Subaortic Stenosis

Subaortic stenosis is the second most common reported canine congenital defect.  The left ventricular outflow tract obstruction ranges from an incomplete fibrous ridge to a fibromuscular tunnel to generalized narrowing and dynamic obstruction.  Breeds such as Newfoundlands, Golden Retrievers, Rottweilers, Boxers, and German Shepherd dogs are most commonly affected.

The degree of stenosis varies from patient to patient and is an important determinant of outcome. In general, the tighter or more narrow the outflow tract, the worse a patient will do.  The murmur is best ausculted at the left base, is systolic in nature, may radiate into the carotid/thoracic inlet/right base, and may have a high pitched ejection quality to it.  The murmur can get louder during the first 6 months of age.  Pulse quality can be reduced.

Symptoms are variable depending upon degree of stenosis.  Stunted growth, exercise intolerance, fainting/seizure, arrhythmia, congestive heart failure and sudden death can occur.

Diagnostic workup may consist of thoracic radiographs, ecg, blood pressure measurement, echocardiography with doppler interrogation and occasionally cardiac catheterization.  Echocardiography with doppler is a tool that often gives us the information necessary to make a diagnosis and quantitate severity.  The flow of blood over the stenotic area is quantified.  The tighter or more narrow the stenosis, the faster the blood must flow or “jet”.  This velocity can be correlated to a pressure gradient.  Disease is categorized based upon the pressure gradient.  Any dog with this disease should not be bred.

Pressure gradient categories based on awake patients, doppler study

0-30mmHG – Mildly affected dogs.  Generally, can live a normal life with mild exercise intolerance.

30-75mmHG – Moderately affected dogs.  These patients have a more unpredictable course as some exhibit very little symptoms, while others may go on to manifest some of the more serious problems such as congestive heart failure and sudden death.

75mmHg and higher – Severely affected dogs.  These patients usually have symptoms that develop within the first two years of life.

Therapy of mildly affected dogs is generally not needed except for prophylactic antibiotics for potential bacteremic episodes (dental procedures, skin wounds, surgery).  Moderately to severely affected patients are frustratingly limited.  In patients with symptoms, calcium channel blockade or beta blockade is thought to slightly improve the patient’s status.

Surgical repair is associated with high cost, high mortality, restenosis and is very technically demanding currently.

Balloon valvuloplasty can be considered in severe  or symptomatic patients.  This technique will often reduce the gradient, but the long term staying power of such a procedure is somewhat limited.  Some patients get worsening gradients over time after the procedure.