Cutting the second order chordae
Tethering of the second-order chordae has been recognized as one of the
most important mechanisms at the basis of the onset and of worsening of
secondary MR. Moreover its persistence is recognized as the cause of
failure of isolated mitral reductive annuloplasty (MRA) (fig. 2).
Transection of the second-order chordae has been proposed by Messas et
al9 as an adjunct to MRA to improve leaflet coaptation
and to reduce MR return. However, even if introduced in 2001, there is
no general agreement on the efficacy of this technique.
The curvature of the leaflets is designed to reduce the stress on the
leaflets during the cardiac systole. The presence of tethered
second-order chordae eliminates the curvature and increases the stress
on the leaflet. Salgo et al10 demonstrated, in a
finite study, that, compared with a flat leaflet, 10% curvature reduced
leaflet stress by 100% from 335 to 3 MPa. However, cutting the
second-order chordae from one side recovers curvature, but on the other
side transfers the tension on the first-order chordae, that now have to
face a tension higher than usual. The risk of rupture of first-order
chordae under excess of tension and the role of the second-order chordae
in maintaining the annulo-papillary stability and LV geometry, casted a
shadow on the benefit that chordal cutting (CC) could provide,
especially due to controversial experimental results. The positive
results obtained by Messas et al9,11,12 were
questioned by other experimental works13-17, that
showed impairment of the LV function after CC. However, the first
clinical reports, appeared in 200518,19, showed good
results and no reduction in LV function after a follow-up of 6 months
and 2 years, respectively. Furthermore, Fayad et al19identified an aortotomy as the best surgical approach for CC, because of
the optimal access to the ventricular side of the AL. In an
observational comparative study, Borger et al20 showed
that, at 2-year follow-up, MR return was reduced in the CC group with
improved ejection fraction (EF) by +10%. Our group, in a propensity
matched study, reported, at a follow of 35 months. reduced MR in the CC
group with increased EF (from 31% to 40%)21, showing
the safety of the procedure. Nevertheless, CC is slowly absorbed by the
surgical community, still concerned about possible harmful effects of
the technique.
We postulate that the role of the second-order chordae is different in a
heart with previous acute myocardial infarction and ischemic MR than in
a normal heart. In a normal setting, the second-order chordae,
connecting the PMs to the trigones through their insertion to the MV
leaflets, absorb the tension generated by the PMs themselves. By their
insertion into the MV leaflets, they contribute to reduce the peak
systolic stress. In a pathologic situation, the second-order chordae,
which insert on the AL, being tethered, eliminate its curvature,
increasing the systolic peak stress by 100-fold10. By
cutting the second-order chordae, the AL recovers its curvature,
reducing the systolic peak stress by 100-fold, but now the transmission
of the systolic stress falls completely on the first-order chordae. In
an experimental setting, Padala et al22 found that,
displacing the PMs in three different directions (apically,
apico-laterally and apico-lateral-posteriorly), the tension on the
marginal chordae, compared to the basal values, increased more on the
right-sided (till 4.6-fold) than on the left-sided chordae (till
2.1-fold) and were generally lower when the PMs were displaced apically
and higher when displaced apico-lateral-posteriorly. When the anterior
second-order chordae were cut, the increase of tension, compared with
basal values, was respectively till 7.3-fold for the right-sided and
till 3.7-fold for the left-sided marginal chordae. Then the tension on
marginal chordae increases with PMs displacement, with a further
increase after CC. As the first-order chordae are stiffer but less
elastic than the second-order chordae, the increased tension can bring
the marginal chordae closer to the rupture point. However, in
experimental settings, the measured forces supported by the first-order
chordae of the AL after CC were not higher than 0.4
N22,23, significantly smaller than the failure load of
6.8 N required for rupture of the anterior marginal chordae reported by
Sedransk et al24. The risk of rupture of the marginal
chordae seems then to be only theoretical, and anyway overwhelmed by the
benefits (recover of the AL curvature and improvement of the
coaptation).