CO2 Absorbents in Anaesthesia

Why CO2 Absorbents Are Needed

Ideal Properties of a CO2 Absorbent

• No reactivity with common volatile anaesthetics
• Non-toxic to patients and personnel
• Low resistance to gas flow
• High absorptive capacity
• Low cost and ease of handling
• Reliable indicator of exhaustion

1. Soda Lime (Most Commonly Used)

Composition

• Silica is added to form calcium and sodium silicate, making granules harder and reducing dust. However, efficiency varies inversely with hardness, so only small amounts are used in modern soda lime.
• Granule size: 4-8 mesh (compromise between absorptive surface area and resistance to gas flow - the smaller the granule, the greater the surface area but the higher the resistance).

Chemical Reactions

CO2 + H2O → H2CO3

H2CO3 + 2NaOH → Na2CO3 + 2H2O + Heat

Na2CO3 + Ca(OH)2 → CaCO3 + 2NaOH

Absorptive Capacity

• Up to 26 L of CO2 per 100 g of absorbent (Morgan & Mikhail states 23 L/100 g; Barash states 26 L/100 g)
• Practical/functional capacity is lower due to channelling (gas follows low-resistance paths through the canister)

pH Indicator: Ethyl Violet

• A substituted triphenylmethane dye with a critical pH of 10.3
• Fresh absorbent: pH > 10.3 → ethyl violet is colourless/white
• Exhausted absorbent: pH falls below 10.3 → colour changes to violet/purple
• Replace when 50-70% of the granules have changed colour
• Caution: Prolonged exposure to fluorescent lights causes photoactivation - the dye appears white despite exhaustion (false negative). Capnography is the most sensitive indicator of exhaustion (inspiratory CO2 > zero = suspect exhaustion).
• Granules that revert to white on resting do NOT regain significant absorptive capacity.

Advantages of Soda Lime

• High absorptive capacity (~26 L CO2/100 g)
• Low cost, widely available
• Good efficiency in circle breathing systems
• Reliable for most clinical scenarios

Disadvantages of Soda Lime

• Contains strong bases (NaOH, KOH) - the root of most problems
• Reacts with volatile anaesthetics when desiccated:
  • Desflurane ≥ Enflurane > Isoflurane >> Halothane = Sevoflurane (order of CO production)
  • Desiccated soda lime + desflurane/enflurane/isoflurane → Carbon monoxide → clinically measurable carboxyhaemoglobin
  • Soda lime + sevoflurane → Compound A (fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether) - nephrotoxic in rats but no proven human toxicity at clinical concentrations; worsened by low-flow technique, high concentrations, and fresh absorbent
• Hydroxide salts are irritating to skin and mucous membranes
• Can absorb and later release volatile anaesthetics (especially when dry)
• Desiccation risk: continuous fresh gas flow (FGF) over weekends can desiccate the absorbent without visible colour change

2. Calcium Hydroxide Lime (Amsorb / Amsorb Plus)

Composition

• ~80% Calcium hydroxide Ca(OH)2
• Calcium chloride CaCl2
• Calcium sulfate (hardening agent)
• Polyvinylpyrrolidone (enhances hardness and porosity)
• Water: 13-18%
• No NaOH, No KOH - this is the key distinction

Advantages

• No strong bases → eliminates CO production from desflurane/isoflurane
• Eliminates or greatly reduces Compound A from sevoflurane
• Cannot cause breathing circuit fires (the Baralyme/sevoflurane fire mechanism does not apply)
• Safer for low-flow and closed-circuit anaesthesia
• Permanent, reliable colour change indicator

Disadvantages

• Lower absorptive capacity: ~10.2 L CO2/100 g (approximately 50% less than soda lime)
• Higher cost per unit
• Requires more frequent canister changes

3. Baralyme (Barium Hydroxide Lime) - OBSOLETE

Composition

• ~73% Calcium hydroxide
• Barium hydroxide Ba(OH)2 (~4-5%)
• KOH < 5%
• Water: 11-16%

Why Discontinued (August 2004)

• Desiccated Baralyme + sevoflurane → breathing circuit fires (temperatures of several hundred degrees)
• Combustible degradation by-products (formaldehyde, methanol, formic acid) + O2/N2O-enriched environment = fire
• Produced the highest concentrations of Compound A and CO
• No longer available in the USA or most markets

4. Newer/Modified Absorbents (Low or No Strong-Base Formulations)

• Uses a lithium catalyst for CO2 absorption
• No NaOH or KOH → no CO or Compound A production
• Absorption is minimally exothermic
• Indicator changes permanently from off-white to violet (cannot revert, so exhaustion/desiccation is reliably indicated)

Comparison Table for Exam

APSF 2005 Recommendations (Key for Exams)

1. Turn off all FGF when the machine is not in use
2. Change absorbent on Monday mornings (desiccation over weekends)
3. Change when colour indicator signals exhaustion
4. Change both canisters in a two-canister system
5. Change if FGF was left on for an extensive/indeterminate period
6. Consider more frequent changes with compact single canisters

Clinical Signs of CO2 Absorbent Exhaustion

• Hyperventilation, hypertension, tachycardia (signs of hypercapnia)
• Inspiratory CO2 > 0 on capnography (most sensitive sign)
• Arterial blood gas: respiratory acidosis