15 Pulmonary Valve



Farhood Saremi

15 Pulmonary Valve



Introduction


Knowledge of pulmonary valve and root anatomy is useful in understanding the spectrum of complicated conotruncal anomalies that arise from abnormal formation of the major vessels in this region. Despite the frequency of pulmonary valve diseases including congenital malformations, the pulmonary valve is the least studied valve by imaging. Along with the evolution of surgical techniques and introduction of new percutaneous procedures in recent years, imaging assessment of the pulmonary root and related pathologies has attracted more attention than before.


Pulmonary valve assessment is primarily dependent on echocardiography. However, because of their retrosternal location, the pulmonary valve and right ventricular outflow tract (RVOT) can be difficult to assess with transthoracic echocardiography especially in adolescents and adults. Furthermore, since the RVOT and pulmonary valve are anterior structures, transesophageal echocardiography is not the best tool for assessment of the pathology. With rapid advancement in imaging technology, cardiac computed tomography (CT) and magnetic resonance imaging (MRI) are being used increasingly for anatomical evaluation, functional assessment, and pathological diagnosis of the pulmonary valve. Postoperative evaluation of the pulmonary valve and outflow tract is among common MR referrals. MR is especially helpful in postoperative follow-up of artificial pulmonary valve function. Anatomical detail of the pulmonary valve and perivalvular structures can be optimally studied with CT scan.


The goal of this review is to offer a general perspective on the development of right outflow tract (OFT) and associated structures with a focus on the morphology and function of the pulmonary valve. Pathologies including congenital heart disease (CHD) are briefly discussed.



Embryology


The two fields of cardiac progenitors are now recognized as the primary, and secondary, or anterior, heart fields. 1 ,​ 2 In mouse, there is firm evidence that the primary heart field gives rise to the left ventricle, with the secondary field forming both the right ventricle (RV) and the OFT. 1 Development of the semilunar valves occurs simultaneously with completion of the secondary (anterior) heart field.


The primordial outflow tract extends proximally from the distal ventricular groove to the pericardial reflections and demonstrates a characteristic dog-leg which divides it into two myocardial subsegments, a proximal subsegment or the conus (infundibulum) and a distal subsegment or the truncus. 1 ,​ 3 The truncus arteriosus is a short segment interposed between the conus and the aortic sac (Fig. 15‑1). With further development, the aortic sac transforms into the extrapericardial ascending aorta and pulmonary trunks, and the truncus is remodeled into the intrapericardial portions of the aorta and pulmonary trunk. The boundary between the two parts of the primordial OFT becomes the sinotubular junctions. The primordial OFT is mainly myocardial. Gradual disappearance of the myocardium by apoptosis, transdifferentiation initially involves the truncus wall and the tissue surrounding the developing arterial sinuses. Later, further absorption of the left conus myocardium produces fibrous continuity between the leaflets of the aortic and mitral valves. On the right, conus myocardium remains as subpulmonary infundibulum. With extension of the developing fibroadipose tissue plane that already separates the aortic and pulmonary trunks, the muscularized partition becomes converted into the free-standing infundibulum of the pulmonary valve, and the supraventricular crest of the RV. As development proceeds, the single OFT undergoes remodeling into separate pulmonary and aortic arteries (Fig. 15‑1). This process involves interactions between diverse cell types, including myocardium, endocardium, and neural crest cells. Endocardial cells respond to signals from the overlying myocardium and undergo an epithelial-to-mesenchymal transformation to form the conotruncal cushions. Neural crest cells invade the extracellular matrix of the cushions and participate in aorticopulmonary septation. 4 OFT undergoes rotation during its remodeling. Rotation of the myocardium at the base of the OFT is probably essential to achieve normal positioning of the great arteries with respect to each other at the ventriculoarterial junction (VAJ). 5 ,​ 6

Fig. 15.1 The developing outflow tract in embryonic chicken hearts at stages 12, 24, 30, and 36 H/H. The area of the outflow tract (OFT) extends between the distal ventricular groove of the right ventricle and the junction with the aortic sac at the pericardial reflections and is divided into the conus (proximal OFT shown in red) and the truncus (distal OFT in light blue). In experiments in which myocardialization of the proximal OFT was compared with that of the distal OFT, the OFT was separated into two parts, the conus and the truncus and the junction between the two will be the distal myocardial border (DMB). These images show that the OFT is initially mainly myocardial (red part) in its entirety and increases in length up to HH24. The OFT myocardium, subsequently, shortens as a result of ventricularization, contributing to the trabeculated free wall, as well as the infundibulum, of the right ventricle (RV). Note the absolute reduction in the length of the OFT between 30 and 36 H/H stages, as well as the relative reduction in relation to the ventricles, which have increased in size by cardiomyocyte proliferation. The OFT has also been divided by septation into pulmonic and systemic outflows, and the aortic root has rotated to a posterior position, where it connects with the left ventricle (LV). The dotted line around the heart indicates the pericardium. DVG, distal ventricular groove; V, primitive ventricle; SV, sinus venosus; A, primitive atrium; RA, right atrium; LA, left atrium; VG, ventricular groove.

The valves and their supporting sinuses are believed to develop from the conotruncal endocardial cushions around the distal part of the conus. 3 Valves are formed by the formation of cavities within the cushions. The central parts of the cushions form the leaflets and the peripheral parts arterialize to form the sinus walls. The improper fusion or dedifferentiation of the endocardial cushions is thought to be responsible for congenitally abnormal semilunar valves. The aorticopulmonary septation by the endocardial cushions is a complex process and involves interaction between diverse cell types, including myocardium, endocardium, and neural crest cells. 3 ,​ 4 In addition to abnormal OFT septation caused by neural crest cell defects, a spectrum of conotruncal anomalies with abnormally positioned great arteries may arise from a perturbation of myocardial rotation including tetralogy of Fallot (TOF), persistent truncus arteriosus, double outlet right ventricle (DORV), and transposition of great arteries (TGA). 5 ,​ 6



Anatomy


The pulmonary root is the part of the RVOT that supports the leaflets of the pulmonary valve. 7 ,​ 8 It consists of three sinuses of Valsalva confined proximally by the semilunar attachments of the valvular leaflets and distally by the sinotubular junction. This relationship can change in CHD. Different nomenclature has been used to define the anatomical location of the pulmonary valve sinuses based on their spatial location in relation to the thorax or the heart itself 7 (Fig. 15‑2 , Fig. 15‑3). The pulmonary valve is in the left anterior of the aorta and forms an angle of approximately 30 degrees with the aortic trunk. In normal individuals, this angle is related to the length of the ascending aorta. In elongated tortuous ascending aorta, the angle between the two arteries will be increased (Fig. 15‑4). Because of the semilunar shape of the pulmonary leaflets (similar to the aortic valve) this valve does not have a ring-like annulus. The sinotubular junction separates the pulmonary valvular sinuses from the tubular component of the pulmonary trunk and demarcates the level of the zones of apposition (commissures) between the annuli (Fig. 15‑5 , Fig. 15‑6). Compared to the aortic root, the pulmonary sinotubular junction is less obvious on CT images. A second junction exists at the VAJ between the infundibular muscle and the fibroelastic arterial wall. The anatomical VAJ forms the annulus. The semilunar attachment of the valvular leaflets, which forms the hemodynamic VAJ, crosses the anatomic VAJ. The leaflets are thickened along their semilunar line of attachment. The fibrous interleaflet triangles are the areas of arterial wall proximal to the semilunar attachments of the leaflets, and therefore are incorporated within the ventricular cavity. The fibrous triangle tips point toward the commissures (Fig. 15‑6). The pulmonary valve is surrounded by the ventricular muscle. 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 (Fig. 15‑7 , Fig. 15‑8). 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. 8 The length of the free-standing infundibulum varies and some cases may be too short to resect (Fig. 15‑7 d).

Fig. 15.2 Upper row: pulmonary valve sinuses. When heart is viewed in attitudinal anatomical position as sitting in the thorax (i.e., axial views), the pulmonary leaflets and sinuses are viewed as posterior (P), right anterolateral (Ra), and left anterolateral (La). However, in relation to the heart (i.e., short-axis views), the pulmonary sinuses can be named anterior (A), left posterior (Lp), and right posterior (Rp). Lower row: same rule can be applied to the aortic valve as seen in the above examples. N, noncoronary sinus. The relationship of the pulmonary and aortic valve (blue and red circles) as well as their orientation in relation to the body (axial) and the heart (SAX) are drawn; the black line shows the location of the interatrial septum. LA, left atrium, SAX, short axis; R, right coronary sinus; L, left coronary sinus.
Fig. 15.3 Upper row showing the position of the pulmonary valve sinuses and relative nomenclature based on their relative location in the body or in relation to heart. Relative position in the body (thorax) are posterior (P), right anterolateral (Ra), and left anterolateral (La). Relative positions in the heart are anterior (A), left posterior (Lp), and right posterior (Rp). Lower row images showing the relative spatial position of the pulmonary valve to the aortic valve. The pulmonary valve is in the left anterior of the aorta. This relationship can change in congenital heart disease. LA, left atrium; R, right; L, left; N, noncoronary aortic sinuses.
Fig. 15.4 Relative positions of the pulmonary trunk and the aortic root. The pulmonary trunk is located anterior to the aorta and forms an angle of approximately 30 degrees with the aortic trunk. In normal individuals, this angle is related to the length of the ascending aorta. In elongated tortuous ascending aorta, the inclination angle of the aortic root will be increased (inlay coronal CT) and the angle between the aorta and pulmonary artery will be increased. LVOT, left ventricular outflow tract; RVOT, right ventricular outflow tract.
Fig. 15.5 Opened pulmonary root before (a) and after removal of valvular leaflets and endocardial dissection (b). The red line shows the sinotubular junction (STJ). The blue line demarcates the anatomical ventriculoarterial junction (A-VAJ) which is the border between the muscular infundibulum and the arterial wall of pulmonary trunk. The semilunar attachments of the three arterial valvular leaflets are marked by the green lines, corresponding to the hemodynamic VAJ (H-VAJ). There is no “annulus” supporting the attachments of the leaflets. (c) Histological section of the pulmonary valve showing the different colored lines in (a) and (b). Stars, intervalvular trigones; CSV, crista supraventricularis; SMT, septomarginal trabeculations; TV, tricuspid valve.
Fig. 15.6 Pulmonary valve (PV) anatomy shown by CT. (a) Anterior view of volume rendered, (b) inlet–outlet (in–out) of the right ventricle (RV), and (c) coronal views of the heart magnified at the right ventricular outflow tract. The anatomical ventriculoarterial junction (A-VAJ) which is demarcated by the blue line in (a) and arrows in (b) and (c) are the anatomical junction between the muscular infundibulum (m) and the elastic arterial wall. The green line in (a) and arrows in (b) and (c) mark the hemodynamic VAJ (H-VAJ), corresponding to the semilunar fibrous attachments of the leaflets to the wall or the “pulmonary annuli” which is much less sturdy than the aortic annuli. It is usually difficult to separate A-VAJ from H-VAJ on three-dimensional volume-rendering images. The yellow line connecting the commissures marks the sinotubular junction (STJ). Interannular fibrous trigone is shown by black star. The commissures between the annuli form peripheral apposition of the leaflets. The pulmonary sinuses including the right anterior (R) and left anterior (L) annuli are seen. The pulmonary valve sits above the ventricular septum and is the most superiorly situated of the cardiac valves. This anatomical feature makes safe resection of the pulmonary valve possible. M, muscle; P, posterior pulmonary valve sinus; RV, right ventricle.
Fig. 15.7 The length of free-standing infundibulum. (a) Sagittal CT images through the right ventricular outflow tract (RVOT). (b) Three-dimensional view of the RVOT. Sections are obtained at three levels (1 through 3) from left to right. Images showing gradual increase in length of the free-standing muscle of the infundibulum with shortest on the left side. (c, d) Sagittal histological sections of the right ventricular infundibulum, pulmonary valve root, left ventricular outflow tract, and aortic root. Note the differences in length of the free-standing right ventricular outflow infundibulum and in the contact area between the right and left outflow tracts (black dotted lines) depending on the level of the section: at the right posterior pulmonary cusp (c) or left posterior pulmonary cusp (d). The subendocardial fibers in the infundibulum run longitudinally. At subendocardial levels of the left ventricular outflow tract, the orientation is mainly spiral and circumferential. Note that there are connections (star) between myocytes in the contact area between both outflow tracts. Blue arrows show the length of free muscle of the infundibulum. R, right; L, left; N, noncoronary aortic sinuses; Lp, left posterior; Rp, right posterior pulmonary sinuses; LA, left atrium; LBB, left bundle branch; LCA, left coronary artery; MV, mitral valve.
Fig. 15.8 The length of free-standing infundibulum. Upper row: Short-axis (SAX) CT views of the aortic valve from inferior to superior. Lower row: Sagittal CT images through the right ventricular outflow tract (RVOT) at three levels from left to right. Images showing gradual increase in length of the free-standing muscle of the infundibulum with shortest on the left side. Red arrows show the length of free muscle of the infundibulum. R, right; L, left; N, noncoronary aortic sinuses; Lp, left posterior; A, anterior pulmonary sinuses; LAD, left anterior descending artery; LVOT, left ventricular outflow tract; MV-AL, mitral valve anterior (aortic) leaflet.


Arterial Supply


The conotruncal structures including the pulmonary valve are normally vascularized by anterior and posterior arterial branches from the right and left coronary arteries 9 (Fig. 15‑9). On the right side, the branches arise from the conal branch of the right coronary artery (RCA) or directly from the aorta. On the left side, they arise from the left anterior descending artery (LAD), the left main, or directly from the aorta. The right anterior conal branch is the most constant and conspicuous branch participating in the preconal circulation, also known as Vieussens’ arterial ring. 9 This collateral intercoronary connection extends between the conus artery and first right ventricular branch (left anterior conus branch) of the LAD artery. The Vieussens’ arterial ring will become dilated when there is proximal LAD artery occlusion or, less frequently, RCA occlusion 10 (Fig. 15‑10). Generally, three major collateral pathways at the conotruncal level provide circulation between the right and left coronary system in all congenital or acquired forms of one-sided coronary occlusion and are used as the basis for different classifications 10 (Fig. 15‑9). These three collateral circulation pathways include preconal (precardiac), retroconal (interarterial), and retroaortic.

Fig. 15.9 Aortopulmonary trunk arterial anastomotic circulation is provided by three arterial anastomotic rings between the proximal right and left coronary arterial systems. The right pulmonary conal branches arise from the RCA or the aorta and the left conal branches from the left main or proximal LAD arteries. The right anterior conal branch exists in almost all individuals and in 50% it may arise from the right aortic sinus. The left anterior conal artery exists in 85%, the right posterior in 15%, and the left posterior in 15%. The retroaortic anastomotic ring is mainly related to the Kugel atrial anastomotic network which connects the proximal right and left coronary systems (as shown in this images) or on one side may communicate with the distal coronary system through the interatrial septum (not shown). AA, ascending aorta; LA, left atrium; DA, descending aorta; LAD, left anterior descending artery; LCx, left circumflex artery; RCA, right coronary artery; SVC, superior vena cava.
Fig. 15.10 Vieussens’ arterial ring. Anterior arterial anastomotic rings (asterisks [*]) between pulmonary conus branches arising from the RCA and the LAD artery are shown (a) participating in vascular supply to the infundibulum as well as the proximal right ventricle. These collaterals may be enlarged in acquired (b) or congenital (c) obstructive coronary disease. AA, Ascending aorta; MPA, main pulmonary artery; RCA, right coronary artery; LAD, left anterior descending artery.

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Mar 16, 2021 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on 15 Pulmonary Valve
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