How is hypoventilation assessed in pulmonary function testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Answer

Answer

Evaluation of hypoventilation as a cause for a reduced PaO2 can be made using the ideal alveolar air equation, which allows computation of an alveolar oxygen tension and calculation of an A-a oxygen gradient.

The simplified ideal alveolar gas equation is as follows:

PaO2 = PIO2 – (PaCO2/R)

PaO2 is the ideal alveolar oxygen tension, PIO2 is partial pressure of inspired oxygen, PaCO2 is partial pressure of arterial carbon dioxide, and R is the ratio of carbon dioxide production to oxygen consumption (generally assumed to be 0.8 at rest).

The measured partial pressure of oxygen in the arterial blood is subtracted from the ideal alveolar oxygen tension to calculate the A-a PO2 gradient. This gradient can be compared to a normal age-adjusted gradient (normal resting, room air A-a oxygen gradient = (age + 4)/4) to determine the effects of hypoventilation and hyperventilation. Reduced arterial oxygen tension secondary to pure hypoventilation exhibits a normal A-a PO2 gradient.


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