Pathophysiology of chordae tendinae
The CT function to transmit the contractions of the PMs to the leaflets.
The first-order CT serve to secure the leaflets to maintain valve
closure and prevent MV prolapse. Their section causes acute mitral
regurgitation (MR). On the contrary the section of the second-order CT
does not produce MR and it is believed that they play a role in
maintaining normal LV size and geometry. To perform these functions, the
CT must contain a high degree of elasticity, as well as considerable
strength and endurance. A study showed that CT are composed of multiple
layers of elastic fibers, an inner collagen core, and an outer layer of
endothelial cells (fig. 1)4. During stretching, when
PM contraction straightens the chords, the collagenous wavy pattern
disappears and transfers the peak stress during contraction to the
leaflets. The sleeve‐shaped elastic network that surrounds the internal
collagen suggests that on relaxation of tension, the elastic fibers
would tend to restore the collagen to their wavy configuration.
From the mechanical point of view, the smaller chordae are less
extensible than the larger ones5. Then marginal
chordae are half as extensible as second-order chordae, over 4 times
stiffer in the pre-transitional range, and 1.6 times stiffer in the
post-transitional range, than second-order chordae6.
The reason is that collagen fibrils are more highly crimped in ticker CT
and hence have a smaller period, as supported by direct
measurements5.
CT are instrumental for the reciprocal exchange of force between the
PMs, the mitral annulus and the fibrous skeleton. The PMs, anatomically
and functionally distinct from the LV wall, function as shock absorbers
that compensate for the geometric changes of the LV wall while
maintaining their tips at a constant distance to their
hemi-annulus7. Marginal chordae may be fundamental for
leading edge control and for correct leaflet coaptation, but, since
their shape and tension change continuously during the cardiac cycle,
the mechanism responsible for maintaining constant this annulo-papillary
distance relies on the second-order chordae, and in particular on the
two, thick strut chordae of the AL, that, for their characteristics, can
support this annulo-papillary distance. The AL collagen fibers are
oriented from the insertion of the strut chords toward the fibrous
trigons, in direction of the load. This suggest that the strut chordae
connect the PMs to both trigons under tension. The tension distributed
towards the second-order chordae has been demonstrate to be more than
three-fold that in the first-order counterpart8. The
correct distribution of the tension does not make the mitral annulus
distance vary during the cardiac cycle, while the apex to mitral annulus
distance changes significantly at the expense of the changes in the apex
to papillary muscle tips7.