Ventilation-Perfusion Mismatch

 Ventilation-Perfusion ( V̇/Q̇ ) mismatch is similar to intrapulmonary shunt (V̇/Q̇ = 0) , with some important distinctions.

In V̇/Q̇ mismatch, disparity between the amount of ventilation and perfusion in various alveoli leads to areas of high V̇/Q̇ (i.e., well-ventilated alveoli) and areas of low V̇/Q̇ (i.e., poorly ventilated alveoli). Because of the shape of the oxyhemoglobin dissociation curve, the improved oxygenation in well-ventilated areas cannot compensate for the low PO2 in the poorly ventilated areas, resulting in lower PaO2 or arterial hypoxemia.

Clinically, in V̇/Q̇ mismatch, administering 100% oxygen can achieve a PO2 on the plateau of the oxyhemoglobin dissociation curve even in poorly ventilated alveoli. Conversely, administering 100% oxygen in the presence of an intrapulmonary shunt only adds more dissolved oxygen in the normally perfused alveoli, although as seen in Fig.1, this may result in more improvement in oxygenation than is often appreciated. Arterial hypoxemia remaining despite administration of 100% oxygen is always caused by the presence of an intrapulmonary shunt.