If PetCO2 remains unchanged after PEEP adjustment, what does this suggest?

• A. PetCO2 will soon rise
• B. PetCO2 should be ignored
• C. You are near or at optimal PEEP
• D. Ventilation is inadequate

Answer: (C)

When end-tidal CO2 does not change after adjusting PEEP, it tells you ventilation is already meeting the patient’s CO2 removal needs at the current level of PEEP. In other words, the alveolar units are recruited enough and aren’t becoming overdistended with the present PEEP, so there’s no further improvement (or deterioration) in CO2 elimination. That implies you’re near or at the optimal PEEP: enough to keep alveoli open and reduce shunt, but not so high as to cause overinflation or increased dead space that would alter CO2.

If you expected a CO2 change to guide you, a rise or fall would indicate that ventilation efficiency is changing with the PEEP level, but a stable CO2 suggests you're already at a balance where ventilation is adequate and focus can shift to oxygenation and other parameters.