In volume-controlled breaths, how does pressure behave with changes in compliance and resistance?

• A. Pressure remains constant
• B. Pressure changes with compliance and resistance
• C. Pressure is fixed by ventilator
• D. Pressure equals tidal volume

Answer: (B)

In volume-controlled breaths, you set a fixed tidal volume, but the pressure needed to deliver that volume varies with the patient’s lung mechanics. Compliance is how easily the lungs can expand; lower compliance (stiffer lungs) means you must generate higher pressure to achieve the same volume. Resistance is how hard it is for air to flow through the airways; higher resistance also requires more pressure to push the same volume in during inspiration. So, pressure rises when compliance decreases or resistance increases, and it falls when compliance improves or resistance decreases. The ventilator doesn’t fix pressure regardless of mechanics, and pressure isn’t simply equal to the tidal volume, because pressure reflects both the volume you’re delivering and the lung and airway properties involved.