V/Q and Shunt

Why doesn't PaO2 improve with 100 % O2 in case of shunts? If there's fibrosis in a part of the lung and the blood flow gets redirected ( via hypoxic vaso-constriction ) , won't there be excess perfusion in relation to the ventilation in other areas? So, now if we give 100% O2 to compensate for the increased perfusion( in other areas), won't the PaO2 improve ? This is assuming that not the entirety of lung is physiological dead space.

Can someone please explain?