Aortic (subaortic) stenosis (SAS) is a form of genetic heart disease in dogs that is very rare in cats. It is characterized by an obstructive band or ridge of tissue that prevents the normal ejection of blood from the left ventricle of the heart. The defect is located in the outlet of the left ventricle, immediately below the aortic valve, hence the name "subaortic" stenosis (or SAS). This genetic defect develops very early after birth, but the severity of blood flow obstruction may increase as the dog matures. Such progression can be particularly prominent in giant breeds such as the Newfoundland.

The left ventricle reacts to a stenosis (narrowing) by increasing the pressure developed during pumping (systole) and generating a pressure difference that propels the blood out of the heart at a much faster rate. These changes cause disturbed blood flow that, in turn, creates a heart murmur, which can be detected by a noninvasive ultrasound diagnosis using Doppler echocardiography. The increased heart work in moderate to severe obstructions is associated with left ventricular hypertrophy or thickening, myocardial fibrosis (scarring), and coronary artery lesions. Coronary blood flow back to the heart is abnormal and poor blood supply to the heart muscle (ischemia) predisposes to irregular heart rhythms and sudden death.

Clinical surveys have indicated that SAS is now the most common congenital heart disease in the dog in many areas. Commonly affected breeds include the golden retriever dog, Newfoundland, boxer, and German shepherd. Recent clinical and breeder surveys indicate the emergence of SAS in other breeds including the Rottweiler, shar-pei, bull terrier, English bulldog and Bouvier de Flanders. Breeding experiments have confirmed that SAS is inherited in these breeds, but the precise mode of transmission has not been proven. The disease does not appear to be transmitted by a gene or as a sex-linked trait. It is important to note that even mildly affected or clinically normal parents may transmit the defect to future generations.

The presence of a heart murmur in a puppy, especially in breeds at risk should be investigated; SAS is only one cause of a murmur. In diagnosed cases, any of the following symptoms should prompt a visit to your veterinarian:

Poor exercise capacity

Cough

Difficult breathing

Collapsing or fainting

Illness with fever

Advanced cases of SAS can lead to complications including heart failure, heart arrhythmia (irregular heart beating), fainting (syncope) or sudden death. Also, heart valve infections can occur in this condition leading to fever, illness and joint ache.

Other heart problems can appear similar to aortic stenosis. These are differentiated based on the history, physical examination, and the echocardiogram. These diseases include:

Patent ductus arteriosus

Pulmonic stenosis

Ventricular septal defect

Mitral valve and tricuspid valve malformation

Complex congenital (birth defect) heart disease

SAS can occur concurrently with other heart defects.