A 10-month-old female domestic shorthaired (DSH) cat was presented with peracute respiratory problems. Physical examination revealed dyspnoea, tachypnoea, cyanosis, weak pulse and bradycardia. Auscultation showed pulmonary crepitation and attenuated heart sounds and a pansystolic grade V/VI murmur. The electrocardiogram showed atrioventricular dissociation identified as third-degree sinoatrial block. X-rays showed increased density in the ventral and middle zones of the thorax and loss of definition of the cardiac silhouette and increased diffuse radiographic density of the entire abdomen.Echocardiography revealed dilatation of the right atrium and concentric biventricular hypertrophy. A type 1 persistent truncus arteriosus was diagnosed at necropsy. This is the first case report of this type of arrhythmia in a cat with persistent truncus arteriosus, and its relationship with the described congenital cardiac anomaly is discussed.