Anatomy, Thorax, Heart Pulmonic Valve

Article Author:
Joshua Sundjaja
Article Editor:
Bruno Bordoni
Updated:
9/24/2019 12:40:40 PM
PubMed Link:
Anatomy, Thorax, Heart Pulmonic Valve

Introduction

The heart is a four-chambered organ hemodynamically functioning as a reservoir and a pump; it receives deoxygenated blood from the systemic circulation through the superior and inferior vena cava in the right atrium and oxygenated blood from the lung via the four pulmonary veins. The heart pumps deoxygenated blood from the right ventricle to the lungs and at the same time pumps oxygenated blood from the left ventricle into the aorta. These processes are orchestrated by the electric conduction system which coordinates the rhythmic contractions of the atria and ventricles, and the opening and closure of the heart valves. The heart valves are especially important to effectively maintain the systolic and diastolic phase of the cardiac cycle. There are two types of heart valves; the atrioventricular valves (mitral, tricuspid) and the semilunar valves (aortic and pulmonic). The pulmonic valve physically separates the right ventricle from the pulmonary trunk. While there is more literature on the other heart valves, little is known about the intricate function of the pulmonary valve and its role in various disorders.

Structure and Function

The pulmonary valve is the semilunar valve that separates the right ventricle from the pulmonary trunk. Anatomically, the annulus (ring-like connective tissue) of this valve delimits the right ventricle chamber at the junction of the pulmonary arterial trunk.[1] The annulus and the cardiac fibrous skeleton, a structure which connects the pulmonary valve to other heart valves, plays an essential role in anchoring all the heart valves in the myocardium. The pulmonary valve consists of three cusps; anterior, left, and right cusp. Each of these cusps is separated from one another by a commissure. Below the free margin, these cusps overlap one another for about several millimeters to ensure adequate closure of the lumen. The overlap area is called lunula. At the central portion of the lunula, there is a local fibrous thickening that forms a nodule (nodule of Arantius) that maximize closure of the lumen. At the superior border of the pulmonary valve, there is a pocket formed by the valve cusp and adjacent arterial wall called the sinus of Valsalva.[2] 

The pulmonary valve is separated from the tricuspid valve by a muscular fold, known as the ventriculoinfundibular fold. At its septal margin, the fold forms the supraventricular crest and this fold inserts between the anterior and posterior limbs of the septomarginal trabeculation. This same fold also forms the subpulmonary infundibulum of the right ventricular outlet.[1]

Histologically, the pulmonary valve consists of stratified extracellular matrix compartments of four layers which are the arterialis, fibrosa, spongiosa, and the ventricularis layer. The arterialis faces the artery, and the ventricularis layer faces the ventricle.[2][3] Each of these layers has a different composition and function. The arterialis layer is the thinnest layer whose function remains. The fibrosa layer is the backbone of the semilunar valves and consists of circumferentially arranged dense collagen networks that merge with the annulus and the cardiac fibrous skeleton. The spongiosa layer is the layer that allows shear stress between layers during flexure and provides compressive strength because it consists of proteoglycans and glycosaminoglycans. The ventricularis layer contained the most elastic fibers and assists with the elastic recoil of the cusps.[2][4]

The pulmonary valve opens at the systolic phase of the cardiac cycle enabling the deoxygenated blood to be pumped from the right ventricle to the pulmonary circulation. It closes at the diastolic phase of the cardiac cycle, allowing sufficient filling of the right ventricle. The pulmonary valve has a diameter of about 20 mm.

Embryology

The semilunar valves develop as the endocardial cushion forms in the outflow tract (OFT) of the primordial looped heart tube; this is the first sign of valvulogenesis. Later this endocardial cushion fold becomes a primordial heart valve with highly proliferative valve progenitor cells. During remodeling and maturation, the valves are rudimentary. The primordial valve will grow and elongate into a thin fibrous cusp of the semilunar valve. These leaflets will become stratified into compartments consisting of highly organized collagen, proteoglycan, and elastin rich extracellular matrix during late gestation and soon after birth. This valves continues to mature and undergoes remodeling well into the juvenile stages.[3]

The embryonic leaflet from which the valve derives is the splanchnic mesoderm.

Blood Supply and Lymphatics

The pulmonary valve has no distinct blood supply, nor does it possess any lymphatic drainage.[5][6][7]

Nerves

The pulmonary valve receives its innervation from the nerve branch of cardiac ventricular plexus. The entire leaflets contain these nerve terminals except for the coapting edge (the ventricular layer or the lower region of each leaflet). Compared to the aortic valve leaflets, the pulmonary valve leaflets contained more numerous nerve terminals. The activity of acetylcholinesterase or tyrosine hydroxylase and neuropeptide Y are observable in these nerve terminals. With age, the density and distribution of innervation in the aortic valve leaflets decrease, but this does not occur in the pulmonary valve leaflets.[8]

Muscles

The pulmonary valve attaches to infundibulum basally, and the muscle of the subpulmonary infundibulum raises this valve above the ventricular septum; so it is positioned superiorly and becomes the most superior of the cardiac valves. This anatomical configuration enables the surgeon to resect the pulmonary valve and its basal attachment within the infundibulum from the rest of the right ventricular outflow tract (RVOT) safely.[9]

Physiologic Variants

There are several physiologic variants of the pulmonary valve. The valve may be unicuspid, bicuspid, or even atretic. Any abnormality of these components can cause congenital semilunar valve disease (CSVD).[2] 

Surgical Considerations

Surgical valvotomy:

There are two types of pulmonary valve valvotomies: closed and open valvotomy. The surgeon performs a closed valvotomy by making an incision at the right ventricular outflow tract, and introducing a cutting valvulotome is introduced. This process allows one to cut the valvar diaphragm transversely, and the stenosed pulmonary valve gets converted into a bicuspid valve. The open valvotomy is done through the pulmonary artery and aided by hypothermia or by cardiopulmonary bypass. The main indication for this surgery is a relief of right ventricular obstruction.[10]

Ross procedure:

The pulmonary valve is usable as a replacement for the diseased mitral or aortic valve.[11] Because this procedure uses the patient's own tissue, it offers growth potential with increasing size of the aortic root according to age; hence, there is no need for anticoagulants. Results indicate that the Ross procedure is more durable than the prosthetic heart valves.[12] Although it is associated with a high reoperation rate, this procedure confers a survival advantage for the pediatric group and young adults.[12][13][14] A common complication of the Ross procedure, autograft dilatation, and regurgitation, has been reported and may be prevented by modifying the surgery technique.[15] A ten-year report about this procedure has revealed positive results in the short and long term, but caution is necessary for the setting of endocarditis.[16]

Clinical Significance

Bicuspid pulmonary valve 

This valve anomaly has been reported in several case reports and may present as an isolated finding or associated with other anomalies such as pulmonary artery aneurysm, transposition of great arteries, or an atrial septal defect.[17],[18],[19],[20],[21] The isolated bicuspid pulmonary valve is a benign finding and associated with good clinical outcomes, and no intervention is required if there is no valve obstruction or insufficiency.[22]

Quadricuspid pulmonary valve

This valve anomaly is even rarer, and on autopsy, the incidence of this anomaly varies from 1 to 5 cases per 20000 necropsies.[23] Other reports indicate that this anomaly may be slightly more in males compared to females.[24][25] Based on one review of 121 quadricuspid valves, the anatomical variations of this valve based on frequency from the highest to the lowest as follows: 72 (60%) had 3 equal sized-cusps and 1 smaller cusp; 18 (15%) had 2 equal large cusps and 2 equal small cusps; 7 (6%) had 1 larger size cusp; 2 intermediate cusps and 1 smaller cusp; 4 (3%) had 3 equal cusps and 1 large cusp; 3 (2%) had 2 equal cusps and 2 smaller unequal cusps; 2 valves (2%) had 4 unequal-sized cusps.[26] Because it derives from the same embryology, many patients also have an aortic valve anomaly at the same time.[23][26] With advances in imaging modalities, researchers will discover the real prevalence of quadricuspid pulmonary valve.[27]

Pentacuspid pulmonary valve 

To date, there are only two case reports of the pentacuspid pulmonary valve.[28],[29] The first one was found incidentally on the autopsy performed on a 50-year-old man who died because of a ruptured aortic abdominal aorta. On a closer look, he had three equal-sized cusps and two smaller ones.[28] Despite this anomaly, he was clinically and pathologically free of heart failure. The second one was a report about a successful repair of the pentacuspid pulmonary valve of a woman with dyspnea on exertion that caused a pulmonary artery aneurysm and severe regurgitation. The valve consisted of 4 equally-sized cusps and one hypoplastic one.[29] 

Pulmonary stenosis

This is a common congenital defect which affects 7% of children with congenital heart disorders.[30][31] There is a female preponderance, and the prevalence is 2% in the absence of genetic disorder.[32] There are three types of pulmonary stenosis: valvular, supravalvular, and subvalvular. Although it can be an isolated lesion, sometimes it may be associated with tetralogy of Fallot, Noonan syndrome, congenital Rubella syndrome, and carcinoid syndrome.[33],[34],[35]

Pulmonary atresia

This is the most severe end of the spectrum for pulmonary stenosis, and it can occur with or without VSD. Other abnormalities such as tricuspid atresia or right ventricular free wall dysplasia usually accompany this disease.[36] Reports exist of functional pulmonary atresia in neonates with normal intracardiac anatomy.[37]

Pulmonary regurgitation, when it occurs as a congenital disorder, may be accompanied by a VSD. Other etiologies such as congenital anomalies (quadricuspid or bicuspid valve), hypoplasia, post-repair of tetralogy of Fallot or prolapse of the pulmonary valve may also be present. Carcinoid syndrome can also produce abnormalities due to thickening and shortening of the valve.[38] When it occurs as an isolated congenital anomaly, pulmonary regurgitation is tolerable for decades, but according to one meta-analysis, symptoms of heart failure developed in 20% of patients after 40 years.[39][40]

Pulmonary valve endocarditis

Pulmonic valve involvement in endocarditis accounts for 1.5 to 2% of total cases.[41] The pulmonary valve shares demographic, clinical, and microbiological characteristics with the more commonly involved tricuspid/aortic valve.[42] Risk factors for endocarditis include congenital anomalies of the heart or injection drug use.[43][44] However, rarely endocarditis can occur in a healthy person without any risk factors.[45] The diagnosis of infective endocarditis is possible with Duke criteria which have proven to be highly specific.[46][47]

Pulmonary balloon valvuloplasty: 

Pulmonary balloon valvuloplasty is a procedure to repair the stenotic pulmonary valve of the heart percutaneously. The patient groin is sterilized and draped, and the patient is anticoagulated during the procedure. A catheter is inserted to the left femoral vein and advanced to the right ventricle to monitor its pressure. An 18 gauge cannula is inserted to the left femoral artery to monitor systemic pressure. Then a wedge catheter is inserted to the right femoral vein and advanced into the pulmonary artery. A "J" guidewire is then inserted through this catheter, and once in place, the wedge catheter gets removed. The dilation catheter advanced through the guidewire and positioned across the stenotic pulmonary valve. While being stabilized by the guidewire, the balloon is dilated by hand with dilute contrast material at the pressure of 45 PSI.[48] Although surgical valvotomy produces low longer-term of gradients and reduced number of reinterventions, balloon valvuloplasty is the preferred therapy in isolated valve disease because it is less invasive, less expensive, and shorter length of stays and good long term results.[49],[50]

Electrocardiography:

Based on an electrocardiography study of 100 subjects with isolated pulmonary stenosis, i.e., with an intact atrioventricular septum grouped into 4 (based on right ventricle systolic pressure), no diagnostic pattern was apparent that could aid diagnosis, irrespective of the severity of the lesion. The most common ECG finding is right ventricular hypertrophy.[51]

Echocardiography:

For pulmonary stenosis, this diagnostic tool is useful in detecting the site of the stenosis, determine the cause, quantify the severity, and determine treatment strategies. The transesophageal echocardiography can visualize the pulmonary valve and right ventricular outflow tract more clearly.[52] In pulmonary regurgitation, the role of TEE is limited because the valve is more difficult to image. However, echocardiography can help the evaluation of cusp number, motion, or structure.[38]

On X-ray, the abnormalities that may present are right ventricular enlargement, right atrial enlargement, prominent pulmonary trunk, and signs of congestive heart failure.[53] Chen sign can also be visible on X-ray, which is vascular fullness at the left lung base more than the right lung base because of the preferential flow of the turbulent jet to the left pulmonary artery.[54] 

 CT & MRI

Nowadays, these modalities are increasingly being used to evaluate the anatomy and functional assessment of the pulmonary valve and right ventricle. CT scan depicts with great detail the pulmonary valve, perivalvular structures, and pulmonary artery branches. Meanwhile, MRI is used postoperatively to evaluate the pulmonary valve function and grading of pulmonary regurgitation.[55]

Other Issues

There are rare cases in which the pulmonary valve is absent; in particular in the absent pulmonary valve syndrome (APVS).[56]



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