It has been widely reported that some absorbents are known to degrade anaesthestic vapour into potentially toxic compounds of which carbon monoxide, formaldehyde and compound A have been reported. Furthermore, in a phenomenon known as adsorption (or disappearance of anaesthetic vapour), desiccated soda lime and new generation absorbents containing molecular sieve zeolites or silica entrap anaesthetic vapour as well as carbon dioxide.

Adsorption is characterized by a condensing and accumulation of the vapourised agent on the absorbent granules and re-vapourization of the agent when canister temperature rises during carbon dioxide absorption. It is thought to be greater at low fresh gas flow rates and when soda limes and absorbents containing zeolites or silica are used.

Clinical signs of adsorption may include inspired concentrations of the anaesthetic agent being markedly different to that of the vapouriser setting. Recollection, by the patient, of the surgical event or pain during surgery is possible, given inadequate anaesthetic delivery. The concomittent use of muscle relaxant drugs could mask a patient’s response to surgical stimuli, allowing patient awareness to go unchecked. During routine anaesthesia, the source of patient awareness would rarely be attributed to a malfunction of the absorber canister but this source should be considered if soda lime or absorbents containing molecular sieve zeolites or silica are in use.

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