventricular node, and right ventricle, causing atrioventricular block and ventricular arrhythmias in a dog.

A 13-year-old male neutered miniature poodle was presented for evaluation following multiple syncopal episodes. A six-lead electrocardiogram revealed high-grade atrioventricular (AV) block and ventricular ectopy presenting as periods of accelerated idioventricular rhythm and runs of ventricular tachycardia. The echocardiogram showed severely increased interventricular septum wall thickness with several cystic anechoic cavities. The owner elected humane euthanasia due to poor prognosis. The postmortem examination revealed the presence of hemangiosarcoma involving the interventricular and interatrial septum, the AV node, and the right ventricular free wall. This report describes an unusual presentation of cardiac hemangiosarcoma causing AV block and ventricular arrhythmias.