In a flow sheet for a patient on pressure-controlled continuous mandatory ventilation (PC-CMV), if exhaled tidal volume is decreasing, which parameter is most likely changing?

• A. Airway resistance is increasing
• B. Dynamic compliance is decreasing
• C. Mean airway pressure is increasing
• D. Plateau pressure is decreasing

Answer: (B)

In pressure-controlled ventilation, the amount of air you deliver with each breath depends on how much the respiratory system can expand in response to the set inspiratory pressure. That relationship is roughly Vt ≈ driving pressure × dynamic compliance, where driving pressure is the difference between the peak inspiratory pressure and PEEP, and dynamic compliance reflects how easily the system inflates during flow (it includes both the elastic properties of the lung and the resistance of the airways).

If dynamic compliance decreases, the lungs become stiffer or the airways more constricted, so the same driving pressure yields a smaller tidal volume. That’s why a falling exhaled tidal volume on a flow sheet under PC-CMV is most directly explained by a drop in dynamic compliance.

While changes in airway resistance can also affect delivered volume, the most direct and common explanation for a reduced tidal volume with a fixed inspiratory pressure is decreased dynamic compliance. Mean airway pressure rising or plateau pressure changing in opposite directions wouldn’t explain a reduced tidal volume as clearly, since those factors relate to pressure and timing differently and don’t tie as directly to the volume delivered under a fixed pressure.