The changes in the arterial blood gases are most interesting when you observe the progression of asthma from mild to moderate to severe to extreme disease.
During an Asthma Attack
At the start of an asthma attack the normal PaO2 of 100 mm Hg falls (e.g. to 60mm Hg), the PaCO2 of 40 mm Hg falls (e.g. to 30 mm Hg) and the pH of 7.40 rises (e.g. to 7.50).
Slowly but surely the PaO2 and the PaCO2 continue to fall and the pH continues to rise as the disease worsens. For example, the PaCO2 reaches 20 mm Hg and the pH reaches 7.60.
Eventually, a state is reached wherein the lungs are unable to blow off more carbon dioxide. At this point the PaCO2 starts to rise and the pH starts to fall, but the PaO2 continues to fall. As the asthma attack gets worse and worse the low PaCO2 and the high pH start to move back toward their normal values (PaCO2 of 40 mm Hg and pH of 7.40).
For example, the PaCO2 may become 30 mm Hg and the pH may become 7.50. However, the PaO2 continues to fall (e.g. to 50 mm Hg). The PaO2 is still falling and for example may now be 40 mm Hg.
Eventually as the asthma attack becomes extreme, the PaCO2 rises above 40 mm Hg and may reach for example 50 mm Hg or more and the pH falls below 7.40 and may reach for example 7.30. The PaO2 continues to fall and may reach for example 20 mm Hg.
The small graphic below represents a chart I developed in the mid-1970s to demonstrate the changes in arterial blood gases. My students and residents have called it "The Fish" since then.
After studying this data, you can see that if you draw an arterial blood gas on admission before any oxygen is given and find the PaCO2 at 50 mm Hg and the pH at 7.50, you can't tell whether the asthma attack is mild or severe. You must look at the PaO2 to distinguish these two states of asthma severity.
This presentation is modeled in part after the following:
National Asthma Education and Prevention Program Expert panel Report 3: Guidelines for the Diagnosis and Management of Asthma Full Report 2007
U. S. Department of Health and Human Services National Institutes of Health National Heart, Lung and Blood Institute