A drop in PetCO2 after increasing PEEP most likely indicates that you have:

• A. Self-limited alveolar collapse
• B. Passed optimal PEEP
• C. Increased cardiac output
• D. Sensor malfunction

Answer: (B)

PetCO2 tracks how much CO2 is delivered to and removed from the lungs, so it depends on both ventilation and pulmonary blood flow. When you increase PEEP, you can recruit more alveoli and improve ventilation, but higher PEEP also raises intrathoracic pressure. If you push PEEP past the optimal level, the increased pressure reduces venous return and cardiac output, which lowers pulmonary blood flow and the amount of CO2 reaching the lungs. The result is a drop in end-tidal CO2, signaling that you have passed the optimal PEEP.

This isn’t explained by self-limited alveolar collapse, which would more likely improve with recruitment. It isn’t explained by increased cardiac output, which would tend to raise PetCO2. A sensor malfunction is possible but less consistent with a physiologic response to PEEP changes.