During a low-flow anaesthesia technique, the inspired CO2 concentration on the capnograph begins to show a steady baseline rise above zero. The MOST likely cause is:

• A The FGF has been set too high, causing washout of CO2 from the absorber
• B Endobronchial intubation causing V/Q mismatch and CO2 retention
• C Exhausted CO2 absorber (soda lime) with CO2 channelling through spent granules ✓
• D Faulty APL valve remaining closed during exhalation, preventing CO2 elimination

Explanation

A rising inspired CO2 baseline (rebreathing of CO2) in a circle breathing system indicates failure of CO2 absorption. When soda lime (or Baralyme) granules are exhausted, their colour indicator changes (from pink/white to violet) and CO2 passes through without absorption. At low FGF, the proportion of rebreathed exhaled gas is high, so any absorber failure rapidly elevates inspired CO2. This is differentiated from alveolar hyperventilation (where end-tidal CO2 falls) or endobronchial intubation (where ETCO2 remains elevated but inspired CO2 remains at zero). The remedy is replacing the absorber canister.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.