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As part of a research study, patients with asthma have asthmatic episodes induced and the resulting drop in arterial O2 levels is measured. Which describes the primary mechanism of hypoxemia in these patients?
A) Right-to-left shunt
B) Oxygen diffusion impairment
C) Hypoventilation
D) Low partial pressure of inspired oxygen
E) V/Q mismatch
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The correct answer is E.
Explanation: In asthmatics, bronchospasm leads to increased airway resistance. This raises the dead space as patients are unable to move adequate volumes of air through the smaller airways. This is V/Q mismatch.
R-L shunts occur with PNA, pulmonary edema In this category, blood is shunted away from areas that don't ventilate. R-L shunts also occur with ASD/VSD (cardiac causes).
There is no evidence for oxygen diffusion impairment in asthma; the disease is of the bronchioles.
Hypoventilation usually doesn't occur in asthmatics; this is reflected in the normal or decreased serum CO2 concentrations.
Low FiO2 is seen at elevated altitudes.
