Pulmonic stenosis in the dog

Pulmonic stenosis (PS) is the most common canine congenital disorder, with a prevalence of 23%; it is occasionally encountered also in the cat. No sex predilection has been found. The most commonly affected breeds are the Beagle, Samoyedo, English Bulldog, Miniature Schnauzer, Chihuahua and Terrier. In the Beagle, the existence of a hereditary form of valvular dysplasia with polygenic inheritance has been confirmed.

PATHOLOGY

Pulmonic stenosis was initially described in the Beagle, with two forms being identified. Grade I stenosis is characterized by a normal annulus and by moderate valve leaflet alterations, which may appear slightly thickened and with a minimal degree of leaflet fusion. Grade II stenosis is instead characterized by annulus hypoplasia and by valve leaflet fusion or hypoplasia. This latter form is associated with a more severe stenosis and is the more common of the two.

Supravalvular stenosis is quite rare; various forms of subvalvular stenosis are more common and may be associated with infundibular muscular bands (double-chambered right ventricle) (Fig. 1) or with the presence of coronary artery anomalies. The presence of coronary artery anomalies is important as it is a contraindication for interventional procedures.

PATHOPHYSIOLOGY

PS is the cause of increased right ventricular afterload and of the consequent concentric ventricular hypertrophy. Decreased right ventricular volume is accompanied by increased ventricular pressure, with consequent flattening of the interventricular septum. Furthermore, increased ventricular pressure causes an increase in right atrial diastolic pressure, a condition which may evolve into right congestive heart failure with ascites and pleural effusion.

DIAGNOSIS

Clinical
Dogs with PS are usually asymptomatic. Weakness, fatigue or right congestive heart failure are occasionally present, however such symptoms are usually more common in dogs over one year of age. Symptoms are more frequently encountered when PS is associated with other defects. When PS is accompanied by interatrial or interventricular defects, with a right-to-left shunt,  the dog may present rest or stress-induced cyanosis and polycythaemia. The jugular pulse may be prominent (a wave) while the femoral pulse is usually normal.

Chest auscultation reveals a heart murmur of variable intensity, with  peak intensity coinciding with the pulmonary artery stenosis (4_^th left intercostal space); it may also irradiate cranially or to the right. The murmur is usually harsh, with a diamond-shaped pattern (crescendo-decrescendo).

Electrocardiogram
The ECG provides information on the possible presence of arrhythmias and of right ventricular hypertrophy, with right axis shift of the mean electrical axis.

Radiology
In the presence of mild stenosis, chest X-rays may appear normal or may show an enlargement of right-sided structures (more evident in VD or DV views).  Radiology may reveal post-stenotic dilatation of the main pulmonary artery,  dilatation of the left proximal artery and pulmonary hypoperfusion.

Ultrasonography
Ultrasonography can be used for both diagnosis and staging of the disease. B-mode ultrasound examination allows investigation of the possible presence of right ventricular outflow tract lesions, hypoplastic pulmonary annulus and valvular lesions. Valve leaflets may be partially fused and thickened. Leaflet systolic movement may appear reduced due to the presence of fused cusps; in such cases doming of the valve is present. Coronary artery anomalies may be already evident with two-dimensional ultrasonography, however a selective coronary angiography is often necessary to confirm the diagnosis. Pulmonary stenosis may be associated with other congenital disorders such as atrial septal defect, patent foramen ovale or, more commonly, ventricular septal defect or tetralogy of Fallot.

Doppler echocardiography allows assessment of the pressure gradient through the stenotic ostium with a good level of precision.  Using the peak pulmonary flow velocity and the modified Bernoulli equation it is possible to calculate the pressure gradient between the right ventricle and the pulmonary artery.  PS is considered moderate when the gradient is < 40-50 mmHg and severe if the gradient is > 80-100 mmHg. The accuracy of Doppler measurement depends on the alignment with the flow to be measured. The views necessary to guarantee good alignment are the right parasternal short axis view, optimized for the pulmonary artery, or the left cranial short axis view.

PS is often associated with valvular insufficiency, however right volume overload caused by pulmonary insufficiency is usually well tolerated.

Angiography and cardiac catheterization
The right ventricle is easily accessible with a minimally invasive approach, with the introduction of venous catheters through a jugular vein.

Right heart catheterization allows to measure the stenotic valve pressure and to precisely assess the severity of the existing stenosis. In addition, cardiac catheterization also allows selective angiography of the right ventricle and of the right ventricular outflow tract (RVOT). Angiography is not used for diagnostic purposes; it is to be considered as a pre-operatory investigation whenever valvuloplasty is planned; angiography does in fact allow to asses the anatomy of the right ventricle and of the RVOT, as well as to accurately measure the pulmonary annulus size. Finally, coronarography allows to determine the possible presence of coronary artery anomalies in breeds at risk.

TREATMENT

Balloon valvuloplasty is the elective treatment of pulmonary valve stenosis and is recommended in all patients with a pressure gradient greater than 60 mmHg. Standard heart failure treatment is to be reserved for those patients with signs of congestive heart failure and in whom surgery is not possible. The use of beta-blockers is an option in subjects with right outflow tract obstruction and with a dynamic pulmonary obstruction. Beta-blockers may reduce the incidence of arrhythmias, reduce syncopal events and prevent sudden death.

Supravalvular pneumonic stenosis is a rare finding. The treatment of two dogs with supravalvular PS by means of a vascular stent has recently been reported, one with a congenital and the other with an acquired condition.

Valvuloplasty

The first pulmonary valvuloplasty, described by Buchanan, was successfully performed in 1980 in an English Bulldog, and was repeated two years later in a child. Today, pulmonary balloon valvuloplasty is the elective treatment for valvular PS in the dog and is recommended in all patients with moderate/severe PS. The procedure is to be performed as soon after the diagnosis as possible, in order to avoid further right ventricle remodelling and failure. In the last decade many cases of dogs successfully treated with pulmonary balloon valvuloplasty have been reported in literature (Fig. 2). The type of stenosis may influence surgical success; in the presence of annulus hypoplasia and non-mobile leaflets, surgery is less effective. According to a recently published paper, the pressure gradient before surgery and the residual post-ballooning pressure gradient are two independent predictive factors for long-term prognosis. Surgery is usually considered effective when it allows for a 50% pressure gradient reduction.

Balloon selection is another fundamental element. The pulmonary annulus and balloon size ratio should be between 1.2 and 1.5. In measuring the annulus a good correlation exists between transthoracic ultrasonography and angiography, however angiography allows for a more accurate measurement and must always be performed before final balloon selection.

A double ballooning technique for pulmonary balloon valvuloplasty has also been described. This technique requires two venous accesses (femoral veins) and allows to overcome existing limitations in patients in whom the use of large-size introducers is not possible in view of the small body size.

The most severe reported complications include myocardial perforation, pulmonary artery rupture and the onset of fatal arrhythmias. In patients with severe dynamic pulmonary stenosis a condition known as “suicidal right ventricle” may occur, in which the stenosis caused by hypertrophy of the right outflow tract worsens the moment in which the valvular pressure gradient is reduced, causing a sudden increase in ventricular pressure. Reported minor complications include lesions of the tricuspid valve, arrhythmias, right bundle branch block and vascular access-site haemorrhages.

Suggested readings

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3. Buchanan JW. Pathogenesis of single coronary artery and pulmonic stenosis in English Bulldogs. JVIM 2001; 15(2): 101-4.
4. Patterson DF, Haskins ME, Schnarr WR. Hereditary dysplasia of the pulmonary valve in Beagle dogs. Pathology and genetic studies. Am J Cardiol 1981; 47(3): 631-41.
5. Bonagura JD, Lehmkuhl LB. Congenital heart disease. In: Fox P, Sisson DD, Mois NS. Textbook of canine and feline cardiology. 1999; WB Saunders, Philadelphia, pp 478-85.
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7. Locatelli C, Domenech O, Silva J et al. Independent predictors of immediate and long-term results after pulmonary balloon valvuloplasty in dogs. J Vet Cardiol 2011; 13(1):21-30.
8. Estrada A, Moise NS, Erb HN et al. Prospective evaluation of the balloon-to-annulus ratio for valvuloplasty in the treatment of pulmonic stenosis in the dog. JVIM 2006; 20(4):862-72.
9. Estrada A, Moise NS, Renaud-Farrel S. When, how and why to perform a double ballooning technique for dogs with valvular pulmonic stenosis. J Vet Cardiol 2005; 7: 41-51.
10. Griffiths LG, Bright JM, Chen Chan K. Transcatheter intravascular stent placement to relieve supravalvular pulmonic stenosis. J Vet Cardiol 2006; 8: 145-55.
11. Johnson MS, Martin M, Edwards D et al. Pulmonic stenosis in dogs: balloon dilation improves clinical outcome. JVIM 2004; 18(5): 656-62.