During pressure-controlled ventilation on an anaesthesia machine, the compliance of the breathing system is 4 mL/cmH2O. At a set peak inspiratory pressure of 20 cmH2O above PEEP 5, what volume is 'lost' to circuit compression per breath?

• A 20 mL
• B 60 mL
• C 100 mL
• D 80 mL ✓

Explanation

Volume lost to circuit compression = circuit compliance × driving pressure. The stem states PIP is 20 cmH2O above PEEP, so driving pressure = 20 cmH2O. Volume lost = 4 mL/cmH2O × 20 cmH2O = 80 mL per breath. This volume distends the compliant tubing and reservoir bag rather than being delivered to the patient. In neonates and small children with tidal volumes of 15–40 mL, uncorrected circuit compression losses can dominate delivered volume; modern anaesthesia machines perform a compliance check pre-use and electronically compensate tidal volume delivery to account for these losses.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.