Soda Lime

BT_SQ 1.15  Outline how CO2 is absorbed in a circle system, and the hazards associated with CO2 absorption.

The other day I saw a picture of some jacaranda trees on a Facebook post, and (tragically) it reminded me of the colour of the ethyl violet pH indicator in our CO2 absorbers.

I also recently did a tutorial on equipment for the registrars at my hospital, who were all panicking about the chemistry of soda lime.

For both these reasons, I though I’d do a post on soda lime / CO2 absorption.

No doubt you’ve realised that the need for chemical CO2 absorption is unique to anaesthesia (i.e. it’s not needed/used in ICU). This is because we use circle breathing systems which permit rebreathing. And the reason for this is to conserve N2O and volatile agents, by allowing them to be rebreathed. Without a rebreathing circuit, the cost of N2O and volatiles would be astronomical. To permit safe rebreathing, the patient’s exhaled CO2 has to be eliminated.

When you first look at the soda lime equations, you can get a bit daunted. Your goal should be to look beyond the equations, so that you understand what is happening. Simply writing out equations in an exam, with no explanation, will score hardly any marks. You could pass a question on this topic without using the actual equations, but by providing an explanation in words of what is happening. (Ideally, you should be able to do both).

Soda Lime Reactions

NB…. the above equations are not balanced. I’ve done this deliberately to make them look simpler. If you want to balance them, you will need to add some numerical coefficients (large numbers to the left of some of the chemical formulae). 

T / F  fresh soda lime is about 15% water – without water, the reactions can’t start

T / F  A strong base (sodium hydroxide) is needed to speed things up. Reaction #1 is very slow. But #1 will go faster if its product (carbonic acid) becomes one of the reactants in a very fast reaction (reaction #2).

T / F  the product of reaction #2 is sodium bicarbonate, which reacts with calcium hydroxide (another strong base) in reaction #3

T / F  the NaOH produced in reaction #3 is recycled through reaction #2 to keep things going

T / F  the CO2 absorber becomes “exhausted” when all the calcium hydroxide is used up

T / F  the basic aim of the CO2 absorber is to turn CO2 into chalk (calcium carbonate)

Discussion Questions

Is NaOH really necessary? If you feel inspired by this post, you might like to read about “Amsorb”. What advantages does it have over soda lime? Is its CO2 absorption capacity the same?

Does soda lime really “absorb” CO2? Can you think of a better word?

BT_PO 1.41a Discuss oxygen therapy including methods of delivery, indications and contraindications, physiological and pathophysiological effects.

T/F  oxygen therapy devices are classified as “fixed performance” (delivering a fixed flow of oxygen) and “variable performance” (able to deliver a wide range of oxygen flow rates)

T/F  a Venturi mask can deliver a known FiO2 because the total flow into the mask (oxygen flow plus entrained room air) should exceed the patient’s peak inspiratory velocity *

T/F  a Venturi mask works by the Bernoulli principle **

T/F  a Hudson mask will deliver a lower FiO2 if the patient’s respiratory rate increases ***

T/F  hyperoxia in the post-resuscitation phase has been shown to improve outcome

T/F  hyperoxia can cause tissue damage by (i) forming reactive oxygen species, and (ii) causing vasoconstriction

T/F  giving 100% oxygen can cause atelectasis

* can you think of some conditions under which a Venturi mask would no longer deliver a known FiO2?

** the answer to this is contentious – if you’re interested in physics and equipment, you might enjoy reading more about it

*** can you explain the answer?

Now that you have thought about oxygen therapy – why not have a go at a past SAQ, asked in the 2017.1 exam (with only a 39% pass rate unfortunately) …
Compare and contrast oxygen delivery via nasal prongs, Hudson mask, and Venturi mask.

BT_SQ 1.13 Describe and classify breathing systems used in anaesthesia (episode 2).


I photographed this little guy in Hong Kong – he’s (or perhaps she’s??) looking bit ragged. Following yesterday’s post I did a bit of research. It tuns out that sea jellies have no breathing system at all. They meet their oxygen requirements through diffusion across their bodies….

Today we will turn our focus to the circle system. This is something most of us use every day. I asked a viva on it in the last exam and was surprised by many of the answers I was given.

The circuit is circular with unidirectional valves – does that mean that gas flows in only one direction throughout the entire system?

BT_SQ 1.13  Describe and classify breathing systems used in anaesthesia. Evaluate their clinical utility and hazards associated with their use

The circle circuit, as commonly used in current anaesthetic practice, is a closed system  TRUE/FALSE

With the standard circle arrangement, fresh gas commonly flows through the CO2 absorber   TRUE/FALSE

Placing the APL valve before the CO2 absorber (on the expiratory limb) helps to conserve the CO2 absorbent  TRUE/FALSE

When running the circle as a closed circuit, minimal gas monitoring is required, as it is a stable system        TRUE/FALSE

A safe circle system requires the fresh gas flow to be placed between the patient and the expiratory valve  TRUE/FALSE

BT_SQ 1.13 Describe and classify breathing systems used in anaesthesia.


What sort of  breathing system do those guys have? Certainly none of the ones we will be discussing today…

Today I will focus on the Mapleson classification of breathing systems. Here is an article written about them by Mapleson himself. He is still alive and in his 90s. You can read a little more about him here. Textbooks on the ANZCA primary exam reading list generally cover this topic adequately too.

BT_SQ 1.13 Describe and classify breathing systems used in anaesthesia. Evaluate their clinical utility and hazards associated with their use

The Mapleson A circuit is more efficient for a spontaneously ventilating patient compared with Mapleson D    TRUE/FALSE

A Mapleson E circuit is also referred to as classic T-piece     TRUE/FALSE

In the Mapleson D circuit the reservoir bag is located off the expiratory limb    TRUE/FALSE

The Mapleson D circuit is more efficient for controlled ventilation (CV) compared with spontaneous ventilation, due to the longer expiratory phase with CV    TRUE/FALSE

The Mapleson C circuit is the most efficient of these systems for spontaneous ventilation TRUE/FALSE