Pretty in Pink : Soda Lime. When should it be changed?

Where does the CO₂ come from?

Your anaesthetised patient exhales carbon dioxide.

Carbon dioxide is produced within the body through cellular function and transported to the lungs via the circulatory system for elimination.  During respiration, in the pause between inspiration and expiration gaseous exchange occurs at the alveolar level within the lungs. Carbon dioxide is excreted and oxygen is absorbed into the blood. The carbon dioxide is then exhaled from the patient and will travel into the circuit and into your soda lime.

If your soda lime has exceeded its capacity to absorb carbon dioxide, and is still being used, these expired CO₂ gases will recirculate to your patient through the anaesthetic circuit. This will likely result in hypercapnia, increased use of inhalation agent and oxygen, as your patient will require more of these to stay in an appropriate anaesthetic plane. Hypercapnia is a serious issue and can lead to hypotension, reduced organ perfusion, cerebral oedema, renal failure or even death¹.

Does patient size matter?

As a general rule, the canister volume should be twice the patients’ tidal volume^2,6, so smaller compact canisters will inherently have less capacity to absorb CO₂. You may consider changing soda lime closer to 6 hours of use, if your clinic is using compact AAS KAB canisters or similar. For prolonged anaesthesia, large patients or when you have a smaller canister (or a combination of all), you may need to change your soda lime during the anaesthetic. This should be performed when a 50% colour change has occurred or if your ETCO₂ baseline is increasing or if persistent hypercapnia is evident (ensure you have a spare canister ready!).

But what about the colour change?

Relying on the colour change alone to indicate exhaustion is not reliable³. Soda lime changes colour due to a different chemical reaction involving the pH (the manufacturers add this magical property in to help you determine when it might be exhausted). Soda lime often reverts back to its original colour after use (as the pH changes again) but this does not indicate that the soda lime has a restored capacity to absorb CO₂ – it is still exhausted!

Dehydrated soda lime won’t change colour, as the chemical reaction cannot occur without moisture.  This could lull you into a false sense of security, as no colour change will occur. You will notice that your patient is requiring increased amounts of inhalation agent to maintain anaesthesia and tachypnea with increased effort may be evident.

How else can I tell that the soda lime is exhausted?

Fresh soda lime is easily crumbled if you crush it between your fingers (whilst wearing gloves!). Exhausted soda lime (which can absorb no more CO₂) will feel hard if handled.

Active soda lime heats up as the chemical reaction is occurring. If you feel the canister during an anaesthetic, it should be warm. If it is not warm, it is not working!

Can capnography help?

Capnography will allow you to measure CO₂ concentration during patient expiration. This provides you with great feedback on the efficacy of your soda lime, as well as critical data on your patient’s status. If you are lucky enough to work in a clinic that uses capnography, you should be aiming to maintain ETCO₂ between 35-40mmHg. Many capnographs will also show inspiratory CO₂ (PiCO₂) – you need to maintain this below 6mmHg⁴. Any higher and you should check your patient, check the circuit for kinks, look at the amount of dead space present and then assess your soda lime for exhaustion, as higher levels of inspired CO₂ can indicate that your soda lime is exhausted.

Exhausted soda lime capnogram

When your soda lime is exhausted, your Capnogram will look like the image above – whereby the baseline increases from 0 to higher, the higher the baseline sits at the more exhausted your soda lime is. The shape of the capnogram would otherwise look normal⁷.

So the question now is: How do you measure usage?

When you have demonstrated 6-10 hours of active use, the soda lime needs to be changed.  If you do not reach 6-10 hours of use within a week you may consider changing your soda lime anyway, as it will be slowly drying out. Remember, moisture is required for the chemical reaction in the soda lime to occur, ensuring that carbon dioxide is absorbed ⁶.  Check the soda lime manufacturer’s instructions for storage criteria and length of time the bottle can be ‘open’ on the shelf.