The Lung and The Heart As The Source of Hypoxemia
In addition to the two common reasons (ventilation-perfusion mismatch and shunt), a diffusion limitation in the lung may also cause hypoxemia. (Incidentally, this order of presentation is different from that found in a textbook because - didactically - it is more effective for them to present the ideas in the reverse order of how common they really are.)
Key to understanding whether the lung is involved in a particular case of hypoxemia is the difference between the alveolar and the arterial oxygen levels; this A-a difference is often called the A-a gradient and is normally small. The arterial oxygen partial pressure is obtained directly from an arterial blood gas determination. The oxygen contained in the alveolar air can be calculated because it will be directly proportional to its fractional composition in air. Since the airways humidify (and so dilute) the inhaled air, the barometric pressure of the atmosphere is reduced by the vapor pressure of water.
Read more about this topic: Hypoxemia
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