Soda lime chemistry and co₂ absorption

How CO₂ is absorbed (mechanism + key reactions)

CO₂ absorption is a two-stage process: (1) CO₂ hydration to carbonic acid in the presence of water, then (2) neutralisation by alkali hydroxides to carbonates + heat.
- Step 1 (requires water): CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid).
- Step 2 (neutralisation): H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O + heat (similar with KOH).
- Regeneration step: Na₂CO₃ + Ca(OH)₂ → CaCO₃ + 2NaOH (NaOH acts as a catalyst and is regenerated).
Net effect: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O + heat (overall reaction often quoted).
The reaction is exothermic and produces water; temperature and humidity rise across the absorber during use.

Composition and physical design (what matters clinically)

Typical soda lime constituents (approximate): Ca(OH)₂ ~80%, H₂O ~15–20%, NaOH ~2–5%, KOH 0–2% (modern formulations often minimise/omit KOH), silica/hardener small amount, indicator dye.
Granule size: designed to maximise surface area while limiting resistance and channelling (commonly 4–8 mesh).
Absorber canister design aims for even gas flow to reduce channelling; packing too loosely or too tightly impairs performance.

Determinants of CO₂ absorption capacity and failure

Key determinants: absorbent alkali content, water content, temperature, fresh gas flow patterns, minute ventilation, and presence of channelling.
Dry absorbent: reduced CO₂ absorption efficiency and increased risk of toxic by-products with volatile agents (especially carbon monoxide and compound A).
End of life signs: inspired CO₂ rise, reduced temperature gradient across canister, exhausted indicator (but beware indicator limitations).

Indicator dyes (colour change and limitations)

Indicator changes colour with pH drop as hydroxides are consumed (e.g., ethyl violet: white → purple; other dyes exist).
Colour reversion: exhausted absorbent may revert back to original colour after resting (CO₂ desorbs and pH transiently rises) but capacity is not restored.
Indicator may be unreliable with desiccated absorbent, some modern formulations, or uneven gas flow (patchy colour change).

Heat, water, and circuit effects

Exothermic reaction warms inspired gases; water production humidifies gases, reducing airway drying compared with non-rebreathing systems.
Water can condense in the circuit and absorber; manage to reduce resistance/valve sticking and infection control concerns.

Toxic by-products and anaesthetic interactions

Carbon monoxide production: occurs when volatile agents contact desiccated absorbents, historically greater with strong base content (NaOH/KOH) and certain agents (classically desflurane/enflurane/isoflurane > halothane; modern practice emphasises risk with dry absorbent rather than agent ranking).
- Clinical clues: unexpectedly high carboxyhaemoglobin, pulse oximetry overestimation of SpO₂, metabolic acidosis, unexplained tachycardia.
Compound A (sevoflurane degradation): increased with low fresh gas flows, high absorber temperatures, and strong bases; associated with renal toxicity in animals (human relevance low with modern absorbents and recommended practices).
Modern absorbents: aim to reduce/omit strong bases (NaOH/KOH) to reduce CO and compound A formation (e.g., calcium hydroxide-based absorbents).

Capacity and practical numbers (useful in vivas)

CO₂ production in adults: typically ~200 mL/min at rest (higher with fever, sepsis, pregnancy, paediatrics).
Absorbent capacity depends on formulation and conditions; a rough working estimate is that 1 kg soda lime can absorb on the order of ~100–150 L CO₂ (varies widely).
Practical implication: absorber exhaustion is usually detected clinically by inspired CO₂ rise rather than time alone; always interpret with ventilation settings and capnography.

Failure modes and troubleshooting (capnography-led)

Rising inspired CO₂ (FiCO₂) suggests rebreathing: causes include exhausted absorbent, incompetent unidirectional valves, inadequate fresh gas flow for the system, or circuit misassembly.
Absorber-related causes: exhausted soda lime, channelling, desiccation, incorrect canister seating/bypass leaks.
Immediate actions: increase fresh gas flow, switch to non-rebreathing mode/system if possible, change absorbent, check valves and canister seals, confirm capnograph sampling integrity.

Describe the chemistry of CO₂ absorption by soda lime.

Aim: show the two-stage mechanism and the role of water and catalysts.

CO₂ must first dissolve in water: CO₂ + H₂O ⇌ H₂CO₃.
Carbonic acid is neutralised by strong bases: H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O + heat (similar with KOH).
Na₂CO₃ then reacts with Ca(OH)₂: Na₂CO₃ + Ca(OH)₂ → CaCO₃ + 2NaOH (regenerates NaOH).
Overall: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O + heat.

Why is water important for CO₂ absorption, and what happens if the absorbent is dry?

Common FRCA viva: link hydration step to performance and toxicity.

Water is required for CO₂ hydration to carbonic acid, enabling the neutralisation reactions.
Dry absorbent reduces effective CO₂ absorption and increases degradation of volatile agents.
Desiccated absorbent increases risk of carbon monoxide formation and (with sevoflurane) compound A formation.

What is the composition of soda lime and what is the function of each component?

Expect approximate percentages and roles (absorber, catalyst, water, hardener, indicator).

Ca(OH)₂: main CO₂ absorbent (forms CaCO₃).
NaOH (and sometimes KOH): accelerates reaction (catalytic role; regenerated).
Water: essential for CO₂ hydration; supports reaction kinetics and reduces volatile degradation when appropriately hydrated.
Silica/hardener: reduces dusting and improves granule integrity.
Indicator dye: pH-dependent colour change to suggest exhaustion (with limitations).

Explain how soda lime can become exhausted and how you would detect it clinically.

Focus on capnography and limitations of colour indicators.

Exhaustion occurs as hydroxides are consumed and the absorbent becomes predominantly carbonate (reduced capacity).
Primary clinical sign: rising inspired CO₂ (FiCO₂) on capnography; may see rising ETCO₂ despite adequate ventilation.
Canister temperature gradient reduces as reaction slows/exhausts.
Indicator colour change may be patchy; colour reversion can mislead after rest.

A patient on a circle system develops inspired CO₂. Give a structured differential diagnosis and immediate management.

Often asked as a troubleshooting viva. Use a system-based approach.

Differential: exhausted absorbent; incompetent inspiratory/expiratory unidirectional valves; inadequate fresh gas flow for the system; circuit misassembly or leaks causing bypass; faulty capnograph sampling.
Immediate management: increase fresh gas flow; consider switching to non-rebreathing; deepen anaesthesia/ensure ventilation; check valves movement and seating; check absorber canister seals; replace soda lime.

What is channelling in a CO₂ absorber and why does it matter?

Links physical packing to functional failure.

Channelling is preferential gas flow through low-resistance pathways in the granules, reducing contact with absorbent.
It causes early breakthrough of CO₂ (FiCO₂ rise) despite apparently normal indicator colour in other regions.
Risk factors: incorrect packing, vibration/settling, uneven granule size, canister design issues.

Describe the interaction between sevoflurane and soda lime. What is compound A and when is it produced?

Classic FRCA topic: degradation products and risk reduction.

Sevoflurane can degrade in CO₂ absorbents to produce compound A (fluorinated vinyl ether).
Production increases with low fresh gas flows, higher absorber temperatures, and absorbents containing strong bases (NaOH/KOH), and with dry absorbent.
Clinical relevance: nephrotoxicity demonstrated in animals; human risk is low with modern absorbents and adherence to recommended practice.

How can carbon monoxide be produced in an anaesthetic circuit and what are the clinical implications?

Examined as a patient safety scenario.

CO can be produced when volatile agents contact desiccated absorbents, especially those containing strong bases (NaOH/KOH).
Clinical implications: COHb formation; pulse oximetry may overestimate oxygenation; risk of tissue hypoxia and myocardial ischaemia.
Management: discontinue volatile, increase FiO₂ to 1.0, change absorbent, consider COHb measurement/co-oximetry; supportive care and consider hyperbaric oxygen depending on severity.

What are the limitations of soda lime colour indicators?

Frequently tested as a ‘trap’ question.

Colour reversion after rest can make exhausted absorbent appear fresh without restoring capacity.
Patchy colour change with channelling or uneven flow; parts of the canister may remain unreacted while CO₂ breaks through.
Indicator performance depends on moisture and formulation; do not rely on colour alone—use capnography and clinical signs.

Give a rough estimate of CO₂ production and how this relates to absorber life in low-flow anaesthesia.

Tests ability to combine physiology with equipment.

Typical adult CO₂ production ~200 mL/min at rest; higher in hypermetabolic states.
Absorber life depends on absorbent mass, formulation, and channelling; use inspired CO₂ as the practical endpoint rather than time alone.
Low-flow techniques increase reliance on the absorber; ensure absorbent is fresh, correctly packed, and not desiccated.