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Carlos M.G. Duran, MD, PhD.
Anatomic knowledge has been driven by the surgeon. When entering a new area, the surgeon usually finds that the available knowledge is wanting. We here review the progressive evolution of our knowledge of the mitral valve based incompletely on what I consider the key authors responsible for these changes. As a whole this review shows that the initial simplistic view of the mitral valve as a one way valve consisting of two leaflets held by chordae that opens and closes following pressure differences is far too simple and consequently, mostly wrong. Closed commissurotomy stimulated a number of studies on the mitral orifice and leaflet sizes. Chechi and Baily (1) showed that the normal total leaflet area doubles the mitral orifice. This tissue abundance explains the success of mitral repair compared with aortic valve repair where lack of available tissue is a constant limitation.
The direct observation of the diseased valve offered by open commissurotomy and early repair maneuvers centered the surgeons interest on the mitral annulus. Initially it was though to be a continuous fibrous structure surrounding the whole mitral orifice. Becker and associates (2) in a necropsy study of human hearts showed that only 10% had a complete fibrous annulus. The only constant fibrous tissue was found at the trigones. The importance of the trigones was demonstrated by Kunzelman et al. (3) who showed in a finite element study that the maximum stress on the mitral valve was at the trigones. The clinical implication of these studies is that all annuloplasties must be anchored to the trigones. The surgeon who uses partial bands that only support the posterior annulus, must ensure that their extremities are properly sutured to the trigones. Tzakiris (4) experimentally, and Ormiston (5) clinically, showed that contrary to our earlier belief, the shape of the annulus varied constantly during the cardiac cycle. This knowledge motivated the development of flexible rings. It was also thought that the annulus was in a single plane until Salgo and associates (6) showed that the annulus resembles a saddle and that this hyperbolic paraboloid shape reduces significantly its mechanical stress. This shape however changes continuously during the normal cardiac cycle and therefore, the design of a saddle shaped rigid ring is only physiologic during a particular moment of the cardiac cycle. Recently, several authors have shown that the expansion and shortening of the annulus during the cardiac cycle are not homogeneous. In a simultaneous sonometric study of the mitral and aortic orifices we found that the intertrigonal distance that was considered to remain constant during the cardiac cycle, did in fact change in parallel to the changes in the aortic valve orifice (7). In a postmortem study of hearts with ischemic and dilated cardiomyopathies the intertrigonal distance was significantly increased compared with normals. This information is of clinical relevance since the standard method to determine the correct size of an annuloplasty is based on the intraoperative measurement of the intertrigonal distance.
The concept of the mitral valve as formed by an anterior and posterior leaflet has also changed. At present the mitral valve is understood to be formed by six distinct leaflets with one anterior, two commissural and three posterior scallops. The diagnostic and therapeutic need to define, identify and record precisely the location of each of the mitral scallops has given rise several classifications and terminologies useful to echocardiologists and surgeons (8).
The importance of the chordal system has been recognized since the early days of cardiac surgery. Although the presence of marginal and basal chords was well recognized, only the marginal were considered of practical importance. While the marginal chords were known to be essential to avoid leaflet prolapse, the basal chords were ignored and even sectioned with apparent impunity. In a recent anatomic study of human, sheep and pig hearts we identified in 100% of the cases two anterior and two posterior chords that because of their thickness and function we termed them as 'stay chords'. The anterior stay chordsr AS and PS were in the three species were inserted into the central part of the anterior mitral leaflet at the junction of its smooth and rough zones and the two posterior stay chords were inserted into a single fibrous band located close to the base of the central portion of the posterior annulusthe central part of PM. PS1 and PS2 endings into the PML ventricular aspect were connected together. These particular ending helped to recognize them see f. These chords remain taught during the whole cardiac cycle and sustain three times more stress than their corresponding marginal chords (9). We and other authors (10) have shown that the distance between tips of papillary muscles and mitral annulus remains constant during the whole cardiac cycle. They probably are the main mechanism for maintaining the essential annulo-papillary continuity. We have applied this information in mostly rheumatic patients who have such a degree of mitral distortion that the whole mitral apparatus must be resected. To maintain annulo-papillary continuity two anterior and two posterior PTFE neo-chords are placed between the papillary muscles and the trigones and posterior annulus. A significant number of recent clinical and experimental studies have implicated the anterior stay chords not only as an important component of the mitral valve but also essential for LV function (11).
The papillary muscles have been considered as a left ventricular muscular protuberance that anchors the chordae to the left ventricular wall. Rupture of one of their heads resulted in severe valve regurgitation. However, Levine's group (12) has recently shown the very important role played by the papillary muscles in the genesis of functional ischemic regurgitation. In these cases the increase in LV sphericity displaces laterally the papillary muscles. This displacement tethers apically the normal leaflets which reduce their coaptation resulting in regurgitation. The papillary displacement pulls down the body of the anterior leaflet through the anterior basal 'stay' chords. This information has given rise to a variety of surgical maneuvers such as section of the culprit stay chords, approximation of the papillary muscles with an internal sling or external devices, implantation of PTFE neo-stay chords that approximate the posterior papillary muscle to the annulus or placing a restraining jacket around the ventricles.
A number of important points need clarifying today. Guy and associates (13) have questioned whether functional ischemic mitral regurgitation is the main cause of LV remodeling or simply its consequence. This is important since it questions the validity of the surgical maneuvers specifically directed towards the reduction of regurgitation such as annuloplasties. May be other techniques at ventricular level are necessary. On the other hand, we have shown in cases of myxomatous anterior leaflet prolapse that placement of a flexible ring immediately resulted in an increase in the angle between the anterior leaflet and annulus. In other words it displaced the papillary muscles reducing spontaneously anterior leaflet prolapse. Similarly Tibayan et al. (14) have shown in an acute ischemic sheep model that a suture annuloplasty not only corrected the annulus enlargement but also induced a relocation of the papillary muscles. The actual mechanism of this phenomenon is still unknown.
In a recent sonometric study of the opening and closure of the normal mitral valve in sheep, we found that the mitral valve started to open before the end systole by 3.5±0.8% and continued opening by an additional 59.9±11.3% until end of isovolumic relaxation. At the beginning of isovolumic contraction 57.2±4.3% of the mitral valve was still open. Closure was only completed at the end of isovolumic contraction (15). These surprising findings were also found in the other cardiac valves. For instance, the aortic valve started to open when the pressure in the ascending aorta was still higher than in the LV. These findings not only question the classic definitions of the phases of the cardiac cycle but more importantly, they show that the predominant paradigm that tries to understand heart valve function with staccato mechanical parameters should probably be shifted towards a smoother, flow-directed behavior. Cardiac flow dynamic studies with three-dimensional magnetic resonance and velocity mapping might provide a more physiologic and real understanding of the blood fluid dynamics of the heart leaflet movements (16)
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11. Goetz WA et al. the aortomitral angle is suspended by the anterior mitral basal stay chords
12. Otsuji Y et al. Mechanism of mitral regurgitation with segmental left ventricular dysfunction. Three-dimensional echocardiographic studies in models of acute and chronic progressive regurgitation. J Am Coll Cardiol 2001;37:641-8
13. Guy IV TS et al. Prevention of ischemic mitral regurgitation does not influence the outcome of remodeling after posterolateral myocardial infraction. J Am Coll Cardiol. 2004;43:377-83
14. Tibayan FA et al. Mitral suture annuloplasty corrects both annular and subvalvular geometry in acute ischemic mitral regurgitation. J Heart Valve Dis 2004;13:414-20
15. Goetz WA et al. Left ventricular endocardial and transverse changes during isovolumic contraction and relaxation: a challenge. Am J Physiol Heart Circ Physiol 2005;
16. Kilner PJ et al. Asymetric redirection of flow through the heart. Nature 2000;404:759-61. carlosduran
