A 5-month-old female teacup poodle was presented to the Cornell University Hospital for Animals Emergency Service for evaluation of respiratory difficulty. The patient had been evaluated by her primary care veterinarian for lethargy and inappetence and thoracic radiographs reportedly showed cardiomegaly and dorsal displacement of the trachea. The veterinarian recommended follow up with a cardiologist, but due to development of respiratory difficulty, the patient was brought to the Emergency Service. On presentation, the patient was tachycardic and tachypneic with pale mucous membranes, prolonged capillary refill time, and mild crackles bilaterally. Point of care bloodwork revealed a hyperlactatemic metabolic acidosis with respiratory compensation and hypoxemia. Point of care thoracic ultrasound showed B-lines bilaterally and possible right heart enlargement. After initiation of supportive care (oxygen and diuretics), repeat thoracic radiographs confirmed right sided cardiomegaly and demonstrated severe pulmonary artery enlargement. The differential diagnosis list was focused on congenital heart diseases prioritizing a right-to-left shunting abnormality. The patient was admitted for further treatment and diagnostic investigation. An echocardiogram was performed which was consistent with intracardiac right-to-left shunt. Due to poor prognosis, the patient was euthanized. Post mortem examination revealed a 0.75 cm atrial septal defect in the region of the coronary sinus and cardiopulmonary changes indicating pulmonary hypertension (right ventricular hypertrophy and dilation, pulmonary trunk dilation, and pulmonary arterial arteriosclerosis). This report focuses on diagnosis, development, and pathophysiology of atrial septal defect.