An increase in inspired carbon dioxide (CO2 rebreathing) is detected on the capnograph waveform during general anaesthesia with a circle system. The MOST likely cause is:

• A Increased tidal volume
• B Hyperventilation
• C Disconnection of the circuit
• D Exhausted soda lime absorber ✓

Explanation

In a circle breathing system, CO2 from expired gas is absorbed by soda lime (calcium hydroxide, sodium hydroxide, potassium hydroxide) before being re-inspired. When the soda lime absorber is exhausted, CO2 accumulates in the circuit and the inspired baseline on the capnograph trace rises above zero (normal inspired CO2 = 0 mmHg). The characteristic sign is an elevated and raised baseline in the capnograph. Disconnection would prevent CO2 detection entirely; hyperventilation lowers EtCO2 but does not elevate baseline; increased tidal volume per se does not cause rebreathing.

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

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Written and medically reviewed by the StethoPrep medical team.