Third-degree atrioventricular block from metastatic poorly differentiated sarcoma suggestive of osteosarcoma in a dog.

A nine-year-old, female spayed, mixed-breed dog was presented with a history of lethargy, anorexia, and bradycardia. Diagnostic evaluation revealed third-degree atrioventricular (AV) block secondary to an infiltrative mass in the septal AV junctional region, with additional masses and nodules in the right third rib, lungs, and kidneys. Necropsy confirmed multifocal neoplastic lesions within the pericardium, myocardium, septal AV junction, rib, kidneys, pancreas, and mesentery. Histopathological examination demonstrated a poorly differentiated sarcoma with an osteosarcoma prioritized.