2018A04 Outline the hazards associated with the use of CO2 absorbents within a circle
breathing system and how the risks can be minimized.



·      Physical hazards

·      Degradation products

·      Other

·      Ingredients: each type


Physical hazards of CO2 absorbent:


·   Worst with baralyme + sevo (150°C)

·   Risk of burns, fires, explosions

·   Fix: use a well sealed well-insulated canister. Change when depleted.

·   Fix: turn off when not in use. Change regularly, after excessive use, or if in doubt.


·   Risk of injury to lungs, skin, eyes

·   Fix: use a well sealed canister, careful not to spill.


Harmful degradation products:

Compound A

·   Reaction sevo + absorbent

·   Baralyme>Sodalime>>Amrisorb and Litholime)

·   More likely if high sevo%, low fresh gas flow rate, high temp, fresh absorbent

·   Nephrotoxic in rats at 150-200ppm

·   Fix: don’t use baralyme

·   Fix: use FGFR>2L/min; limit sevo to 2MAC-hours if FGF 1-2L/min

Carbon monoxide

·   Reaction absorbent + CHF2 (difluormethyl) moiety in volatile

·   Isoflurane/enflurane > desflurane >> other

·   More in baralyme > sodalime

·   More if absorbent hot and dry (e.g. left on over weekend)

·   HbCO can rise to 35%. Binds to Hb with 250x higher affinity than O2.

·   Fix: avoid baralyme; avoid continuous use without break

·   Fix: ensure machine turned off after use; check CO2 canister before use; replace if in doubt

·   Fix: use sevo


·   Formaldehyde

·   Methanol



Unrecognised exhaustion

·   When near depleted, lime colour can change back to “fresh” if left unused

·   Unused OH- ions in the core migrate to the surface.

·   Fix: check before each case

·   Fix: use capnography in every GA

Airway resistance

·   4-8 mesh optimal for balance between ↓ airway resistance and ↑ surface area

·   Risk of breathstacking in children

·   Fix: use a Mapleson circuit instead


·   Risk if canister changed during anaesthesia

·   Fix: change between cases if possible

·   Fix: administer IV anaesthetic temporarily


Ingredients: (unlikely to score marks)


·   80% Ca(OH)2

·   <4% NaOH

·   16% water(to accelerate reaction)

·   Indicator (ethyl violet or phenolphthalein)

·   Silica (hardening agent)


·   80% Ca(OH)2

·   <20% BaOH

·   Water is incorporated into lime, making it less efficient

Amsorb plus

·   >75% Ca(OH)2

·   CaCl2

·   15% H2O


·   >75% Ca(OH)2, small amount LiOH




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