Isovolumetric Contraction.
It has been documented that during this phase there is a mechanical
shortening of the transverse fibers belonging to the free wall of the RV
in continuity with the LS, conditioning the narrowing of the cavity.
This forms an ”external armor” keeping the base of the ventricles fixed,
and compressing the AL (the cross-sectional diameters of both ventricles
decrease) causing a brief temporary longitudinal lengthening of the LV
apex during the pre-ejection interval (increase in the longitudinal
diameter of the LV)19,20,23. The global translation of
these movements of BL is the counterclockwise rotation of the entire
heart as seen from the apex as reported by MRI
studies23,24. LV elongation during the pre-ejection
phase and compression secondary to BL contraction can generate a
”loading” effect on the AL to produce a Starling effect through the
titin mechanism for posterior ejection26. Until this
moment, the contraction of the subendocardial DS is prepared, which is
continuous to the LS but directionally opposite, which during the
isovolumetric contraction phase does not produce enough force to cause
the longitudinal shortening movement as it happens in the next phase
(ejection), the lack of chamber shortening during the isovolumetric
phase implies that the counterclockwise rotation of the heart is due to
circumferential muscle shortening of the BL.