Chordal fibroblasts are metabolically active cells that synthesize a variety of matrix proteins including collagen and elastin, which enable the chords to withstand the repetitive contractile stress exerted by the PMs. The chords convey blood to and from the mitral leaflets in vessels coursing the length of their shafts. The absence of branching vasculature suggests the chordae derive their nutrient supply by diffusion.
Three types of chords have been delineated: (1) primary (marginal), (2) secondary (basal), and (3) tertiary. Primary chords attach to the free margins of the leaflets, and secondary chords connect just beyond these to the undersurface of the mitral leaflets. Both types of chords, however, may arise from a common bifurcating stem. Each PM distributes chords to the ipsilateral half of both leaflets. The tertiary chords arise directly from the LV wall or from the trabeculae carnae and insert exclusively into the posterior mitral leaflet. In all, the human heart contains >100 chords arising from about 25 major chordal trunks.
Primary and secondary chords serve very distinct functions. The thinner primary chords maintain leaflet apposition and facilitate valve closure and acute mitral regurgitation results when these are sectioned. Sectioning secondary chords, in contrast, does not produce mitral regurgitation, and it is believed that they play a role in maintaining normal LV size and geometry. The function of the tertiary chords has not been determined.
Among the secondary chords, the thicker and longer ones, variably referred to as strut, stay, or principal chords (Figure 6), insert into the 4 o'clock and 8 o'clock positions on the undersurface of the anterior mitral leaflet. No well-developed strut chords attach to the posterior mitral leaflet. The strut chords constitute the anatomic interface between the musculature of the LV myocardium (at the PMs) and the mitral annulus (at the fibrous trigones) maintaining papillary-annular or ventricular-valvular continuity. From their insertion at the base of the LV, epicardial fibers descend along the anterior LV wall toward the apex gradually becoming subendocardial where they give rise to the PMs.[19,20] The strut chords emerging from the PMs attach to the dense collagen network of the anterior mitral leaflet,[21,22] which provides fibrous continuity that ultimately terminates at the fibrous trigones (completing the ventricular-valvular loop). The strut chords, like a stretched rubber band, are under continuous tension, which is transmitted to the PMs and fibrous trigones.[23,24]
Strut chords. Two prominent strut chords are seen (arrows), one from each PM, inserting into the anterior mitral leaflet (with permission from Lam et al.).
The tension in the strut chords is made evident from the retraction of the PMs that follows their division. It has been proposed that this loss of tension is transmitted from the PMs to the vertical epicardial fibers, with which they form a continuous syncytium. The resultant changes in fiber orientation are associated with a measurable decrease in longitudinal strain. Interruption of the ventricular-valvular loop by strut chord transection also causes a number of perturbations in LV geometry, including an increase in the length of the LV major axis and local remodeling in the region of the PMs. These findings, as well as reported abnormalities in LV systolic performance, including reduced torque generation, stroke volume, end-systolic elastance, and preload recruitable stroke work, are controversial.
Another alteration in LV geometry observed after strut chord transection is narrowing of the aortomitral angle, a phenomenon originally described after surgical insertion of rigid mitral prostheses. The aortomitral angle is formed by the intersection of the mitral and aortic annular planes at the intervalvular fibrosa (Figure 7). Magnetic resonance imaging indicates that this angle is normally about 90 °.[30,31] The aortomitral angle is suspended by the tension in the strut chords, which pull on the fibrous trigones. If this tension is released by strut chord transection, the annuli are drawn together, narrowing the aortomitral angle with effects on intracavitary blood flow patterns. Figure 8 is a color flow Doppler image from a patient with a narrow aortomitral angle due to a rigid mitral prosthesis. Note that LV inflow is diverted anteriorly along the interventricular septum. After encircling the apex, blood returns along the posterior wall toward the base of the LV. Because it lies in the path of this returning stream, the mitral valve can obstruct the outflow tract.[29,31,32]
Aortomitral angle. The aortomitral angle is formed by the intersection of the mitral annulus and the aortic annulus at the intervalvular fibrosa. AMA, aortomitral angle; MA, mitral annulus; AA, aortic annulus; IVF, intervalvular fibrosa; PC, primary chords; SC, strut chords; PM, papillary muscle.
Color flow Doppler images obtained in an apical long axis view. Panel (A) is from a subject with a normal aortomitral angle and demonstrates normal intracavitary flow. Left ventricular inflow travels along the posterior wall toward the apex (blue arrow), and LV outflow travels along the septum (red arrow). In panel (B), there is reversed intracavitary flow due to narrowing of the aortomitral angle after insertion of a mechanical mitral prosthesis. Left ventricular inflow is diverted anteriorly toward the septum (blue arrow) and blood returning from the apex travels along the posterior wall (red arrow).
Am Heart J. 2009;158(6):887-895. © 2009
Cite this: Contemporary Insights into the Functional Anatomy of the Mitral Valve - Medscape - Dec 01, 2009.