Conventional carbon dioxide absorbents, containing sodium- and potassium hydroxide are liable to react with inhalation anaesthetics by destructing, degrading or absorbing the agents. A vast number of investigations, review articles and editorials were devoted to this topic in the recent ten years. To summarize the results: Desiccated absorbents are more liable to react with anaesthetics than absorbents containing the normal amount of water. The newer agents sevoflurane and desflurane are more liable to react with the absorbents than enflurane, halothane or isoflurane. Gaseous compounds generated by destruction and / or degradation of the anaesthetics by desiccated  absorbents are those really threatening the patients: with desflurane carbon monoxide, with sevoflurane, besides other still unidentified substances, methanol, fomaldehyde, and formic acid are formed. Barium hydroxide lime is much more reactive than soda lime: unanimously in all investigations the toxic compounds reached maximum concentrations whenever this absorbent was used. Absorbents containing the normal amount of water do still react with halothane, but much more vigorously with sevoflurane by forming haloalkenes. The formation of compound A no longer should be a matter of clinical concern: even high loads with this compound, gained during long lasting low flow anaesthesia, never did result in any significant impairment of renal function. Any restriction with respect to the fresh gas flow used in sevoflurane low flow anaesthesia does not seem to be justified.

The admixture of both, potassium and sodium hydroxide to the absorbents were found to be mainly responsible for destruction and / or degradation of inhalation anaesthetics.  New less reactive absorbents were introduced into clinical practice: Potassium hydroxide free soda lime proved not to be completely satisfying, as compound A concentrations reached the same if not higher levels as with conventional soda lime.  Absorbents containing neither potassium nor sodium hydroxide, referred to as calcium hydroxide lime or non-caustic lime, in vitro and in vivo investigations proved to be significantly less reactive than conventional - or potassium free soda lime, likewise in dehydrated and normally hydrated condition. However, first reports were already published that also alkali metal hydroxide free absorbents in desiccated condition are liable to degrade sevoflurane to several up to now unidentified compounds, leading to delayed wash-in of the desired concentration and increasing the  consumption of the anaesthetic. Whether lithium hydroxide will become the alternative remains a question. Its high absorption capacity and its property to sufficiently absorb carbon dioxide even in dehydrated condition makes it  an ideal absorbent, however, the material is expensive and highly aggressive which makes handling difficult in clinical practice.

Refering to these findings following rules should be observed in routine clinical practice: Any use of barium hydroxide lime as a carbon dioxide absorbent in anaesthetic rebreathing systems should be given up completely. All measures have to be taken to safely avoid any accidental drying out of the absorbent, which holds likewise for conventional and postassium hydroxide free soda lime, but also for calcium hydroxide - or non-caustic lime. Careful maintenance and handling provided, soda lime, and especially potassium free soda lime, still can be used in clinical practice. If, however, sevoflurane and desflurane are the mainly used agents, or proper maintenance of the absorbents can not be guaranteed calcium hydroxide - or non-caustic lime should be the absorbent of choice.  The increase of costs, resulting from lower absorption capacity, seem to be negligible if judged in relation to the increase in safety for the patients, which can be gained by using these new alkali metal hydroxide free absorbents.

Measures to prevent from desiccation of carbon dioxide absorbents
 
1.  All flow controls should be turned off after each anaesthetic. In addition when the daily list is finished, the pipeline connectors should be disconnected from the sockets of the central gas piping system.
2.  A breathing system equipped with an absorber canister must never be dried out by switching on a continuous gas flow at the flow control system, or by opening the Y-piece and switching on the ventilator, while the machine is idle during night or at the weekend.
3.  Whenever the canister is newly filled with absorbent it should be labelled with the filling date written on an adhesive strip.
4.  The absorbent should be changed routinely in fixed intervals, at least once a week. This, however, only can be recommended if the anaesthetic machine is equipped with a gas monitor reliably measuring the inspired carbon dioxide concentration.
5.   If carbon dioxide monitoring is not available, the absorbent should be changed routinely every morning before starting the list. For economical reasons, the use of small 1 litre canisters is recommended.
6.   The absorber canisters of anaesthetic machines left idle for a longer period of time should not be filled with absorbent. To safely protect the absorbent from desiccation it should be stored in the unopened original packaging near to the machine. The canister should then be filled only in case of use. If, however, the machine has to be used urgently, leaving no time for filling of the canister, a fresh gas flow rate equalling the minute volume will provide adequate elimination of expired carbon dioxide out off the breathing system via the exhaust port.
7.   As recommended by the manufacturers, opened original packaging containing carbon dioxide absorbents should be carefully closed again after use.
8.   Low flow anaesthesia should be performed consistently to preserve the water contained in the absorbent.