2.8 Sodalime

Question 1

Name the compounds used to absorb
carbon dioxide in the breathing system.

Answer 1

1. Soda lime
2. Baralyme or barium lime
3. Litholyme or lithium lime
4. Amsorb
5. Amsorb Plus
6. Dragersorb and Medisorb

Question 2

Soda lime

Answer 2

Commonly used, it has been proven that presence of sodium hydroxide,

at any level,
provides the basis for anaesthetic agent dehalogenation,

which can lead to formation of

compound A and carbon monoxide.

Question 3

Baralyme or barium lime

Answer 3

It is made of 80% calcium hydroxide and 20% barium hydroxide.

This is less efficient than soda lime and also

produces compound A more quickly.

Question 4

Litholyme or lithium lime

Answer 4

Also an effective carbon dioxide absorbent,

litholyme is free of the strong bases (NaoH, KoH)

and does not produce compound A or carbon monoxide.

Lithium chloride acts as the catalyst to accelerate the

formation of calcium carbonate.

Question 5

Amsorb

Answer 5

Contains calcium hydroxide and calcium chloride

and is not associated with the formation of

carbon monoxide or compound A

Question 6

Amsorb Plus

Answer 6

It is a new generation carbon dioxide absorbent,

free from strong alkali metal hydroxides.

It utilizes calcium hydroxide as the

active base with minor constituents

that promote speed and capacity of absorption.

Amsorb Plus is specifically designed for low
and minimal flow anaesthesia.

Question 7

Dragersorb and Medisorb

Answer 7

• These are produced by Drager and GE healthcare, respectively, with
  varying proportions of constituents.

Question 8

Describe soda lime in detail.

Answer 8

1. Constituents

• 94% calcium hydroxide
• 5% sodium hydroxide,
• small amount of potassium hydroxide (0.1%)
• silica to prevent disintegration.

2. A dye or colour indicator such as
   ethyl violet is also added,
   which changes the colour when
   the soda lime is exhausted.
   This happens at pH < 10.
3. pH 13.5
   Moisture content 14%–19%
4. 1 kg absorbs 120 L of carbon dioxide
5. When exhaled gases reach the canister,
   carbon dioxide absorption takes
   place and heat and water are produced.
6. The warmed and humidified gas
   rejoins the fresh gas flow (FGF).
7. In a patient with tidal ventilation of 6 L/min
   and Co2 production of 250 ml/min,
   the soda lime will be exhausted in 6 hours at FGF of 1 L/min
   and in 8 hours if the FGF is 3 L/min.

Question 9

What is the chemical reaction during the absorption of co2 by soda lime?

Answer 9

Co2 + H2o → H2Co3

then

H2Co3 + 2 KoH →K2Co3 + 2 H2o + Energy

then

K2Co3 + Ca(oH)2→ CaCo3 + 2KoH

Each mole of Co2 (44 g) reacted produces one mole of water (18 g).

________

The overall reaction is

Ca(oH)2 + Co2 → CaCo3 + H2o + heat

Question 10

What are the harmful products that are formed when using soda lime?

Answer 10

1. Compound A
2. Carbon Monoxide

• Other insignificant substances like methane, acetone, ethanol, etc.

Question 11

Compound A

Answer 11

Is a fluoro methyl ether

produced when sevoflurane is used

with soda lime due to dehydrohalogenation

in the presence of KoH.

Question 12

Factors that increase the production of compound A are

Answer 12

° High sevoflurane

° Increasing temperature

° Low FGF

° Use of baralyme

Question 13

Carbon Monoxide

Answer 13

Occurs when inhalational agents with CHF2 moiety

such as desflurane,
enflurane,
and isoflurane

are used with desiccated soda lime granules.

This happens when the system is left unused for a long time.

This can lead to formation of carboxyhaemoglobin

and can be significant in smokers

especially when very low flows are used.

Question 14

• Factors increasing the production of CO include

Answer 14

1. ° Type of inhaled anaesthetic agent

magnitude of Co production from greatest to least is
desflurane > enflurane > isoflurane > sevoflurane)

2. ° High absorbent dryness
3. ° Type of absorbent
   (at a given water content, baralyme produces more CO than soda lime)
4. ° Increased temperature
5. ° Higher anaesthetic concentration

Question 15

What is the size of the soda lime granules?

Answer 15

The typical size is expressed as between 4–8 mesh.

It means the granules will
pass through a mesh with 4–8 strands per inch in each axis.

Question 16

How does this affect its performance?

Answer 16

The uniformity in size is necessary to provide a smooth flow.

They should provide larger surface area
but lesser resistance to flow.

Bigger molecules would cause gas channelling,

and smaller granules can increase the resistance to gas flow.

Question 17

What are the uses of using soda lime?

Answer 17

• Anaesthetic use
• Non anaesthetic use—
  in submarines and recompression chambers
• Metabolic monitoring—
  in alkaline fuel cells to extract carbon dioxide as it affects the measurement

Question 18

What are the advantages of Soda lime?

Answer 18

Advantages

1. • Permits low flows without rebreathing carbon dioxide,
     making the system cost effective
2. • Less waste and pollution
3. • Humidification of inspired gases due to the inherent exothermic reaction

Question 19

is the colour indicator always accurate and represent the usage of soda lime?

Answer 19

No.

The decrease in pH causes the indicator to change colour,

so any product that causes a decrease in pH
can mimic an exhausted soda lime.

In partial exhaustion,
the carbonic acid levels increase and cause the change
in colour of the indicator.

If the soda lime is unused,

then the free hydroxyl ions from the depth of the canister
migrate to the surface and neutralise the acid,

reverting the colour change.

As a result, the soda lime appears fresh
although it is partially exhausted.

Question 20

What are the clinical signs of soda lime exhaustion?

Answer 20

1. • Increased spontaneous respiratory rate in the absence of muscle relaxant
2. • Increase in sympathetic drive
3. • Respiratory acidosis
4. • Increased surgical bleeding due to hypertension and coagulopathy

Question 21

What are the pros of using a circle system?

Answer 21

Advantages

1. • Economy of anaesthetic consumption
2. • Warming and humidification of the inspired gases
3. • Reduced atmospheric pollution

Question 22

What are the cons of
using a circle system?

Answer 22

Disadvantages

1. • Unstable if closed-system is used
2. • Slow changes in the inspired anaesthetic concentration
     with low flows and out-of-circuit vaporiser

Question 23

What are the components of a circle system

Answer 23

• CO2 absorber canister
• Breathing bag
• Unidirectional inspiratory and expiratory valves
• Fresh gas supply
• Pressure-relief valve
• Corrugated hoses and a Y-piece

Question 24

Circle system - rough dimensions of tubing?

Answer 24

The body of the absorber is connected to the patient

by means of inspiratory and expiratory tubes

and a Y-piece the size of which is important

when anaesthetising paediatric patients

as very small tidal volumes may not generate
enough pressure to open the valves effectively.

The effective dead space of the Y-piece is larger than it appears,
and so there can be rebreathing of exhaled gas.

These difficulties are overcome by the use of
purpose-built infant absorbers and paediatric tubing and Y-pieces.