Third-degree atrioventricular block associated with suspected arrhythmogenic cardiomyopathy in a three-year-old Cocker spaniel.

A three-year-old spayed female English Cocker spaniel was presented with diarrhea, abdominal distension, and exercise intolerance. On physical examination, bradycardia, abdominal distension with positive undulation, and a soft systolic right-sided heart murmur were detected. Electrocardiography showed a third-degree atrioventricular block with monomorphic idioventricular rhythm and premature ventricular ectopic beats. On echocardiography, an extremely thin right ventricular wall, severe right ventricular and atrial dilation, and severely depressed right ventricular systolic function were observed, and ascites was confirmed. Despite pacemaker implantation and standard treatment for congestive heart failure, the dog's condition deteriorated quickly, leading to euthanasia. Pathological examination showed a thin right ventricular free wall with fibrosis and largely absent cardiomyocytes, along with myocardial infiltration of adipocytes and lymphocytes. An arrhythmogenic cardiomyopathy complicated with a third-degree atrioventricular block was considered the most likely diagnosis.