Which lab value should be monitored for a client on mechanical ventilation to assess for ventilator-associated hypercapnia?

• A. Arterial PaO₂
• B. Arterial PaCO₂
• C. Serum potassium
• D. Serum bicarbonate

Answer: (B)

When evaluating ventilator performance, the key issue is how well carbon dioxide is being cleared from the bloodstream. Ventilator-associated hypercapnia means CO2 isn’t being eliminated adequately, so the best indicator is the arterial carbon dioxide level (PaCO2). This value directly reflects ventilation efficiency and CO2 removal, guiding adjustments to the ventilator to prevent or correct hypercapnia.

PaO2 shows how well oxygen is being put into the blood, not how effectively CO2 is removed, so it doesn’t tell you about ventilator clearance of CO2. Serum potassium is unrelated to ventilation status, and while serum bicarbonate can change in response to chronic CO2 retention, it isn’t a direct measure of current ventilatory adequacy.