Valvular Heart Disease

The four cardiac valves consist of either cusps or leaflets that close to prevent the blood from flowing backwards. When pressure behind the valve builds up, the valve opens, after blood has passed through, the pressure is reduced and the valve closes, actively or passively.

Epidemiology

Rheumatic valve disease used to be the most prevalent etiology of valvular cardiac diseases worldwide. In developing countries, rheumatic heart disease remains the most common cause of valvular heart disease. Over the past 60 years, the etiology of most valvular heart disease in industrialized countries shifted towards degenerative etiologies, mainly because of a decrease in acute rheumatic fever. However cardiac valve diseases remain common in industrialized countries, mainly because the decrease in rheumatic valve disease is compensated by an increase in degenerative valve disease, with an important contributing fact being the aging population of industrialized countries. This shift in pathologic etiology accounts for differences in patient characteristics and distribution of type of valvular lesions. [1]

In the US population, the national prevalence of moderate and severe valve disease was estimated at 2,5%, determined by echocardiography. In another cohort, prevalence based on clinical signs and symptoms, confirmed by echocardiographic imaging, the estimated prevalence of at least moderate valvular diseas was estimated at 1,8%. This difference indicates the under diagnosing of valvular heart disease, and illustrates the fact that diagnosis on the basis of clinical information alone is not reliable. [2]. Prevalence did not change according to gender, but increased substantially with advancing age, with 13,2 % after the age of 75 years, versus <2% prior to 65 years old. The predominance of degenerative etiologies accounts for the higher prevalence in the elderly. Moreover, the prevalence of degenerative valve disease is expected to rise with the aging population of Western countries.

Mitral regurgitation was found to be the most frequent valvular disease, with a prevalence of 1,7 %, followed by aortic regurgitation (0,5%), aortic stenosis (0,4%) and mitral stenosis (0,1%). Mean age of patients presenting to the hospital in the Euro heart survey was 65 years [3]. In this survey, 63% of all cases of native valve disease were of degenerative etiology. Aortic stenosis was found to be the most frequent valvular disease of patients referred for treatment. In 22% of all patients etiology was rheumatic heart disease.

In developing countries, approximately 30 milion cases of rheumatic fever occur annually, in general before the age of 20. [4]. Approximately 60% of patients will develop rheumatic heart disease, which becomes clinically evident 1 to 3 decades later [5]. Rheumatic heart disease remains the most common cause of valvular heart disease in third world countries. In western countries, rheumatic heart disease is the second most common cause of valvular heart disease.

Pathophysiology

Normal valves

All cardiac valves have similar well defined interstitial cell layers, covered by endothelium. The three cell layers have specific features, and are named fibrose, spongiosa, and the ventricularis. During the cardiac cycle, the spongiosa rich in glycosaminoglycans, facilitates the relative rearrangements of collagenous and elastic layers. Valvular interstitial cells (VIC) are abundant in all layers of the cardiac valves and comprise a diverse, dynamic population of resident cells. Regulation of collagen and other matrix components is ensured by enzymes, synthesized by VICs. Integrity of valvular tissue is maintained by interaction of valvular endothelial cells (VECs) with VICs. Changes and remodeling of valvular interstitial and endothelium cell leads to changes in properties and potentially function of the valve.

Aortic valve

The tricuspid aortic valve separates the left ventricle outflow tract from the aorta. Behind the three semilunar shaped cusps of the aortic valve are dilated pockets of the aortic root, called sinuses of Valsalva. The right coronary sinus gives rise to the right coronary artery, the left coronary sinus gives rise to the left coronary artery. The commissures are the areas where attachments of two adjacent cusps to the aorta meet.

The commissure between the left en non coronary leaflets is positioned along the area of mitro-aortic continuity. The three cusps ascend towards the commissures and descend to the basal attachment with the aorta. The aortic valve has passive valve mechanism, in contrast to the mitral valve. Opening and closure of the valve is a passive, pressure driven mechanism. Tissue of the aortic cusps is stretched via a backpressure in diastolic phase with elongation and stretching of elastin. In the systolic phase, recoil of elastin ensures relaxation and shortening of the cuspal tissue [6]. Optimal functioning of the valve requires perfect alignment of the three cusps.

Mitral Valve

The mitral valve was named after a Mitre, by Andreas Vesalius [7][8]. This active valve is located at the junction of the left atrium and left ventricle. The mitral valve apparatus contains five functional components; leaflets, annulus, chordae tendineae, papillary muscles and subajacent myocardium. The annulus is a junctional zone of discontinuous fibrous and muscular tissue portion that joins the left atrium and ventricle. The anterior leaflet spans about one third of the primary fibrous, anterior part of the annulus. Part of the mitral valve anterior leaflet is in direct fibrous continuity with the aortic valve annulus, the mitro-aortic continuity. The posterior, ventricular leaflet is attached to the posterior predominantly muscular half to two third of the annulus. The mitral valve orifice is funnel shaped due to the asymmetric leaflets.

Chordae tendinae from both the anterior and posterios papillary muscles are attached to each leaflet. The papillary muscles contract and pull the chordae tendinae during systole, which closes the two mitral valve leaflets.

The mitral valvular complex comprises the mitral valve apparatus and left atrial en ventricular myocardium, endocardium and the mitro-aortic continuity. The timed passage of blood through the valve as well as the tight closure during systole is facilitated by combined actions of the mitral valvular complex. Furthermore, the mitral valvular complex contributes to the formation of the left ventricular outflow tract and facilitates the accommodation of blood, followed by the rapid, forceful ejection through into the aortic root. [9]

Pulmonary Valve

The structure of the pulmonary valve is analogous to the aortic valve structure. The leaflets are semilunar shaped, with semilunar attachments. The pulmonary valve has no traditional annulus. Anatomically, three rings can be distinguished, superior at the sinotubular junction, at the musculoarterial junction and a third ring at the base of the sinuses. [10]

Tricuspid valve

The tricuspid valve is located at the junction between the right atrium and right ventricle. The tricuspid valve apparatus consists of 3 leaflets, chordae tendinae, anterior, posterior and often a third papillary muscle. The peripheral ends of the septal, anterosuperior and inferior or mural leaflets are referred to as commissures. The tricuspid valve has no well defined collagenous annulus. The three leaflets are attached to a fibrous elliptic shaped annulus. The direct attachment of the septal leaflet is a distinctive feature of the tricuspid valve. The prominent papillary muscles support the leaflets at the commissures.

The anterior papillary muscle provides chords to the anterior and mural leaflets, the posterior papillary muscle provides chords to the mural and septal leaflets.

Normal valve function requires structural integrity and coordinated interactions among multiple anatomic components. [11] A variety of pathophysiologic mechanisms can cause cardiac valve disease.

Valvular stenosis, defined as inhibition of forward flow secondary to obstruction caused by failure of a valve to open completely, is almost always caused by a primary cuspal abnormality and a chronic disease process.

Valvular insufficiency is defined as reverse flow caused by failure of a valve to close completely, may result from either intrinsic disease of the valve cusps or from damage to or distortion of supporting structures without primary cuspal pathology.

Rheumatic valve disease

Chronic rheumatic valve disease is characterized by chronic, progressive deforming valvular disease. Anatomic lesions combine to varying degrees fibrous, or fibrocalcific distortion of leaflets or cusps, valve commissures and chordae tendineae, with or without annular or papillary muscle deformities.

Stenosis results from fibrous leaflet and chordal thickening and commissural and chordal fusion with or without secondary calcification. Fusion of a commisure in an open position can cause regurgitation, as well as scarring induced retraction of chordae and leaflets.

Aortic valve Stenosis

Obstruction of the left ventricle outflow can occur at subvalvular level (eg hypertrophic cardiomyopathy), supravalvular level or valvular level. Aortic valve stenosis is left ventricle outflow obstruction at valvular level.

In industrialized countries, aortic stenosis is the most common lesion among patients referred for treatment of valvular disease [3]. Age-related degenerative calcified aortic stenosis is the most common cause of aortic stenosis in adults in North America and Western Europe. The second most common cause is calcification of a congenitally bicuspid aortic valve. Other rare causes of calcified aortic stenosis include Fabry disease, lupus erythematosus, Paget disease, and ochronosis with alkaptonuria. The most common etiology of aortic stenosis worldwide remains rheumatic heart disease.

Prevalence of aortic valve abnormalities increases due to age-related pathology in the ageing population. The first detectable macroscopic modifications of the calcification process is named aortic valve sclerosis. [6] Aortic sclerosis, seen as calcification or focal leaflet thickening with normal valve function, was detected in 25% of people at 65 years of age, this increases to 48% in people aged >75% in a population-based echocardiographic study. [12] [13]

The prevalence of calcified aortic stenosis is estimated at 2 % of people 65 years of age, increasing to 3-9% after the age of 80 years. [2] [12]

Calcified degenerative aortic valve stenosis was previously considered to be the result of a passive degenerative process due to longterm mechanical stress in combination with calcium accumulation. Recently this concept is revised. Calcified degenerative aortic stenosis is considered an active pathobiological process, including proliferative and inflammatory changes, lipid accumulation, renin-angiotensin system activation, valular interstitial cell transformation, ultimately resulting in calcification of the aortic valve [14][15] [16] [17]. Risk factors for development of calcific aortic stenosis are similar to those for vascular atherosclerosis such as diabetes, hypertension, and cholesterol levels. [18] [19] Progressive calcification leads to immobilization of the cusps causing stenosis.