Pulmonic stenosis can occur at three levels: (1) infundibular or subvalvular pulmonic stenosis may occur secondary to the valvular form and is caused by a ring of fibrous tissue surrounding the right ventricular outflow tract; (2) the valvular form which is the most common form in dogs; and (3) the supravalvular form is rare. Pulmonic stenosis is one of the components of tetralogy of Fallot. Isolated pulmonic stenosis is rare in cats.
Valvular pulmonic stenosis represents a form of pulmonic valve dysplasia. The valve leaflets are often fused or thickened and in most cases there is secondary hypertrophy of the infundibular portion of the right ventricular outflow tract. The clinical signs are related to the degree of right ventricular outflow obstruction. Pulmonic stenosis causes an increased systolic pressure in the right ventricle with a systolic pressure gradient across the valve. Many dogs have mild lesions and show no clinical signs. More severe lesions lead to a pressure gradient across the pulmonic valve and pressure overload in the right ventricle. Concentric hypertrophy of the right ventricle may contribute to the outflow obstruction and a further increase in myocardial oxygen consumption. The jetting of blood through the stenotic valve may damage the valve leaflets resulting in pulmonic regurgitation and right ventricular overload. Some cases of pulmonic stenosis may be complicated by tricuspid valvular insufficiency.
The incidence of pulmonic stenosis is highest in English bulldogs, fox terriers, miniature schnauzers, chihuahuas and samoyeds. A hereditary basis has been postulated in the beagle breed. No sex predisposition has been reported.
Clinical signs associated with pulmonic stenosis often do not become apparent until 6 months of age or older although before this the animal may appear stunted and ill thriven. The most common presenting sign is reduced exercise tolerance; more severe lesions may result in syneopal episodes and occasionally dyspnoea, cyanosis and signs of right-sided cardiac decompensation. A prominent jugular pulse due to increased right atrial pressure may be present. A crescendo-deerescendo systolic murmur can be heard over the pulmonic valve region; the murmur may also beaudible at the thoracic inlet. Pulmonary hypertension may lead to delayed closure of the semilunar valves and a split S2 sound. Severe murmurs may be associated with a precordial thrill.
Moderate to severe lesions consistently cause a right axis shift (> 120°) and signs consistent with right ventricular enlargement (deep S and Q waves in leads I, II, III and aVF). Ventricular arrhythmias are relatively common.
Post-stenotic dilatation of the main pulmonary artery segment may lead to an absence of the cranial waist on the lateral view and a bulge at the two o’clock position on the dorsoventral view. Right ventricular hypertrophy is manifested as a typical reverse D shape with shifting of the cardiac apex towards the left on the dorsoventral view. Pulmonary vascularity may appear decreased.
Echocardiographic findings include right ventricular hypertrophy, post-stenotic dilatation of the main pulmonary artery, abormal thickening of the pulmonic valve leaflets and flattening of the interventricular septum with paradoxical motion due to right ventricular pressure overload. The severity of the outflow obstruction can be assessed by Doppler echocardiography. A systolic pressure gradient of more than 15 mmHg between the right ventricle and the pulmonic artery is suggestive of pulmonic stenosis.
Angiocardiography and intracardiac pressure studies
Angiocardiography and catheterization pressure studies are rarely required since both the diagnosis and the pressure gradient across the pulmonic valve can be assessed non-invasively by echocardiography. Non-selective or selective angiocardiographic techniques may be performed.
Pulmonic stenosis: Treatment
The pressure gradient across the pulmonic valve should be determined before contemplating surgery. Surgery is generally indicated if the pressure gradient across the valve exceeds 70 mm Hg with evidence of severe right ventricular hypertrophy in a dog showing clinical signs. Surgical management consists of balloon dilatation of the pulmonic valve. Dogs with right ventricular systolic pressures greater than 120 mm Hg have higher 5 year mortality than dogs with right ventricular pressures less than 120 mm Hg.