CO2 Absorber in Anaesthesia - 20-Mark Answer

Introduction

• Barash Clinical Anesthesia 9e, p.2003

Definition

The Absorber Canister - Physical Description

• Double canisters in series - arranged to ensure complete CO2 absorption, require less frequent changes, and lower gas flow resistance
• Head plate and base plate - enclose the granules
• Transparent walls - allow visual monitoring of indicator dye colour change
• Baffle system - directs gas through the centre to minimise channeling through loosely packed areas, ensuring greater use of the absorbent
• Dust trap at the base - collects dust and moisture
• Lever release mechanism - for canister removal/replacement

• Morgan & Mikhail Clinical Anesthesiology 7e, p.92-93

Types of CO2 Absorbents

1. Soda Lime (Most Commonly Used)

• Small amounts of silica are added to form calcium and sodium silicate, producing a harder, more stable pellet that reduces dust formation
• NaOH acts as the catalyst for CO2-absorptive properties
• KOH acts as an activator

• Smaller granules = greater surface area = better absorption
• Smaller granules = higher resistance to gas flow

• Barash Clinical Anesthesia 9e, p.2003-2004

2. Calcium Hydroxide Lime (Amsorb / Amsorb Plus)

• Calcium hydroxide Ca(OH)₂ (primary component)
• Calcium chloride CaCl₂
• Calcium sulfate (hardening agent)
• Polyvinylpyrrolidone (increases hardness and porosity)

• Barash Clinical Anesthesia 9e, p.2004

3. Baralyme (Barium Hydroxide Lime)

• ~20% barium hydroxide Ba(OH)₂
• Calcium hydroxide
• Indicator dyes: Mimosa Z and ethyl violet (initially pink, turns blue-gray when exhausted)
• Efficiency is comparable to soda lime per unit volume, but because of its higher density, it is only half as efficient per unit mass
• Produces more heat than soda lime (highly exothermic)
• Largely withdrawn from clinical use due to higher risk of toxic compound formation

4. Lithium Hydroxide (LiOH) - Litholyme

• Lower exothermic reactivity → reduced fire risk
• Does not degrade volatile anaesthetics
• Higher cost, but allows lower fresh gas flows with sevoflurane, offsetting the cost advantage
• Available outside the US; limited availability in some markets

5. SpiraLith Ca (Micropore Inc.)

• Supplied on a polymer matrix base, rolled as a fixed spiral in a cylinder
• Contains no lithium or indicator dye - requires inspired CO2 monitoring
• Advantage: Exhausted absorbent can be recycled by the manufacturer
• Barash Clinical Anesthesia 9e, p.2010

Chemistry of CO2 Absorption

CO₂ + H₂O → H₂CO₃

H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O H₂CO₃ + 2KOH → K₂CO₃ + 2H₂O

Na₂CO₃ + Ca(OH)₂ → CaCO₃ + 2NaOH K₂CO₃ + Ca(OH)₂ → CaCO₃ + 2KOH

• Barash Clinical Anesthesia 9e, p.2003-2004; Morgan & Mikhail 7e, p.91

Indicator Dyes

Toxic Byproducts - A Critical Safety Consideration

Compound A

• Produced from sevoflurane degradation by soda lime (especially desiccated absorbent with strong bases)
• Nephrotoxic in certain animals; not conclusively shown to be harmful in humans at clinical concentrations
• Risk increased by: higher sevoflurane concentrations, prolonged exposure, low fresh gas flows, dry absorbent
• Absent or markedly reduced with calcium hydroxide lime (Amsorb)

Carbon Monoxide (CO)

• Produced when desflurane, enflurane, isoflurane (less so sevoflurane) degrade in desiccated absorbent containing strong bases (NaOH/KOH)
• Greatest risk with desflurane; occurs at higher temperature with sevoflurane
• Can cause clinically measurable carboxyhemoglobin concentrations
• Virtually eliminated by using strong-base-free absorbents (Amsorb, Litholyme)
• Prevention: Never allow fresh gas to flow continuously through unused absorber canisters (desiccates the absorbent)
• Morgan & Mikhail 7e, p.92; APSF guidelines

Signs of CO2 Absorbent Exhaustion

1. Colour change of indicator dye
2. Rising inspired CO2 on capnograph (most reliable)
3. Canister feels warm to touch (exothermic reaction ongoing) - or absence of warmth when exhausted
4. Patient develops signs of hypercapnia (tachycardia, hypertension, increased respiratory effort, arrhythmias)

Ideal Properties of a CO2 Absorbent

1. High CO2 absorptive efficiency
2. Low resistance to gas flow
3. No reactivity with common anaesthetic agents
4. No toxicity to patients or staff
5. Ease of handling
6. Low cost
7. Reliable indicator of exhaustion
8. Adequate moisture content (prevents desiccation and toxic byproduct formation)

• Barash Clinical Anesthesia 9e, p.2003

Advantages of CO2 Absorbers

1. Prevention of Hypercapnia

2. Enables Low Fresh Gas Flow (LFF) Anaesthesia

3. Significant Cost Reduction

4. Reduction of Environmental Pollution

5. Humidification and Warming of Inspired Gases

• Reduces heat loss from the respiratory tract
• Decreases the risk of postoperative hypothermia
• Helps maintain mucociliary function

6. Conservation of Oxygen

7. Reduced Pollution of the Operating Theatre Environment

8. Enables Closed-Circuit Anaesthesia

9. Maintains Anaesthetic Depth Stability

10. Indicator Function

Precautions and Disadvantages

Summary Table: Comparison of CO2 Absorbents

Clinical Bottom Line

• Barash, Cullen, and Stoelting's Clinical Anesthesia 9th edition, Chapter 25 - Anaesthesia Equipment (pp. 2003-2010)
• Morgan & Mikhail's Clinical Anesthesiology 7th edition, Chapter 3 - The Breathing Circuit (pp. 91-94)
• OpenAnesthesia: Carbon Dioxide Absorbers (current clinical guidelines)
• APSF: Low Flow and CO2 Absorbents (patient safety guidelines)

Soda Lime, Baralyme, and Amsorb - 10-Mark Answer

Introduction

1. Soda Lime

Composition (High-Moisture formulation)

• Silica forms calcium and sodium silicate, producing harder, more stable pellets with less dust
• Hardness and absorptive efficiency vary inversely - so minimal silica is used in contemporary preparations
• Granule size: 4-8 mesh - optimal balance between surface area (absorption) and resistance to gas flow

Chemical Reactions

• NaOH/KOH are regenerated (act as true catalysts)
• Final product: Calcium carbonate (CaCO₃)
• Absorbent is exhausted when all hydroxide has been converted to carbonate
• Some CO2 reacts directly with Ca(OH)₂ but this is a much slower reaction

Absorptive Capacity

• 26 litres of CO₂ per 100 g of absorbent (can absorb ~19% of its weight in CO2)

Indicator Dye

• Ethyl violet - colourless/white when fresh → turns violet/purple when exhausted
• Colour can revert to original after a rest period even when absorbent is truly exhausted - capnography remains the primary monitoring tool

Advantages

• Highest absorptive capacity among common absorbents
• Low cost
• Widely available
• Reliable, well-established clinical track record

Disadvantages

• Strong bases (NaOH, KOH) cause degradation of volatile anaesthetics when desiccated:
  • Sevoflurane → Compound A (potentially nephrotoxic, especially in animals)
  • Desflurane/isoflurane/enflurane → Carbon monoxide (CO) when absorbent is desiccated
• Exothermic reaction raises temperature - fire risk with desiccated absorbent
• Dust formation from granule breakdown
• Barash Clinical Anesthesia 9e, p.2003-2005

2. Baralyme (Barium Hydroxide Lime)

Composition

Chemical Reaction

Indicator Dye

• Initially pink → turns blue-gray on exhaustion (due to Mimosa Z + ethyl violet combination)

Absorptive Capacity

• Equal to soda lime per unit volume
• However, because of its greater density, only half as efficient per unit mass compared to soda lime

Advantages

• Efficient per unit volume
• No need for added NaOH/KOH activators (Ba(OH)₂ is itself reactive)
• More robust granules, less dust formation than soda lime

Disadvantages

• More exothermic than soda lime - greater heat generation raises fire risk
• Still contains sufficient alkali to produce Compound A and CO when desiccated, similar to soda lime
• Barium compounds are potentially toxic if granule dust is inhaled or ingested
• Largely withdrawn from clinical use because of its higher fire/explosion risk and greater toxic byproduct formation compared to newer alternatives
• Half as efficient per unit mass (heavier to transport/handle)
• Barash Clinical Anesthesia 9e, p.2004; Sciencedirect CO2 Absorbents review

3. Amsorb (Calcium Hydroxide Lime)

Composition

• Contains NO strong bases (no NaOH, no KOH) - this is its defining and most important feature

Chemical Reaction

Absorptive Capacity

• 10.2 litres of CO₂ per 100 g - approximately 50% less than soda lime
• This is the most significant clinical limitation

Indicator Dye

• Colour-change indicator present; specific formulation varies by manufacturer

Advantages (KEY distinguishing features)

1. No Compound A production - absence of strong bases means sevoflurane is not degraded, making Amsorb safe for low-flow sevoflurane anaesthesia
2. No carbon monoxide production - even when desiccated, does not produce CO from desflurane or other volatile agents
3. Reduced fire risk - lower exothermic reactivity compared to soda lime and baralyme
4. Safer for low-flow and minimal-flow anaesthesia - removing the constraint of toxic byproduct production allows fresh gas flows below 1 L/min
5. Reduced anaesthetic consumption - low-flow techniques save ~50% of volatile agent costs, largely offsetting Amsorb's higher unit price
6. Environmental benefit - lower volatile agent emissions into the atmosphere
7. No desiccation hazard from strong bases

Disadvantages

• ~50% lower absorptive capacity - canisters must be changed more frequently
• Higher cost per unit than soda lime
• Requires larger or more frequent canister changes
• Barash Clinical Anesthesia 9e, p.2003-2004; Morgan & Mikhail 7e, p.92

Comparison Table

Clinical Relevance

• Soda lime remains the most widely used due to low cost and high efficiency, but requires adequate hydration and monitoring for toxic byproducts
• Baralyme is largely obsolete - its combination of high exothermy and greater toxic byproduct formation makes it less safe than alternatives
• Amsorb represents the modern preferred choice where low-flow or minimal-flow anaesthesia is practised, particularly with sevoflurane - the absence of strong bases eliminates the principal safety concerns of CO2 absorbers, and the cost savings from reduced volatile agent use largely offset its higher unit price