In pressure-controlled continuous mandatory ventilation (PC-CMV), breaths are delivered at a set pressure with variable tidal volume depending on lung compliance. Which parameter is fixed?

• A. Set pressure
• B. Set tidal volume
• C. Set inspiratory time
• D. Set rate

Answer: (A)

In this mode, the breath is delivered to reach and hold a predetermined airway pressure during inspiration. Because the pressure is fixed, the actual amount of air that fills the lungs (tidal volume) varies with how compliant or stiff the lungs are and with airway resistance. A more compliant lung will receive a larger volume at the same pressure, while a stiffer lung will receive a smaller volume. Therefore, the parameter that stays fixed is the set inspiratory pressure. Tidal volume is not fixed—it changes with lung mechanics. The rate and sometimes the inspiratory time are set separately, but the defining fixed element in this mode is the pressure.