In a volume-controlled breath, what happens to the volume waveform?

• A. Follows the pressure waveform
• B. Remains constant
• C. Depends on patient effort
• D. Varies with rate

Answer: (B)

In volume-controlled ventilation the machine targets a fixed tidal volume for every breath, so the delivered volume is regulated rather than allowed to fluctuate with the patient or airway conditions. As inspiration begins, the volume rises quickly until the set tidal volume is reached and then plateaus for the remainder of the inspiratory phase. Expiration follows and the volume returns toward baseline. Because the tidal volume is held constant, the volume waveform is essentially the same from breath to breath, assuming no leaks or big changes in lung mechanics.