Subaortic stenosis (SAS) is a common developmental defect in dogs that is caused by a fixed obstruction (an abnormal ridge of fibrous and muscle tissue just below the aortic valve) to blood flow leaving the left side of the heart. This increases the workload on the left ventricle, causing it to thicken (hypertrophy). As a result, the heart does not fill normally and there is decreased blood flow to the body. Other clinical consequences of severe SAS include abnormal heart rhythms (arrhythmias), exercise intolerance, weakness, or collapse/fainting. Eventually, dogs with severe SAS may develop left sided congestive heart failure (pulmonary edema – fluid in the lungs). Clinical signs of pulmonary edema include cough, exercise intolerance, and labored breathing. Due to thickening of the heart muscle, dogs with SAS are also predisposed to heart arrhythmias and they have an increased risk of sudden death. Clinical signs of arrhythmia include sudden weakness, stumbling or “fainting”. Dogs with SAS are at increased risk of infections of the aortic valve (bacterial endocarditis), so appropriate and early antibiotic therapy of any infections, as well as peri-operative antibiotics, is warranted.

The degree of SAS can progress over time until a dog is fully grown, usually around 1-2 years of age. Since this disease is thought to be genetically linked, it is recommended that dogs with subaortic stenosis not be bred to avoid passing the disease along to future generations.

Treatment with a beta-blocker (such as atenolol) is often recommended in patients with SAS. Beta-blockers are cardioprotective/anti-ischemic by decreasing myocardial oxygen demand (via negative inotropy [decreased contractility] and negative chronotropy [decreased heart rate]). Beta blockade may also help limit arrhythmias (abnormal heart rhythms) secondary to myocardial ischemia and fibrosis. Finally, beta blockade may also help ameliorate secondary dynamic left ventricular outflow tract obstruction.

The majority of dogs with mild SAS have a normal life span and remain asymptomatic. Unfortunately, the prognosis for severe SAS is guarded, and affected dogs are at risk for sudden death.