Background: There is a lack of studies on the occurrence of spontaneous conduction disturbances at atrial level in small animals. Bachmann's bundle block is a particular type of interatrial cardiac conduction disturbance that is characterized by a variable delay on electrical depolarization of both atria resulting in prolonged P waves duration on the electrocardiogram. It is currently classified as partial or advanced depending on the grade of severity of the conduction delay. In humans the advanced form of Bachmann's bundle block has low prevalence but is clinically very important because predicts left atrial enlargement and arrhythmias. The objective of this paper was to report the electrocardiographic findings identified to as intermittent advanced Bachmann's bundle block in a Boxer dog with clinical signs of congestive heart failure. Materials, Methods & Results: An intact nine-year-old female Boxer dog was referred for evaluation because of clinical manifestation of a gradually progressive dyspnea, loss of stamina and abdominal enlargement. The cardiac rhythm was irregular, the mean heart rate was 140 beats per minute and the systolic blood pressure was 158 mmHg. The femoral pulse was strong and synchronous to the heart beats. Thoracic auscultation was unremarkable. A lateral chest radiography showed mild cardiomegaly. Routine blood laboratory tests did not revealed alterations. A transthoracic bidimensional echocardiogram was performed and showed an enlarged left atrium in wich a mass suggestive of neoplasia was seen. Abdominal ultrasound examination showed hepatic passive congestion and mild ascites. Electrocardiographic tracings showed sinus arrhythmia, the mean heart rate was 147 beats per minute and the mean QRS electrical axis was situated between + 60 and + 90 degrees. On the leads D2, D3 and aVF it was observed an irregular intermittency of prolonged P waves with a biphasic plus-minus morphology. The therapeutic management consisted of restriction of physical exercice, commercial dog food, oral benazepril and furosemide. The dog initially showed clinical improvement but subsequently the treatment was negleted and the dog was found dead two months later. Discussion: Partial interatrial electrical conduction delay, defined as broad and sometimes bifid P waves, is an electrocardiographic abnormality that is very frequent in humans with enlarged left atria as seen in rheumatic mitral valve disease, some congenital heart defects and cardiomyopathy. In dogs, broad and eventually biphid P waves can be found in cases of advanced mitral endocardiosis, dilated cardiomyopathy, and even in some congentital heart anomalies as patent ductus arteriosus and mitral valve dysplasia. The advanced form of interatrial conduction delay, also called third degree Bachman's bundle block, however, has not yet been described in the current small animal veterinary literature. The particular abnormal morphology of the P waves observed in this case study was identical to that reported for the human with the advanced interatrial block and for dogs submitted to experimental surgical lesions in the Bachmann's bundle. Although rare, as observed in this case report, the advanced Bachmann's bundle block, when present, could be considered as a potentially non-invasive marker of dilated left atrium, especially if the patient has shown clinical signs of cardiac disease.
