Located at the distal end of the right ventricular outflow tract at the junction of the pulmonary artery, the pulmonary valve (PV) normally is comprised of three equal-sized, semilunar cusps or leaflets (right, left, anterior), which are joined by three commissures. The leaflets are made up of a thin layer of endocardium and are surrounded by a tough, fibrous annular ring (Fig. 1). The PV is thinner and more delicate than the aortic valve. Unlike the atrioventricular valves (mitral and tricuspid) the PV is not attached to chordae tendinae or papillary muscles. Its opening and closing depends on the pressure gradient across the valve. Generally, the size of the PV slightly exceeds the size of the aortic valve, and the PV diameter is, on average, larger in men than in women.1 The PV lacks coronary artery ostia; it lies anterolateral to the aortic valve, and it is separated from the tricuspid valve by the infundibulum of the right ventricle. The principal function of the PV is to deliver deoxygenated blood from the right ventricle to the lung vasculature, and when competent it opens widely during systole, allowing an appropriate fraction of the right ventricular volume to empty into the pulmonary trunk. During diastole it should close completely to prevent regurgitant flow.
Congenital absence or malformation of the PV can cause significant changes to the intracardiac blood flow, often requiring surgical or endovascular repair. PV atresia, insufficiency, stenosis (infundibular, valvular, or supravalvular), or unicuspid, bicuspid, or quadricuspid arrangements may occur in isolation or in the setting of multiorgan syndromes and/or multichamber heart abnormalities.2–4 Depending on the severity and type of anatomic distortion, symptoms may present in infancy, childhood, or adulthood. Acquired defects secondary to systemic illness (infective endocarditis, rheumatic heart disease, or carcinoid syndrome) or primary cardiac valvular tumors (papillary fibroelastoma) may also alter the right ventricular output requiring surgical and/or medical therapy.3
In the healthy individual, the 3 PV cusps coapt when pressure in the pulmonary artery exceeds the right ventricular pressure (Fig. 2). Inadequate apposition of the leaflets leads to regurgitation of deoxygenated blood back into the right ventricular outflow tract during diastole.5 Dilatation of the pulmonary ring and pulmonary artery may be seen on imaging (Fig. 3). Mild pulmonary insufficiency is a common incidental finding and of little clinical significance, whereas moderate to severe insufficiency can lead to right ventricular strain and remodeling.6 There are many causes of PV insufficiency, including pulmonary hypertension, Marfan syndrome, rheumatic heart disease, infectious endocarditis, and carcinoid disease.6–10 Pulmonary insufficiency is best imaged with magnetic resonance imaging (MRI) because of the anatomic detail of the valve and the ability to provide physiological evaluation with phase-contrast imaging (Fig. 3).
CONGENITAL PV ATRESIA AND STENOSIS
PV atresia is a congenital defect in which the valve orifice never becomes patent, and blood cannot exit the right ventricular outflow tract for oxygenation in the lung. The PV remains atretic because of underdeveloped leaflets or overlying membranous tissue preventing opening during systole. In utero, the fetus is relatively unaffected by the defect as blood is oxygenated through the mother’s placenta; however, at birth, pulmonary circulation is necessary for life, and affected patients require an alternative circulation pathway, typically retrograde flow through a patent ductus ateriosus.11 PV atresia can occur in the setting of an intact ventricular septum or with a ventricular septal defect. The former entity is rarer and results in hypoplasia of both right-sided valves.11 Pulmonary atresia with a ventricular septal defect is more common, occurring in 2.5% to 3.4% of cardiac congenital defects.11 It is considered a form of tetralogy of Fallot and results in an abnormally small right ventricle, as all blood is shunted into the left side of the heart. Pulmonary arterial blood flow is provided by major aortico-pulmonary collateral arteries from the left subclavian artery, descending thoracic aorta, and abdominal aorta (Fig. 4). Treatment for PV atresia typically involves close monitoring in the neonatal intensive care unit with administration of prostaglandins to maintain the ductus arteriosus. Corrective surgery is almost always necessary in these patients and depends on the exact morphology of the right ventricle, PV, and pulmonary artery.12
Valvular pulmonary stenosis results from thickening and fusion of the PV, leading to a decrease in the opening area, an increased pressure gradient across the valve, and, often, dilated main and left pulmonary arteries. The degree of stenosis can be classified as mild, moderate, or severe on the basis of the diameter of valve opening, velocity of jet leaving the right ventricular outflow track, and amount of right ventricular remodeling.13 However, right ventricular enlargement is not typically present unless there is significant concomitant pulmonary regurgitation.13 Etiologies of valvular pulmonary stenosis include congenital malformations, postinfectious causes, and rheumatic heart disease. There is also an association with tetralogy of Fallot, congenital rubella syndrome, Noonan syndrome, and carcinoid heart disease.3,10,14,15 Valvular pulmonary stenosis may also be an incidental finding in asymptomatic patients depending on how well the heart compensates for the decreased aperture. On imaging, valvular pulmonary stenosis can be appreciated on posterior-anterior and lateral plain radiographs as well as on cross-sectional imaging with dilation of the main and left pulmonary arteries due to preferential flow of the turbulent poststenotic jet into the left pulmonary artery (Figs. 5, 6). The right pulmonary artery originates at nearly a 90-degree angle from the main pulmonary artery and is therefore not exposed to this high-velocity jet. It is important to note that the poststenotic dilatation observed in the main and left pulmonary arteries does not occur in infundibular and supravalvular stenosis.16 Computed tomography (CT) and MRI in valvular stenosis show a thickened PV and reduced opening area. MRI can be used to calculate the velocity of the flow jet across the area of narrowing (Fig. 6). Cine imaging can show a high-velocity jet producing signal void that extends into the pulmonary artery during systole. Leftward bowing of the interventricular septum due to elevated right ventricular pressure can also be appreciated (Fig. 6).
Supravalvular pulmonary stenosis may be observed at single or multiple sites of the pulmonary artery.17 Lesions vary from focal narrowing distal to the valve to diffuse pulmonary arterial hypoplasia (Fig. 7). Diffuse stenosis is also possible without arterial hypoplasia. Because of increased pressure proximal to the narrowing, right ventricular thickening and a dystrophic PV may be observed. Supravalvular stenosis is often associated with other cardiac or noncardiac congenital defects, including tetralogy of Fallot, congenital rubella syndrome, William syndrome, Noonan syndrome, Alagille syndrome, and Multiple Lentigines Syndrome (LEOPARD Syndrome).3,14,15,18–20 Supravalvular pulmonary stenosis can also occur as a postsurgical complication following the placement of Blalock-Taussig shunts or a Jatene switch in D-transposition of the great arteries.21 Isolated supravalvular stenosis is rare. Treatment can include balloon pulmonary angioplasty for short-term reduction in right ventricular hypertension and alleviation of symptoms.
Stenosis can occur proximal to the PV. Infundibular or subvalvular stenosis is typically caused by diffuse fibromuscular narrowing of the right ventricular outflow tract and is considered part of the spectrum of the double-chambered right ventricle.4 It can be an isolated finding or a component of complex congenital heart disease such as tetralogy of Fallot or D-transposition of the great arteries.3,21 It is well demonstrated on CT or MRI (Fig. 8). Consequences of subvalvular stenosis include right ventricular hypertrophy and eventual right heart failure. Infundibular stenosis is distinct from secondary outflow tract narrowing that occurs as a sequela of pulmonary hypertension or valvular pulmonary stenosis. Secondary outflow tract narrowing can be corrected by valvotomy or valvuloplasty, whereas infundibular stenosis is not corrected by repair of the PV.22
Treatment for PV stenosis includes balloon valvuloplasty and surgical valvulotomy. Endovascular or operative correction of pulmonary stenosis can lead to severe regurgitation. The decision to replace the PV with a mechanical or bioprosthetic device depends on the size and function of the right ventricle in addition to the extent of patient symptoms.22
DEVELOPMENTAL ABNORMALITIES OF THE PV LEAFLETS
Although the PV ordinarily comprise three equal-sized leaflets (anterior, left, and right) arranged in a semilunar pattern, developmental abnormalities of the valve occur, including unicuspid, bicuspid, or quadricuspid arrangements. Such idiosyncratic anatomy can be diagnosed incidentally with imaging or at autopsy, or lead to shortness of breath and jugular venous distention associated with inhibition of blood flow exiting the right ventricular outflow tract.23
Unicuspid valve is a particularly rare anomaly (Fig. 9). The valve has a single commissure and often demonstrates incomplete apposition during diastole and/or a narrowed orifice during systole. Frequently, unicuspid PV can cause pulmonary stenosis, insufficiency, and/or regurgitation. Bicuspid PV is a congenital defect that is most often asymptomatic and usually diagnosed postmortem. Bicuspid PV occurs in <20% of cases of valvular pulmonary stenosis (Fig. 1B).23 The condition is typically associated with other congenital heart defects such as tetralogy of Fallot, pulmonary stenosis, patent ductus arteriosus, and transposition of the great vessels.3,21 Like unicuspid valves, bicuspid valves lead to pulmonary stenosis and regurgitation. Unique to the bicuspid configuration, doming of the valve is commonly visualized. The quadricuspid configuration is a rare anomaly with four valve leaflets (Fig. 9).24 There is little correlation between cusp size and valvular function. This anomaly is typically less problematic than a quadricuspid aortic valve, as the PV does not have coronary ostia at risk for occlusion by addition leaflets.
INFECTIONS OF THE PV
Blood-borne infections that seed the PV leaflets leading to endocarditis can cause severe morbidity and mortality (Fig. 1C). Among the cardiac valves, endocarditis of the PV is least common.25 It occurs most often in intravenous drug users, in patients with congenital heart disease, in patients with automatic implantable cardiac defibrillators, and in those with central venous lines.26 Endocarditis of the valve typically results in friable vegetations that can be seen on echocardiography, CT, or MRI. Gross pathologic correlation reveals macroscopic deposits randomly situated on all valve leaflets. These vegetations cause abnormal flow jets and turbulence that are visible with color Doppler and the cine features available with cross-sectional imaging (Fig. 10). Vegetations can also dislodge from the valve leaflets and embolize to the pulmonary vasculature and lung parenchyma, causing ischemia or spread of infection. Vegetations <1-2 cm usually respond to medical treatment. Surgery for larger lesions may be necessary.
Rheumatic heart disease following pharyngeal infection with Streptococcus pyogenes can also affect the PV. Approximately half of all patients with acute rheumatic fever develop inflammation of the valvular endothelium (Fig. 11). Such inflammation may cause pulmonary insufficiency or stenosis, or it may worsen the existing PV pathology. Rheumatic heart disease affecting the PV predisposes to recurrent infective endocarditis.
TUMORS OF THE PV
Both benign and malignant tumors can arise from or metastasize to the PV. Papillary fibroelastoma is the most common primary valvular tumor (Fig. 1D). It can originate from any endocardial surface, but 80% to 90% are situated on the valves themselves. Other endocardial surfaces account for 10% of papillary fibroelastomas.27 These locations include the left ventricular apex, chordae tendinae, and the right and left ventricular outflow tracts. It is rare for a patient to have multiple papillary fibroelastomas. The valvular frequency of fibroelastomas is aortic (35%), mitral (25%), tricuspid (17%), and pulmonary (13%).27 Most cases of valvular papillary fibroelastoma are discovered at autopsy (Fig. 12). Treatment is surgical excision and reconstitution or replacement of the valve to prevent postoperative stenosis or insufficiency. Fibroelastomas may show delayed enhancement on MRI because of the presence of fibrous tissue. Differential diagnosis includes bland thrombus, valvular vegetations, and tumor. Microscopically they have characteristic gelatinous, branching fronds.
Carcinoid syndrome can also affect the PV. A neoplasia arising from enterochromaffin cells, carcinoid tumors produce biologically active substances, of which serotonin is the most common. Vasoactive substances released from carcinoid metastases to the liver can reach the right side of the heart and result in fibrous deposition on endocardial surfaces (Fig. 13). Right-sided endocardial deposits can be seen in up to 50% of patients with liver-involved carcinoid syndrome.10 A PV that is affected by carcinoid deposits appears thickened, retracted, or stenotic on imaging, with narrowing of the annulus. Symptomatic patients may require surgical intervention. Because replacement with biological valve prosthesis can lead to recurrent carcinoid degeneration, use of mechanical prostheses is recommended. Patients who present late in the disease course are often treated with medical management only.
The PV receives less attention than the tricuspid or left-sided heart valves, but it can be affected by a wide variety of pathologic processes. Congenital abnormalities, most notably pulmonary stenosis, can cause symptoms even in adulthood. Acquired diseases such as endocarditis and tumors such as papillary fibroelastoma can also lead to significant morbidity and mortality.
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