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Outflow Tract Obstructions


This group of congenital cardiac defects includes aortic stenosis, pulmonic stenosis, and coarctation of the aorta. All involve obstruction to either right or left ventricular outflow.

Aortic Stenosis

Left ventricular emptying may be obstructed at 3 locations: 1) subvalvular, also called subaortic, consisting of a fibrous ridge of tissue within the left ventricular outflow tract; 2) valvular; and 3) supravalvular or obstruction distal to the aortic valve. The most common form in dogs is subaortic stenosis. Breed predilections have been identified for Boxers, Golden Retrievers, Rottweilers, German Shepherds, and Newfoundlands.

Fig. 2

Aortic stenosis induces left ventricular hypertrophy, the degree of which depends on the severity of the stenosis. In severe cases, left ventricular output may be decreased, especially during exercise. The major ramification of left ventricular hypertrophy is the creation of areas of myocardium with poor perfusion. Myocardial ischemia is a major factor in the development of serious life-threatening ventricular arrhythmias.

Clinical signs do not consistently parallel the severity of stenosis. There may be a history of syncope and exercise intolerance. Animals with no history of illness may die suddenly and the defect is first detected at necropsy. An ejection-type systolic murmur heard best at the aortic valve area is present. The intensity of the murmur correlates fairly well with the degree of stenosis and may increase as animals mature, reflecting progressive stenosis. Puppies without detectable murmurs should not be considered free of disease until they reach 6 mo of age because the murmur may be very soft in the first months of life. In moderate to severe cases, femoral pulse strength is diminished. Electrocardiography may show left ventricular enlargement (tall R waves in lead II) and ventricular premature complexes that may increase in frequency with exercise. Holter monitoring should be used in syncopal animals or in patients with severe disease to define the presence of any arrhythmias, assess arrhythmia severity, and assist in determining the risk of sudden death. A recheck Holter monitor may be considered following initiation of antiarrhythmic therapy to assess efficacy. Radiographically, there is variable left ventricular enlargement and poststenotic dilatation of the aorta. Doppler echocardiography is recommended to confirm the diagnosis and rule out other cardiac abnormalities. The degree of left ventricular hypertrophy and peak systolic flow velocity through the defect can help determine stenosis severity.

Treatment options include medical management of arrhythmias to reduce the incidence of clinical signs of exercise intolerance or syncope, balloon valvuloplasty (typically not very effective), and surgical resection (high morbidity and mortality, high cost, and lack of significant gradient reduction). The use of β-blockers such as atenolol has been advocated to control ventricular arrhythmias in patients with subaortic stenosis and to presumably reduce the chance of sudden death. Mildly affected animals commonly require no treatment and the prognosis can be fair to good in very mildly affected patients. Affected animals should not be used for breeding.

Pulmonic Stenosis

Pulmonic stenosis is common in dogs and infrequent in cats. It results in obstruction to right ventricular outflow due, in most cases, to dysplasia of the pulmonic valve cusps. The stenosis can also occur in the infundibulum, the subvalvular region, or in the supravalvular area.

The right ventricle must generate increased pressure during systole to overcome the stenosis, which in moderate to severe cases can lead to dramatic right ventricular hypertrophy and dilatation. As the right ventricle hypertrophies, ventricular compliance diminishes, leading to increased right atrial pressure and venous congestion. The increased flow velocity deforms the wall of the main pulmonary artery, resulting in a poststenotic dilatation. In severe cases, right-sided congestive failure may be noted. Supravalvular pulmonic stenosis is uncommon and may be most often observed in Giant Schnauzers. Concurrent tricuspid valve dysplasia is sometimes noted in animals with pulmonic stenosis. Anomalous coronary artery development has been documented in some affected animals with pulmonic stenosis such as Boxers and English Bulldogs. Typically, the left main coronary artery originates from a single right coronary artery and encircles the right ventricular outflow tract.

Affected animals may have a history of failure to thrive and exercise intolerance. Right-sided CHF may be present and is characterized by ascites or peripheral edema. A prominent ejection-type systolic murmur is present and heard best at the pulmonic valve area. A corresponding precordial thrill is usually present. Jugular distention and pulsations may also be present. Electrocardiography will demonstrate evidence of right ventricular enlargement in many cases. Radiographic abnormalities include right ventricular enlargement, an aneurysmal dilatation of the main pulmonary artery, and diminished pulmonary perfusion. Echocardiography is indicated in these cases and may demonstrate right ventricular dilatation and hypertrophy, interventricular septal flattening, and thickened and relatively immobile pulmonic valve cusps. In a few cases, supravalvular or discrete subvalvular stenosis can be noted. Pulmonic insufficiency can sometimes be noted in dogs with pulmonic stenosis. Doppler evaluation is valuable in determining the severity of the stenosis. Based on severity (reported as the pressure gradient across the valve), the need for intervention can be assessed. Animals with moderate or severe pulmonic stenosis can benefit from balloon valvuloplasty or surgical intervention (valvulotomy, patch grafting, partial valvulectomy, or conduits). The choice of surgical procedure depends to some degree on the presence and degree of subvalvular muscular hypertrophy. Palliative therapy with oral medications such as diuretics and vasodilators should be initiated if right-sided CHF is present. The prognosis is typically poor if atrial fibrillation or right-sided CHF is present. If atrial fibrillation is noted, use of a digitalis glycoside may be warranted.

Coarctation of the Aorta

This rare condition of dogs and cats involves narrowing of the aorta distal to the subclavian artery, typically in the area of the ductus arteriosus. Other uncommon congenital abnormalities of the aorta include tubular hypoplasia of the ascending aorta and aortic interruption. Surgical correction has been reported.

Last full review/revision April 2012 by Davin Borde, DVM, DACVIM

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