The pulmonary infundibulum (conus) is a tubular muscular structure that supports the leaflets of the pulmonary valve. Its length, size, and angle vary. The size of the infundibulum is independent of the general size of the RV.

The posterior (paraseptal) wall of the infundibulum is formed by a prominent muscular ridge, known as the supraventricular crest (crista supraventricularis or ventriculoinfundibular fold) which separates the inlet and outlet components of the RV (Figs. 7.3, 7.4, and 7.5). This is in contrast to the LV where the aortic and mitral valves are in fibrous continuity. Although it looks like a ridge from the perspective of the RV cavity, the supraventricular crest is in fact an infolding of the ventricular wall (the ventriculoinfundibular fold) inserting into the ventricular septum. It is separated from the aorta by the epicardial fat, and any incision through it will lead outside the heart into the vicinity of the right coronary artery (Figs. 7.3 and 7.4). Only the central portion of its inferior most part between the limbs of the septomarginal trabeculation contributes to the interventricular septum (muscular outlet septum or conal septum) [17] (Figs. 7.4 and 7.5). In the normal heart, however, this area is exceedingly small and can hardly be distinguished from the septomarginal trabeculation by CT.

The septomarginal trabeculation is a prominent Y-shaped muscular strap reinforcing the septal surface. It bifurcates into anterosuperior and inferoposterior limbs (Fig. 7.6) which clasp the supraventricular crest. The anterosuperior limb extends along the infundibulum to the leaflets of the pulmonary valve. The posterior limb runs onto and overlays the ventricular septum, toward the right ventricular inlet, giving rise to the medial papillary muscle complex. The body of septomarginal trabeculation extends to the apex of the ventricle, where it gives rise to the moderator band and anterior papillary muscle before breaking up into the general apical trabeculations. The body of the septomarginal trabeculation is interventricular rather than supraventricular and when enlarged can appear as a bump on the septum on cross-sectional imaging. When hypertrophied, the septomarginal band can divide the RV into two chambers (double-chambered RV) [19].

It is considered as part of the septomarginal trabeculation, supporting the anterior papillary muscle of the tricuspid valve and, from this point, crossing to the free wall of the ventricle. The moderator band incorporates the right atrioventricular bundle, as conduction tissue fibers move toward the apex of the ventricle before entering the anterior papillary muscle. It is usually located equidistant from the tricuspid valve and the apex and can be identified in 90 % of hearts. In 40 % the band is a short and thick trabeculation. The average thickness of the band is 4.5 mm, and its length is 16 mm, ranging from 11 to 24 mm [20]. The moderator band is supplied by branches of the left anterior descending (LAD) artery named the artery of the moderator band. The artery supplying the band makes anastomotic connections at the base of the anterior papillary muscle with branches of the right coronary artery.

The medial (septal) papillary muscle of the conus presents in 82 % of the hearts, while in the rest, it is replaced by tendinous chords [21] (Figs. 7.4 and 7.6). It is a single papilla in 50 % and double in 30 %. It connects with the septal and anterior leaflets of the tricuspid valve. It represents an important surgical landmark for the location of the right bundle branch to avoid injury to the bundle during surgical correction of certain types of ventricular septal defects.

The pulmonary root is the part of RVOT that supports the leaflets of the pulmonary valve. It consists of three sinuses of Valsalva confined proximally by the semilunar attachments of the valvular leaflets and distally by the sinotubular junction. Different nomenclatures have been used to define the anatomical location of the pulmonary valve sinuses base on their spatial location in relation to the body of the heart itself (Fig. 7.7). Because of the semilunar shape of the pulmonary leaflets (similar to the aortic valve), this valve does not have a ringlike annulus. The sinotubular junction of the pulmonary trunk marks the level of the commissures between the annuli (Fig. 7.7). Compared to the aortic root, the pulmonary sinotubular junction is less obvious on CT images. A second junction exists at the ventriculoarterial junction. The bases of the sinuses within the ventricle cross the anatomical ventriculoarterial junction. The anatomical ventriculoarterial junction forms the annulus. The semilunar attachment of the valvular leaflets which forms the hemodynamic ventriculoarterial junction crosses the anatomical ventriculoarterial junction. The leaflets are thickened along their semilunar line of attachment. The fibrous interleaflet trigones are the areas of arterial wall proximal to the semilunar attachments of the leaflets and therefore are incorporated within the ventricular cavity. Their tips point toward the commissures. The musculature of the subpulmonary infundibulum raises the pulmonary valve above the ventricular septum to position the pulmonary valve as the most superiorly situated of the cardiac valves. This anatomical feature makes possible the safe resection of the pulmonary valve, including its basal attachments within the infundibulum from the rest of the RVOT.

Isolated pulmonary stenosis (PS) is almost always congenital and many can be asymptomatic when first diagnosed. It is not unusual to suspect PS in a young patient on routine chest x-ray or CT by noticing enlarged main and left pulmonary arteries. With severe PS, symptoms of dyspnea, fatigue, chest pain, palpitations, and decreased exercise tolerance may occur. Three morphological types are described [18, 25–28] (Table 7.1). The most common type of congenital PS (40–60 %) is a dome-shaped pulmonary valve, which is characterized by a mobile valve and 2–4 raphes and incomplete separation of valve cusps due to commissural fusion resulting in funnel with a small circular orifice (Fig. 7.12a). The line of basal attachment of the domed valve is not semilunar; instead, the sinuses are shallow and the line attachment appears somewhat circular. A waist-like narrowing of the sinotubular junction may be seen is some cases. Dysplastic pulmonary valve is the second most common PS (20–30 %) and is associated with immobile thickened cusps and in some cases a hypoplastic ventriculoarterial junction [26] (Fig. 7.12b). Cauliflower-like myxomatous thickening is limited to the free margin of the leaflets, and the proximal part of leaflets is intact. The commissures are not fused, the sinuses are deep, and the lines of attachment are semilunar, all as seen in normal hearts. The semilunar attachment of the pulmonary valve leaflets is an essential feature for normal function of the valve. Shallow attachment and lack of “height” of the overall valvular apparatus can cause pulmonary stenosis due to limiting mobility of the free edge of the leaflets even in the absence of commissural fusion. Of those cases with PS who require active treatment whether interventional or surgical, dysplastic valves would be far more common. Milo et al. [25] described a third morphology of PS with deep “bottle-shaped” sinuses and an hourglass deformity due to supravalvular narrowing at the sinotubular junction (Fig. 7.12c). Although the later morphology is reported in 16 % of patients with congenital PS, it is not accepted as a separate variant by every investigator [26]. Dome-shaped valve with dysplastic leaflets is another uncommon variant. Different morphologies can be equally distinguished with cardiac MR and CT angiography. Bicuspid or multicuspid valve is rare [27] (Fig. 7.13). In bicuspid valve, one leaflet can be larger containing a shallow raphe or both leaflets may be equal in size. Stenosis and post-stenostic dilatation are common. Compared to bicuspid valve, quadricuspid pulmonary valve is usually asymptomatic. Mild pulmonary regurgitation (PR) is not uncommon. Congenital variations can be isolated but are often associated with other congenital heart anomalies. For example, tetralogy of Fallot can be associated with a bicuspid pulmonary valve. Congenital pulmonary valve anomalies can also be associated with extracardiac anomalies as in Noonan syndrome and LEOPARD syndrome, which is often associated with a dysplastic pulmonary valve.

Chronic PS results in RV hypertrophy, especially at the RVOT. When prominent, RVOT hypertrophy can lead to secondary dynamic subvalvular stenosis. Distinguishing between valvular stenosis and subvalvular dynamic stenosis secondary to infundibular hypertrophy can become challenging. Subvalvular dynamic obstruction (late systolic stenosis), in fact, often accompanies severe valvular PS and is characterized by a late-peaking jet in MRI similar to that of dynamic LV outflow obstruction. PS can also result in post-stenotic dilatation of the pulmonary trunk and left pulmonary artery. For symptomatic patients with dome-shaped pulmonary valve, balloon valvuloplasty is indicated when a peak instantaneous gradient >50 mmHg is present [28]. A successful procedure is defined by final peak gradient of <30 mmHg and is obtained in >90 % [29]. If the valve is dysplastic, surgery is more likely to be required; if there is annular or pulmonary trunk hypoplasia, a transannular patch may become necessary. In patients with PS and significant pulmonary regurgitation, valve replacement is required. Mechanical valve replacement is used rarely because of thrombosis issues. Bioprosthetic valves and pulmonary homografts are preferred [30].

TOF consists of a large nonrestrictive subaortic ventricular septal defect (VSD), dextroposed aorta riding up over the septal defect, and RVOT obstruction (Fig. 7.14). TOF without PS is called Eisenmenger complex (Fig. 7.15). Subpulmonary stenosis, which is an essential part of TOF, is mainly due to anterosuperior malalignment of the muscular outlet septum relative to the limbs of the septomarginal trabeculation, coupled with thickened septoparietal trabeculations [17, 31, 32] (Fig. 7.14). Stenosis can also occur at subpulmonic level by hypertrophy of the septomarginal trabeculation or the moderator band. This gives the arrangement often described as “two-chambered right ventricle.” The subpulmonary infundibulum itself varies markedly in length and can sometimes be short especially in Eisenmenger complex (Fig. 7.2). In most other instances of TOF, the narrowed infundibular chamber is normal in length but sometimes has considerable length. Absent pulmonary valve syndrome occurs in less than 3–6 % of TOF patients [17]. This syndrome is associated with significant pulmonary artery dilatation and airway compression. Pulmonary atresia in TOF is also due to severe deviation of the outlet septum. However, isolated pulmonary atresia can rarely occur as a result of valve imperforation rather than severe stenosis. In pulmonary atresia blood supply to the right and left pulmonary arteries (if not atretic) will be provided by a large patent ductus arteriosus or multiple aortopulmonary collateral arteries. Extensive reconstructive surgery is required in extreme cases.

Patients with TOF have remarkable intrinsic histological abnormalities and reduced elasticity in both ascending aorta and pulmonary artery, and it appears that TOF repair does not improve these abnormalities [33, 34]. Aortic root dilation with or without aortic regurgitation is common [33]. Cardiac MRI or CT can address these major clinical implications (Fig. 7.14). The concept of aortic overriding is shown in Fig. 7.16. Note that mild overriding above the ventricular septum can be seen in normal instances. Greater than 50 % overriding falls into definition of DORV subgroup. However, in TOF there is always fibrous continuity between the anterior mitral leaflet, while in DORV this may not be the case (Fig. 7.16). The concept of dextroposition is shown in Fig. 7.17.

Double-chambered RV (DCRV) is characterized by subinfundibular stenosis due to aberrant hypertrophied septomarginal trabeculations or abnormal moderator band that divides the RV cavity into a proximal high-pressure and a distal low-pressure chamber [19, 35, 36] (Fig. 7.18). The severity of the DCRV stenosis tends to increase with time [36]. DCRV is usually associated with a perimembranous VSD. MRI and CT are usually diagnostic, identifying the degree and location of the obstruction and the presence of a VSD. The degree of stenosis can be best quantified with MR phase-contrast techniques. The indications for surgery in DCRV are similar to those for pulmonary valve stenosis (peak gradients >50 mmHg). Muscular resection and correction of VSD have excellent long-term results and low rates of recurrence [37].