Subaortic Stenosis in Dogs (SAS): Heart Murmur, Symptoms and Treatment

What is subaortic stenosis?

Subaortic stenosis (SAS) is a congenital heart defect in which fibrous tissue forms below the aortic valve, obstructing blood flow from the left ventricle into the aorta. It is the most common congenital cardiac malformation in dogs. The obstruction forces the heart to work harder, leading to left ventricular hypertrophy, arrhythmias and risk of sudden cardiac death.

Predisposed breeds

SAS has a genetic basis (polygenic or autosomal dominant with variable penetrance):

• Newfoundland — highest prevalence (up to 30–35 % in some studies).
• Golden Retriever, Rottweiler, Boxer, German Shepherd.
• Bernese Mountain Dog, Bull Terrier, Great Dane, Samoyed.

Symptoms

• Mild cases: asymptomatic. Detected via a systolic left-sided heart murmur on routine examination.
• Moderate cases: exercise intolerance, easy fatigue, exercise-induced syncope (fainting).
• Severe cases: frequent syncopes, weakness, dangerous arrhythmias, congestive heart failure. Risk of sudden death.

Diagnosis

• Auscultation: systolic murmur at the left cardiac base radiating to the neck. Grade I–VI correlates imperfectly with severity.
• Doppler echocardiography: gold standard. Measures the peak instantaneous pressure gradient across the obstruction. Mild <30 mmHg; moderate 30–80 mmHg; severe >80 mmHg.
• ECG and Holter monitor: detect arrhythmias, particularly ventricular tachyarrhythmias that raise the risk of sudden death.

Treatment

Medical

• Atenolol (beta-blocker): does not reduce the obstruction but controls heart rate, reduces arrhythmia risk and improves ventricular filling. Treatment of choice for moderate and severe cases.
• Restriction of intense or competitive exercise.

Interventional / surgical

• Balloon valvuloplasty: percutaneous technique; results in SAS are suboptimal as the fibrous lesion remodels and obstruction recurs.
• Open-heart surgery: surgical resection of the fibrous tissue; available only at specialist university centres; high risk.

Prognosis

Dogs with mild SAS (gradient <30 mmHg) have a near-normal life expectancy. Moderate cases: median survival 3–4 years. Severe cases (>80 mmHg) carry an elevated risk of sudden death before age 3 without treatment. Breeding dogs with moderate-severe SAS is not recommended given the hereditary component.