A 13-year-old spayed female domestic short-haired cat presented with sudden onset of right forelimb lameness and dyspnea. Thoracic radiography revealed an enlarged heart and pleural effusion, and congestive heart failure and thromboembolism of the right forelimb were suspected. Echocardiography showed marked dilatation of both atria and loss of A wave in the left ventricular inflow velocity waveform and right ventricular inflow velocity waveform, despite normal ventricular structure and motility. In addition, electrocardiography confirmed absence of P waves and junctional escape or idioventricular rhythm, leading to the clinical diagnosis of unclassified cardiomyopathy and persistent atrial standstill. Postmortem examination revealed that the cat had persistent atrial standstill caused by atrial myopathy.
