Pulmonary Stenosis (PS)

Pulmonary stenosis is the second most common congenital heart disease. It accounts for 7.5-12% of all congenital heart diseases. 50% of all congenital heart diseases include pulmonary stenosis as a component of the defect.

Outflow tract septum:  The cardiac outflow tract include the ventricular outflow tract, the semilunar valves and the aortopulmonary septum.  There has been much debate regarding this process, the following is a summary of various theories [6, 36, 40].

 Kramer (1942) suggested that there are three embryological areas, the conus, the truncus and the pulmonary arterial segments.  Each segment develop two opposing ridges of endocardial tissue, the opposing pair of ridges and those from various segments meet to form the septum separating the two outflow tracts and the aortopulmonary trunks.  The aortopulmonary septum is formed by ridges separating the fourth (future aortic arch) and the sixth (future pulmonary arteries) aortic arches.  The truncus ridges are formed at the area where the semilunar valves are destined to be formed, therefore forming the septum between the ascending aorta and the main pulmonary artery.  The conus ridges form just below the semilunar valves and from the septation between the right and left ventricular outflow tracts.

 Van Mierop (1979), agreed that there are three pairs of ridges forming in the aortopulmonary, truncus and conus regions, however, he stated that the pairs of ridges fuse independently and later on fuse with each other to complete the septation.  His theory indicate that the truncus ridges form  first, and as they fuse they form a truncal septum which then fuses with the aortopulmonary septum which is formed by invagination of the dorsal wall of the aortic sac between the fourth and the sixth aortic arch arteries.  Figure 11

 Asami(1980),  followed Van Mierop’s theory, however, he stated that these ridge fuse in the opposite direction of what Van Mierop has indicated, i.e. from the outflow tract to the aortopulmonary region.

 Pexieder(1978, 1984) and Orts Llorca et al (1982), stated that there are only tow septa, a conotruncal (or bulbar) and an aortopulmonary septum.

 Bartlings et al (1989), introduced a new theory.  They stated that the septation process of the ventricular outflow tracts, pulmonary and aortic valves and the great vessels is mostly caused by a single septation complex, which they termed aortopulmonary septum.  This septation complex develops at the junction of the muscular ventricular outflow tract with the aortopulmonary vessel.  This junction has a saddle shape, i.e. not in one plane which would allow the right ventricular outflow tract to be long with a short main pulmonary artery, while the left ventricular outflow tract become short with a long ascending aorta.  (Figure 12) The ventricular outflow septation is formed by condensed mesenchyme, embedded in the endocardial cushion tissue just proximal to the level of the aorto-pulmonary valves.  The condensed mesenchyme will come in close contact with the outflow tract myocardium, from the area just above the bulboventricular fold, and participate in the septation of the outflow tract by providing an analogue to muscle tissue.  [6-9].  Myocardium in contact with the mesenchymal arch grow rapidly and form the bulk of the outflow septum and is continuous with the primary fold on the parietal  wall of the right ventricle and the myocardium on the right side of the primary septum.

 The pulmonary valve leaflets may be fused or the valve leaflets may be thickened. The opening may be eccentric when the valve leaflets are thick, sometimes the leaflets are immobile with a variably small annulus. The right and left coronary arteries are usually dilated because of poststenotic jet effect. Right ventricular hypertrophy is proportional to the degree of stenosis. Right ventricular outflow tract obstruction may be noted due to muscular hypertrophy of the moderator band. This is usually seen in association with membranous ventricular septal defect, however, the septal defect may close leaving right ventricular outflow obstruction. It is not known if it is possible to develop this pathology without ever having had a VSD. Tetralogy-like pulmonary stenosis with intact ventricular septum is rare. Left and right pulmonary arteries hypoplasia together with pulmonary stenosis is seen after congenital Rubella syndrome. Diffuse pulmonary stenosis is seen with Alagille syndrome where there is pulmonary valve stenosis as well as diffuse main and branch pulmonary artery stenosis. Peripheral pulmonary stenosis is seen in Noonan’s syndrome. Peripheral pulmonary stenosis and main pulmonary artery stenosis is seen with Williamis syndrome.

The right ventricular pressure will increase to overcome the stenosis of the pulmonary valve with an increase in cardiac output due to exercise. The extent of stenosis will be exaggerated resulting in higher right ventricular pressure until the right ventricular muscles can no longer generate enough pressure to overcome the stenosis resulting in right ventricular heart failure. The right ventricular hypertrophy and hypertension will result in right ventricular dilation leading to left ventricular dysfunction.

 Patients are usually asymptomatic. On auscultation the first heart sound is normal followed by an ejection click. The shorter the distance from S1 to the click, the more severe is the pulmonary stenosis. Early systolic click is noted in all cases of pulmonary stenosis except those with dysplasia of the pulmonary valve. Absence of a click makes the diagnosis questionable. The murmur is ejection in type, typically harsh and usually IV/VI in intensity or more. The later the peaking of the ejection systolic murmur the worse is the pulmonary stenosis. The second heart sound is typically widely split and the wider the split the more is the stenosis. A soft P₂ secondary to decreased PA pressure due to severe stenosis may be noted.

Right ventricular hypertrophy with right axis deviation. Right atrial enlargement may be noted in severe cases. If the pulmonary stenosis is quite severe causing right ventricular atresia such as in a near-pulmonary atresia then the right ventricular forces will be decreased.

Typically the maximum instantaneous pressure gradient Doppler is 10% more than the peak-to-peak gradient measured at the cath lab. When planing for balloon dilatation of pulmonary stenosis, the nature of the valve leaflets have to be studied by 2D echocardiography and the pulmonary valve annulus measured to determine the size of the balloon to be used.

• Mild pulmonary valve stenosis does not require therapeutic intervention. SBE prophylaxis is not indicated.
• Moderate to severe pulmonary stenosis: Cardiac catheterization should be performed to balloon dilate the stenotic pulmonary valve. Results of balloon dilatation is better when the pulmonary stenosis is due to fusion of commissures rather than when the pulmonary valve is dysplastic.

In severe pulmonary stenosis with cyanosis due to right-to-left shunting at the atrial level Prostaglandin will be necessary and balloon dilatation of the pulmonary valve may result in relief of much of the pressure gradient across the pulmonic valve.

Suicide right ventricle (severe right ventricular contractility failure) could be seen in patients post balloon dilatation of the pulmonary valve due to right ventricular outflow tract obstruction secondary to musculature hypertrophy of the right ventricular outflow tract. This is also noted post surgical relief and particularly in patients with an initial right ventricular pressure of more than 100 mm Hg. It is rare nowadays to see patients with this kind of pressure in the right ventricle as they are treated early enough.

In patients with a pressure gradient of less than 25 mm Hg only 4% become worse. Patients with a pressure gradient across the pulmonary valve of 26-49 mm Hg have a 21% chance of developing progressive increase in pressure gradient. Patients with a pressure gradient of 50-79 mm Hg across the pulmonic valve have a 79% chance of becoming worse and patients with 80 mm Hg or higher have a 97% chance of becoming more severe. Life expectancy in mild, moderate or severe pulmonary stenosis after repair is within normal limits.

 Critical Pulmonary Stenosis in Neonates

With near atresia of the pulmonic valve the right ventricle is hypertrophied yet with a very small cavity as in pulmonary atresia with intact ventricular septum and this is associated with left axis deviation on the electrocardiogram. With surgical relief or balloon dilatation of the pulmonic valve cyanosis decreases as there is more antegrade flow through the tricuspid valve and pulmonic valve. As time passes the right ventricular size increases and could accommodate for the entire cardiac output with resolution of cyanosis. This is more likely to occur in the case of pulmonary atresia with intact ventricular septum therefore a B-T shunt may not be necessary as treatment with prostaglandin keeping the ductus arteriosus patent may suffice as a bridging condition until antegrade flow through the right ventricular outflow and pulmonic valve is adequate.

Peripheral Pulmonary Stenosis

• This is usually mild with no significant ill consequences. It is seen in neonates due to the smallness of the pulmonary arteries in-utero since they carry a small portion of the combined cardiac output to the lungs (7% of the combined cardiac output goes to both lungs). approximately 10-15% only will require surgical or balloon dilatation procedures.

• In Rubella syndrome there is peripheral tapering of the pulmonary arteries bilaterally from their origin and rarely it is of an intensity requiring therapeutic procedures such as surgery or balloon dilatation.

• In Noonan’s syndrome peripheral pulmonary stenosis may be seen and older patients with this syndrome may also have cardiomyopathy. This syndrome is associated with lymphedema, webbed neck, dysmorphic features and hypotonia.

• Patients with William’s syndrome develop supravalvar pulmonary and aortic stenosis with or without coarctation of the aorta and renal artery stenosis.