CO2 Absorption

Question 1

Why absorb CO2?

Answer 1

1. To reuse volatile anesthetics in an anesthetic circuit carbon dioxide absorption is required (Cannot just pump CO2 and gas out the window- too expensive. Want to rebreathe expensive anesthetics as well)
2. Carbon dioxide absorption is also critical in other enclosed environments such as submarines and space vehicles

Question 2

Carbon dioxide is a end product of?

Answer 2

1. Metabolism that is kept low in the body by its expiration
2. Normal PCO2 levels are:
   - 40 mmHg (arterial)
   - 45 mmHg (venous)

Question 3

Human CO2 production?

Answer 3

1. Approximately 4 ml per minute per kg of body weight

Question 4

Basis of carbon dioxide absorption is its ability to form _____ in water?

Answer 4

1. Form carbonic acid in water

Question 5

Carbon Dioxide Reaction in body

Answer 5

1. CO2 + H2O H2CO3 H+ + HCO3 (bicarbonate)
   In acidic environment, carbonic acid will give up proton
2. H+ + HCO3- —> 2 H+ + CO3 -2 (carbonate)
   In basic enough environment, bicarbonate will release protons to make carbonate

Question 6

Upon reacting with water, carbon dioxide does what?

Answer 6

1. Goes from volatile gas to a non volatile carbonate form

2. Note that CO2 can form 2 different anions

Question 7

Non Volatile Carbonate Salts

Answer 7

1. The basis of irreversible carbon dioxide absorption is its ability to react with water to ultimately form non volatile carbons
2. Non volatile carbonate salts can be formed with various cations
   Examples:
   - Na2CO3 (reversible)
   -CaCO3 (irreversible)
   - BaCO3 (reversible)

Question 8

Soda Lime Reactions (2)

Answer 8

1. 2 H2CO3 + 2 NaOH + 2 KOH —> Na2CO3 + K2CO2 + 4 H2O + Heat
2. 2 Ca (OH)2 + Na2CO3 + K2CO3 –> 2 CaCO3 + 2 NaOH + 2 KOH + Heat
   - CO2 reaction with Ca(OH)2 is slow, but it is faster with NaOH
   - Calcium Hydroxide forms Calcium Bicarbonate with is irreversible*

Question 9

Role of water in CO2 absorption

Answer 9

1. Because water is critical to absorption, decreased CO2 absorption occurs below a water content of 12%
2. Decreased absorption also occurs above 20% water content

Question 10

Temperature and CO2 absorption

Answer 10

1. The core of soda lime canisters usually reaches temperatures of 40-45 C. Sides are cooler
2. Do not use temperature as guide though*

Question 11

Reactions of Carbon Dioxide with Ba(OH)2 in baralyme:

Answer 11

1. Not used much in this country
2. Ba(OH)2 is in the form OF octahedral even in baralyme.
3. Baralyme may perform better in drier conditions

Question 12

Composition of Soda Lime (Sodasorb)

Answer 12

1. Calcium Hydroxide: 79%
2. Sodium Hydroxide: 1.2%
3. Potassium Hydroxide: 2.3 (activator)
4. Moisture content: 13.6%
5. Indicator (ethyl violet): 0.06%
   Plus small amounts of silica to form Ca++ and Na+ silicates for hardness and binding*

Question 13

Composition of Baralyme

Answer 13

1. Calcium Hydroxide: 69%
2. Barium Hydroxide: 10.4%
3. Potassium Hydroxide: 4.6%
4. Water: 13.7%
5. Indicator (ethyl violet): 0.15%
   * also contains silicates for hardness. Baralyme is more dense than Soda Lime and 15% less efficient, releases more water

Question 14

Small facts about Sodalime label

Answer 14

1. Pretty basic actually
2. Reacts with skin a little bit, will want to wash your hands after contact (will become soapy and slippery
3. Prolonged exposure to direct UV light may deplete the ethylviolet indicator

Question 15

CO2 absorption characteristics of soda lime

Answer 15

1. In a properly packed canister, 15L of CO2 will be absorbed per 100 g of soda lime, before exit gas exceeds 1% CO2.
2. For an 8 hour operation, approximately 1 kg of sodalime is needed

Question 16

Factors that can reduce absorption efficiency:

Answer 16

1. Channeling (uneven flow through soda lime)- all gas goes to one side and not the other, gas goes towards big particles because there’s less resistance
2. High gas flows, decreased CO2 residency time
3. Caking- another form of channeling, particles stick together
4. Decreased moisture
   * newer absorbants are better- minimize reactions with anestheticsi

Question 17

Newer CO2 absorbent: Amsorb

Answer 17

1. Greatly decreases sevo and des breakdown
2. Accomplishes this by not containing the strong bases NaOh and KOH
3. Consists of calcium hydroxide with a compatible humectant, calcium chloride. The absorbent mixture does not contain sodium or potassium hydroxide, but includes two setting agents (calcium sulfate and polyvinylpyrrolidine) to improve hardness and porosity
4. Sodasorb LF (low flow) also does NOT contain NaOH**

Question 18

Size of granules:

Answer 18

1. Size of determined experimentally and is a function of resistant to air flow and absorptive efficiency
2. Sodalime and baralyme granules are between 4 and 8 mesh

Question 19

Mesh:

Answer 19

1. Number of openings per linear inch in a sieve through which the particles can pass
   A- 4-Mesh (four 1/4th inch openings per inch)
   B- 8- Mesh (eight 1/8th inch openings per inch)
2. 4-8 mesh allows for maximum surface area and exposure, and low flow resistance*

Question 20

Shape:

Answer 20

1. To increase surface area, and thus absorption efficiency,
   Granules are made porous and irregular shaped. New granules are cylindrical in shape
2. Silica is added to Sodalime to make granules harder and to reduce DUST formation

Question 21

Irregularity vs Cylindral granules

Answer 21

1. Irregularity works to your favor: more twists and turns= more absorption of CO2 capacity

Question 22

Dye Indicator

Answer 22

1. Ethyl violet is used in sodalime and baralyme
2. Critical pH of ethyl violet is 10.3
3. Above 10.3 the dye is colorless, below dye turns purple
4. pH drops as the soda lime is exhausted because the very basic NaOH is depleted

Question 23

Color Change:

Answer 23

1. CO2 absorption is a chemical reaction: removes CO2 from the gas flowing around granules in the canister. Reaction produces heat and moisture
2. Exhaustion of CO2: absorber no longer is able to absorb CO2, indicated by color change (to violet)
   Indicated by increasing ETCO2 monitoring level (>.5% ETCO2)
3. Color change requires moisture: absorbent can dry out (desiccate) if: exposed to constant gas flow for long periods of time, exposed to substantial heat (color change can reverse making it appear like absorbent is unused)

Question 24

Why circuits get wet?

Answer 24

1. Average adult exhales CO2= 200 ml/min at rest

2. When absorbed, this CO2 produces 8.6 ml of H2O/hr or in 8 hours of use= 70 ml of H20

Question 25

Potential Deactivation of ethyl violet: (4)

Answer 25

1. Fluorescent lights can slowly deactivate ethyl violet
2. Intense ultraviolet light can reactive ethyl violet
3. Ethyl violet can deactivate over long periods of time. Oxygen flushes will allow oxidation of the dye. Oxygen flush, in addition to oxidizing the dye will dehydrate the absorbent
4. Severe moisture loss will deplete effectiveness of the dye

Question 26

Color reversion:

Answer 26

1. After setting for several hours following use, subsurface Ca(OH2) may allow dye to revert to colorless. This is due to granule surface pH increasing > 10.3
2. This may occur when soda lime is near exhaustion. Soda lime does not regenerate

Question 27

Color change duration

Answer 27

1. Color change intensity fades, but color is still clearly visible after 12 hours
2. Color can remain visible in the canister for months- color change is long lasting

Question 28

Canister Temperature:

Answer 28

1. As noted, absorption is accompanied by heat. However, do not rely on the heat of the soda lime canister as an indicator of soda lime effectiveness.
2. Localized heat is dependent not only generation, but gas flow rate, temperature of the OR, etc. Canister may remain warm even after soda lime is exhausted

Question 29

Unwanted reactions of anesthetics with carbon dioxide absorbents:

Answer 29

1. Volatile fluoridated anesthetics can react with carbon dioxide absorbents to result in:
   - carbon monoxide formation (particularly w/ desflurane)
   - degradation products for sevo, i.e. Compound A, B, C, D and E, but mostly A
   - Extreme heat and fire has been reported to occur (particularly with sevo and baralyme)

Question 30

Amsorb does not contain the strong bases (what 2)?

Answer 30

1. NaOH

2. KOH