On a bag in bottle ascending bellows ventilator, the set tidal volume is 400 mL. The fresh gas flow into the circuit is increased substantially while all ventilator settings remain unchanged. What happens to the actual tidal volume delivered to the patient?

• A It decreases because excess gas vents through the APL valve during inspiration
• B It increases modestly because fresh gas entering the circuit during inspiration adds to the volume displaced by the driving gas ✓
• C It stays exactly at 400 mL because the ventilator delivers a fixed volume
• D It doubles because the bellows fill completely regardless of setting

Explanation

In an ascending bellows machine the bellows housing receives compressed driving gas while the bellows deliver circuit gas to the patient. Fresh gas continues to flow into the circuit throughout inspiration and this volume joins what the bellows displaces, so delivered tidal volume slightly exceeds the setting, typically by the amount of fresh gas delivered during the inspiratory time. The APL valve is excluded from the circuit during inspiration, so it cannot vent the excess.

Reference: Miller's Anesthesia, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.