The flow sheet for a patient on pressure-controlled continuous mandatory ventilation (PC-CMV) demonstrates which of the following when Exhaled Vt is increasing?

• A. Airway resistance is decreasing
• B. Airway resistance is increasing
• C. Compliance is decreasing
• D. Tidal volume is decreasing

Answer: (A)

In pressure-controlled ventilation, the inspiratory pressure is fixed, so the amount of air that reaches the alveoli (and thus the tidal volume) depends on the ease with which air can move into the lungs. If airway resistance decreases, less pressure is lost across the conducting airways, allowing more pressure to reach the alveoli during the inspiratory phase. That increases the tidal volume that is exhaled. So a rising exhaled tidal volume points to decreasing airway resistance.

If resistance were increasing, or if compliance were decreasing (lungs stiffer), the tidal volume would tend to fall rather than rise. And a tidal volume decreasing on the flow sheet would also not explain the observed increase.