pictured above : the normal hearts shape.
with pulmonary vasoconstriction, the RV overdistends and the LV gets underfilled.
when the RV overdistends, the heart muscle is overstretched and
the intercardiac septum shifts leftward.
This leftward shift of the intercardiac septum compromises some
of the left ventricles volume.
The compromisation of the LV exacerbates the underfilling of the LV.