You are taking care of a patient with sepsis of unclear cause and find hypoxemia. The chest x-ray is normal. What is causing the hypoxemia?
When the history and physical examination do not reveal an explanation for hypoxemia, it helps to think physiologically. Air, chest vasculature, and blood all are essential.
Classically, hypoxemia is caused by:
Low PiO2 (example, altitude)
Lungs: Impaired diffusion (now thought to be an uncommon contribution)
V/Q mismatch (diseases of the chest)
Shunt (a type of V/Q mismatch with theoretically zero ventilation)
Dead space (a type of V/Q mismatch with theoretically zero perfusion)
At the bedside, the relevant differentiation is alveolar hypoventilation from V/Q mismatch. Is the patient alert and breathing adequately? Order an arterial blood gas if there is doubt, and look for elevated CO2 as an indicator of underventilation. For everything else, consider the lungs and the heart.
In this case, further questioning of the patient revealed longstanding tobacco use and subsequent chronic obstructive pulmonary disease. The patient has chronic V/Q mismatch.
Take home points:
For unexplained hypoxemia, assess ventilation
If ventilation is normal, pursue diseases of the chest