First described by Elliotson, infundibular pulmonary stenosis (IPS) refers to obstruction of outflow from the right ventricle (RV) within the body of the RV, as opposed to obstruction at the pulmonary valve, pulmonary artery (PA), or its branches. IPS often occurs in association with other intracardiac anomalies; isolated IPS is rare. In this article, IPS refers to isolated infundibular pulmonary stenosis with an intact ventricular septum.
IPS has 2 forms. In the more common type, stenosis of the proximal portion of the infundibulum is due to a fibrous or muscle band at the junction of the main cavity of the RV and the infundibulum. The band divides the cavity into 2 chambers. The second type is associated with a thickened muscular infundibulum that forms a narrow outlet to the RV. The infundibulum appears shrunken. In this second type, the narrowed area may be short or long and may be located immediately below the pulmonary valve or lower into the outflow tract.
IPS is often associated with a ventricular septal defect (VSD). Common opinion holds that the VSD closes, leaving behind an isolated IPS. Whether some children have excessive muscle without ever having had a VSD is not known.
The hemodynamic consequence of the obstruction is elevated pressure within the RV cavity. The degree of elevation depends on the severity of obstruction. When severe, the resulting RV systolic pressure may exceed that of the left ventricle (LV). This high pressure is limited to the portion proximal to the infundibulum. Lower or even normal pressures are present beyond the obstruction site. Reactive RV hypertrophy follows.
Subsequent elevation of end-diastolic pressure and decreased compliance of the RV, consequent to the hypertrophy, lead to elevated right atrial (RA) pressure and dilatation of that chamber. Greater RA pressures are required to fill the ventricle, and relative right and left atrial pressures may be reversed, favoring persistent patency of the foramen ovale and right-to-left shunting. This gives rise to central cyanosis. Such reversal and cyanosis can occur when the RV is hypoplastic, even with less severe PS.
In neonates with severe PS, the pulmonary blood flow depends on the patency of the ductus arteriosus. The RV becomes hypertrophic and maintains cardiac output to a very advanced stage. With exercise, the requirement for cardiac output increases, and RV pressure proportionately rises until it exceeds the capacity of the RV muscle.
Deformity and malfunctioning of the LV occur in proportion to RV hypertension and can be demonstrated using sophisticated techniques. These LV problems have little practical consequence because they improve with relief of the RV hypertension.
Reduced LV end-diastolic compliance has been chiefly ascribed to displacement of the interventricular septum (IVS) into the LV cavity. The mechanisms responsible for systolic dysfunction are less clear.
Patients with isolated valvular PS may have a reactive infundibular hypertrophy that could elicit a reactive infundibular obstruction. This obstruction might persist for a variable period following relief of the valvar obstruction.
IPS can cause anatomic and, possibly, functional changes in the developing pulmonary vascular bed. Experimental construction of the pulmonary trunk in fetal lambs is accompanied by relatively thin-walled PA resistance vessels. Some researchers postulate this decreased medial muscular layer is caused by a higher oxygen tension of the blood perfusing the fetal pulmonary circulation.
IPS has been noted to develop in a fetus following twin-to-twin transfusion, and authors have, in that case, postulated that the elevated blood volume/pressure may be a factor in the pathogenesis.  Inversin, the product of the inv locus, may have a specific role in cardiac morphogenesis (especially in the development of IPS) in addition to its contribution to situs determination.
In the United States, the relative frequency of IPS among all obstructive lesions of the RV outflow tract is 2-10%.
Internationally, no reliable figures have been reported.
Sex- and age-related demographics
IPS has no sex predilection.
IPS is present from birth, although most patients are asymptomatic. The severity of stenosis can progress with age.
Prognosis mainly depends on the severity of stenosis in the absence of any additional lesions.
Patients with mild-to-moderate stenosis live normal lives with no symptoms, apart from the risk of IE.
Severely affected patients are at risk of death at any time throughout their lives. Patients at highest risk are those with symptoms in the first year of life, as well as those with congestive cardiac failure or paroxysmal dyspneic episodes.
Severe infundibular pulmonary stenosis (IPS) that leads to atresia can result in neonatal death when the ductus arteriosus constricts. Heart failure secondary to IPS is rare in childhood beyond early infancy. Exercise may provoke syncope and even sudden death in severe stenosis. The stenosis may remain asymptomatic but may progress to require intervention in adulthood.
Complications of pulmonary stenosis include the following:
Increasing severity of stenosis
Significant residual pulmonary regurgitation and right heart failure (rare)
RV endocardial fibroelastosis
Pulmonary artery aneurysm formation 
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