Pulmonary (Pulmonic) Stenosis Imaging

The term pulmonic stenosis (PS, pulmonary stenosis, pulmonary valve stenosis) is used to refer to the 2 types of right ventricular outflow obstruction—namely, valvular stenosis and infundibular stenosis. Occasionally, supravalvular stenosis may occur. The primary consequence of PS is pressure overload of the right ventricle, the degree of which is dependent on the severity of the stenosis. Pressure overload of the right ventricle results in increased contractility and dilation leading to increased wall stress and compensatory right ventricular hypertrophy. Increased muscle mass allows for the right ventricle to maintain a normal cardiac output. Right ventricular hypertrophy may cause a decrease in ventricular compliance and result in increased right ventricular end-diastolic pressures and increased right atrial pressures. Right-to-left shunting may occur if there is an interatrial communication. Over time, progressive right ventricular hypertrophy and stiffness can give rise to right ventricular diastolic and systolic dysfunction. ^[1]

Valvular pulmonary stenosis commonly occurs as an isolated lesion in 8–10% of cases of congenital heart disease. Occasionally, supravalvular stenosis may occur.  

The most common cause of valvular PS is congenital. Isolated congenital PS accounts for 7-12% of all congenital cardiac abnormalities but may be associated with other conditions, such as tetralogy of Fallot, congenital rubella syndrome, and Noonan syndrome. ^[2]  Rheumatic inflammation of the pulmonic valve is uncommon. Rheumatic pulmonary valve disease usually involves other valves; it rarely leads to serious pulmonic valvular deformity. One study from New Mexico reported a high incidence of significant pulmonic valve involvement secondary to rheumatic fever. Such involvement may occur in association with the pulmonary hypertension that occurs at high altitudes and the increase in stress that such hypertension places on the pulmonic valve.

Pulmonary valve disease is often accompanied by pulmonary regurgitation (PR) as a result of inherent abnormalities of the valve or from prior intervention. ^[1] In patients with malignant carcinoid, carcinoid plaques, not unlike those affecting the tricuspid valve, may involve the outflow tract of the right ventricle. The plaques result in constriction of the pulmonic valve ring, retraction and fusion of the valve cusps, and either PS or the combination of PS and pulmonic regurgitation. Obstruction in the region of the pulmonic valve may be extrinsic to the valve apparatus and may be produced by cardiac tumors or by aneurysm of the sinus of Valsalva.

Congenital PS is usually treated by balloon dilation; it sometimes requires surgical valvotomy. If there is severe hypoxia (PaO₂< 30 mm Hg), then it may be necessary to create a systemic-to-pulmonary shunt; prostaglandin may be required to maintain patent ductus. ^{[3, 4, 5, 6]} Successful prenatal pulmonary valvuloplasty in the presence of critical pulmonary stenosis has been reported. ^[7]

If not detected at birth, pulmonic stenosis is usually diagnosed by 3 years of age; patients present with dyspnea and fatigue.

Echocardiography

Reliable localization of the site of obstruction and an assessment of its severity are possible with combined continuous-wave or pulsed-wave Doppler and 2-dimensional echocardiography.

The 2-dimensional echocardiogram usually shows prominent pulmonary valve echoes with restricted systolic motion, as well as poststenotic dilation of the main pulmonary artery and its branches.

(See the images below.)

Electrocardiography

Electrocardiography (ECG) may be helpful in assessing the degree of obstruction to right ventricular output.

In mild cases, the ECG often appears normal, whereas moderate and severe stenoses are associated with right-axis deviation and right ventricular hypertrophy. In patients with severe stenosis who are 2-20 years of age, right ventricular pressure may be estimated by multiplying the height of the R wave in lead V4R or V1 by 5. A tall QR wave in the right precordial leads with T-wave inversion, together with ST-segment suppression (right ventricular strain), reflects severe stenosis.

When a regular sinus rhythm is observed in lead V1 (20% of patients), right ventricular pressures are lower than those in cases in which there is a pure R wave of equal amplitude. High-amplitude P waves in leads II and V1 indicate right atrial enlargement; such findings are associated with severe stenosis.

Other imaging techniques

On radiography in some patients with pulmonic stenosis, no recognizable abnormality is noted. The most characteristic finding is enlargement of the main pulmonary artery, which results in convexity of the left, upper cardiac margin below the aortic knob. In patients who have normal peripheral pulmonary vascularity, the characteristic findings of valvular stenosis are right ventricular enlargement and prominence of the main and left pulmonary arteries. The heart may be of normal size, but it is enlarged in about one half of patients. The enlargement is right-sided and results in a rounded right, lower cardiac contour in the frontal projection. The apex may be elevated and blunted.

Computed tomography (CT) scanning of the heart usually requires modification of the standard CT scanning techniques used for investigating other parts of the body. ^{[8, 9]} ​ The degree of confidence is moderate in pulmonic stenosis. The x-ray exposure, the need for contrast medium, and an inability to image in multiple planes limit the usefulness of CT scanning in the evaluation of congenital heart disease. However, the rates of false-positive and false-negative findings are low in pulmonic stenosis.

MRI may be used to determine whether valvular stenosis and regurgitation are present. This is done by using cine MRI to depict the signal void caused by a high-velocity jet flow across a narrow valvular orifice associated with the opened valve in stenosis. In cases of pulmonic stenosis, the high-velocity jet produces a signal void that is projected into the pulmonary artery in systole.

Radionuclide studies (for example, multiple gated acquisition [MUGA] scans) may be used to assess right ventricular function and left ventricular function in patients with pulmonic stenosis and dyspnea. Nuclear medicine studies are nonspecific in pulmonic stenosis, although the rates of false-positive and false-negative findings are low.

Angiocardiography may show enlargement of the right atrium and ventricle. It usually provides a superb demonstration of the actual site and degree of pulmonic stenosis, along with the poststenotic dilatation of the pulmonary artery.

Imaging in infants and neonates

Babies with PS can usually be distinguished from those with tetralogy of Fallot ^{[10, 11]} or tricuspid or pulmonary atresia on the basis of radiographic and electrocardiographic evidence. In typical cases of tetralogy, there is no radiographic evidence of cardiomegaly. For infants with tricuspid and pulmonary atresia, ECGs show a preponderance of left ventricular forces, in contrast to the right ventricular hypertrophy usually observed in patients with critical PS who are without right ventricular hypoplasia. Combined 2-dimensional ECG and continuous-wave Doppler ultrasound examination may be used to characterize the anatomic valve abnormality and its severity. Magnetic resonance imaging (MRI) may be performed in utero. These techniques have essentially eliminated the need for cardiac catheterization and angiographic studies to establish a precise diagnosis. ^{[8, 12]}

In some patients with pulmonic stenosis (PS, pulmonary stenosis, pulmonary valve stenosis), no recognizable abnormality is noted. In patients who have normal peripheral pulmonary vascularity, the characteristic findings of valvular stenosis are right ventricular enlargement and prominence of the main and left pulmonary arteries. The right atrium is sometimes enlarged. Isolated infundibular stenosis is rare. In pulmonic stenosis, the degree of confidence associated with radiography is moderate and the results of a radiographic examination are nonspecific.

The heart may be of normal size, but it is enlarged in about one half of patients. The enlargement is right-sided and results in a rounded right, lower cardiac contour in the frontal projection. The apex may be elevated and blunted.

The most characteristic finding is enlargement of the main pulmonary artery, which results in convexity of the left, upper cardiac margin below the aortic knob. This enlargement of the main pulmonary artery is caused by poststenotic dilatation. The dilatation involves the main pulmonary artery and the left pulmonary artery, producing a prominent arterial silhouette of the left hilum. The change in the left pulmonary artery may be best seen in the lateral projection as it arches over the left bronchus.

The right pulmonary artery may be dilated, but this vessel is hidden by mediastinal opacity. Therefore, in patients with PS, the hilar vessels appear to be of different sizes; by contrast, in patients with pulmonary hypertension, the hilar vessels appear to be of the same size. This is a helpful sign in the differential diagnosis. Poststenotic dilatation occurs in valvular stenosis; by contrast, in infundibular and supravalvular stenosis, the pulmonary artery is not prominent, and radiographic findings indicative of cardiovascular disease may be absent.

The large, main arterial change is associated with the normal size of the vessels in the lungs and in the right hilum. ^{[13, 14]}

Several factors make MRI especially useful for establishing a cardiovascular diagnosis. ^{[8, 9]} First, a high natural contrast exists between the blood pool and the cardiovascular structures because of the lack of signal from flowing blood with the spin-echo MRI technique and because of the brightness of blood with the gradient-echo (cine MRI) technique. When the spin-echo technique is used, blood appears black on images; therefore, internal structures of the heart may be visualized within the signal void of the cardiac chambers. In addition, MRI may be applied to complex pulmonary anatomy such that omniplanar imaging is utilized for evaluation of the vessels directly, as well as their relationships to adjacent vascular and non-vascular mediastinal structures.

The degree of confidence associated with MRI is high in pulmonic stenosis, and the rates of false-positive and false-negative findings are low.

Cardiac imaging usually requires some form of physiologic gating of the imaging sequence. The goal of cardiac gating is to decrease the degree of patient motion articfact that can lead to image unsharpness and reduced diagnostic value of the imaging study. The gating with MRI requires the use of a nonferromagnetic physiologic signal-sensing circuit. An electronically isolated ECG electrode lead circuit that contains little metal has been used for repetitive synchronization (ie, ECG gating) of pulse sequences to fixed segments of the cardiac cycle. ECG gating is used prospectively for some sequences (spin-echo) or retrospectively for others (cine gradient-echo).

MRI may be used to determine whether valvular stenosis and regurgitation are present. This is done by using cine MRI to depict the signal void caused by a high-velocity jet flow across a narrow valvular orifice associated with the opened valve in stenosis. In cases of pulmonic stenosis (PS, pulmonary stenosis, pulmonary valve stenosis), the high-velocity jet produces a signal void that is projected into the pulmonary artery in systole.

The volumes of shunts, valvular function, and pressure gradients across valves and conduits may be estimated by use of velocity-encoded cine MRI (velocity-flow mapping). However, these capabilities are not employed as widely as they might be, owing to the fact that echocardiography and Doppler techniques are used for many of these purposes. Consequently, the current clinical role of MRI is to supplement the information acquired by means of echocardiography.

Because MRI defines the right ventricular myocardium, it may be the preferred technique for the accurate determination of right ventricular function. MRI has shown reasonable accuracy in measuring right ventricular end-diastolic, end-systolic, and stroke volumes, as well as the ejection fraction. ^{[15, 16]}

 

Reliable localization of the site of obstruction and assessment of its severity are possible with combined continuous-wave or pulsed-wave Doppler and 2-dimensional echocardiography. The 2-dimensional echocardiogram usually shows prominent pulmonary valve echoes with restricted systolic motion, as well as poststenotic dilation of the main pulmonary artery and its branches.

In contrast to these findings in classic valvular PS, patients with a dysplastic valve have thickened and immobile leaflets with hypoplasia of the pulmonary valve annulus and absent poststenotic dilatation of the pulmonary artery.

Parasternal and subcostal views are required to most accurately detect maximal blood flow velocity in the pulmonary artery. This velocity is converted to a pressure difference across the valve by using a modified Bernoulli equation: pressure difference (in mm Hg) = 4 × the squared peak Doppler velocity (in meters per second). A semiquantitative estimation of pulmonary and tricuspid regurgitation can be obtained.

The peak systolic velocity of the tricuspid regurgitant jet provides a reliable indirect measurement of the severity of obstruction because the reverse gradient between the right ventricle and right atrium allows the derivation of the ventricular peak systolic pressure. One may assume a value for right atrial pressure, or look at inferior vena cava collapse during sniff, using 5 mm Hg if it clearly narrows, 10 mm Hg if partial narrowing, or 15 mm Hg if no narrowing; or the right atrial pressure can be estimated from vertical distance from midright atrium to pulsation level in jugular veins, converted from centimeters of water to millimeters of mercury. ^{[17, 18]}

The degree of confidence is high in pulmonic stenosis, and the rate of false findings is low.

Angiocardiography may show enlargement of the right atrium and ventricle. It usually provides a superb demonstration of the actual site and degree of pulmonic stenosis (PS, pulmonary stenosis, pulmonary valve stenosis), along with the poststenotic dilatation of the pulmonary artery. Cardiac catheterization and angiocardiography are now used only rarely to establish or preclude other diagnostic possibilities. The usual indication for cardiac catheterization is to provide definitive therapy for the lesion. However, cardiac catheterization may also be used to localize the site of obstruction, to evaluate its severity, and to document the coexistence of additional cardiac malformations.

The resting cardiac output is usually normal, even in cases of severe stenosis; in most children, cardiac output increases with exercise.

Right ventricular dysfunction occurs especially when venoarterial shunting is significant and produces systemic arterial desaturation.

During hemodynamic evaluation of patients with critical stenosis, care must be taken to avoid dangerously occluding the stenotic valve opening with the cardiac catheter.

In PS, the valve gradient is usually obtained by means of a catheter pull-back maneuver from the pulmonary artery to the right ventricle, although multilumen catheters are available for simultaneous pressure recordings.

In typical cases of valvular PS, the angiographic appearance differs from that of a dysplastic valve. In PS, the valve is thickened and domed during systole; the configuration returns to normal in diastole. Poststenotic dilatation of the main pulmonary trunk and sometimes of the left pulmonary artery are usual. Although the leaflets of the dysplastic valve are not fused anatomically, they are thickened and immobile; thus, on angiography, the valve appears to change little during the cardiac cycle. In addition, a small annulus and narrow sinuses of Valsalva are common accompaniments of valve dysplasia.

With either type of valve, systolic narrowing of the right ventricular infundibulum is usually associated with moderate or severe obstruction. ^[19]

The degree of confidence is high in pulmonic stenosis, and the rates of false-positive and false-negative findings are low.