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Partial pressure of arterial O2 (PaO2) is the partial pressure of oxygen in the arteries. This can be measured with arterial blood gas (ABG).

The A-a gradient in a normal person is approximately 10 to 15 mmHg. Values outside this range indicates possible pathology.

Hypoxemia means a decrease in the partial pressure of oxygen in the arteries below normal levels. Hypoxemia becomes very serious when the decreased partial pressure of oxygen in blood is less than 60 mm Hg.

A diffusion problem occurs when the oxygen fails to diffuse across the alveolar wall to enter the blood. This occurs when the walls are thickened or diseased such as in the case of pulmonary fibrosis.

Shunting is a type of V/Q mismatch that happens when the alveoli of the lungs are perfused with blood, but ventilation fails to supply the perfused region. In this case, the ventilation/perfusion ratio (V/Q ratio) is zero. Examples are when the alveoli are filled with fluid like in CHF.

Dead space is a type of V/Q mismatch that occurs when there is a perfusion issue in the lungs, such as a blood clot that is obstructing the blood flow (PE). This causes V/Q to increase abnormally and can lead to an increase in A-a gradient.