Which arterial blood gas value is most directly used to assess ventilation status?

• A. PaO2
• B. PaCO2
• C. HCO3-
• D. pH

Answer: (B)

Ventilation status is about how effectively carbon dioxide is being removed from the body. The arterial CO2 level (PaCO2) directly reflects this: if ventilation drops, PaCO2 rises; if ventilation increases, PaCO2 falls. This makes PaCO2 the most immediate indicator of how well the lungs are ventilating.

PaO2 measures oxygenation, not ventilation, and can be normal or low for reasons unrelated to breathing effort. HCO3- represents metabolic buffering and tells you about metabolic compensation, not current ventilation. pH shows overall acid-base balance and is influenced by both CO2 and bicarbonate, so it’s not a direct read on ventilation alone.

So the value that best indicates ventilation status is PaCO2.