In volume-controlled ventilation with VC-CMV, a trend showing decreasing delivered tidal volume while inspiratory pressure is held constant indicates which change in the patient’s lungs?

• A. Airway resistance is increasing
• B. Lung compliance is decreasing
• C. Tidal volume is increasing
• D. Oxygenation is improving

Answer: (B)

In volume-controlled ventilation, the ventilator delivers a set tidal volume by applying a specific driving pressure. The amount of air the lungs actually receive for that pressure depends on how easily the lungs expand, i.e., their compliance. If the inspiratory pressure is held constant and the delivered tidal volume falls, it means the lungs aren’t expanding as much for the same pressure—lung compliance has decreased. This is what you’d expect if the lungs became stiffer due to edema, atelectasis, pneumonia, ARDS, or fibrosis.

If airway resistance were the main issue, you’d expect changes in flow and possibly peak pressures, and oxygenation changes wouldn’t directly explain a drop in tidal volume. If tidal volume were increasing, that would contradict the observation. Oxygenation improving is a separate outcome and doesn’t account for a reduced volume with fixed pressure.