Why does 100% oxygen help patients with dead space but not pulmonary shunting?

Everything seems to make sense until you take into account hypoxic pulmonary vasoconstriction. For the sake of simplicity, imagine one entire lung has either dead space or pulmonary shunting and the other one is normal. In the case of dead space, the diseased lung will not have any perfusion. All that blood that is supposed to flow through that lung will flow through the normal lung. The normal lung will thus have 2x as much blood flowing through it. As a result, giving a patient in this... Why does 100% oxygen help patients with dead space but not pulmonary shunting?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Tags: Step I Source Type: forums