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CO₂ Absorbents — Soda Lime, Amsorb, Compound A
TEXTEquipment II · 9 min read
The chemistry that pulls CO₂ out of rebreathed gas can also generate Compound A and carbon monoxide if you let it dry out.
After this lesson you can
3 min read8 sections- Explain the chemistry of CO₂ absorption.
- Distinguish soda lime vs Amsorb (KOH/NaOH-free).
- Recognize signs of an exhausted absorbent.
- Identify Compound A and CO risk scenarios.
Soda lime chemistry
Exothermic (~22 kJ/mol CO₂).
Capacity ~25 L CO₂ per 100 g absorbent.
Standard granule size 4-8 mesh — balances surface area + airflow resistance.
Color indicator + exhaustion
Indicator can revert white at rest as residual chemistry continues — observing color during active flow is the reliable check.
Quantitative confirmation: inspired CO₂ >0 on capnography = rebreathing = absorbent failure.
Replace when indicator purple OR inspired CO₂ rises.
Don't wait — exhausted absorbent rebreathes within minutes.
Exothermic risks and dehydration
39-45°C; canister hot to touch is normal.Excessive heat (>60°C) suggests dehydrated absorbent + fresh gas continued post-case overnight (dries it further).
Dehydration matters because dry absorbent generates Compound A (sevo) + carbon monoxide (des, iso, enf).
Turn off fresh gas flow at end of case + replace absorbent if dryness suspected (especially first case Monday morning).
Compound A from sevoflurane
FDA labeling recommends sevo FGF ≥1 L/min for cases >1 MAC-hour.
Human nephrotoxicity not demonstrably linked clinically, but the labeling persists.
Workarounds: Amsorb (no strong base) generates negligible Compound A safe at any FGF; or simply maintain ≥1 L/min on standard soda lime.
Carbon monoxide generation
Risk highest Monday morning if fresh gas left flowing weekend (dries absorbent).
- cherry-red mucosa
- elevated COHb on co-oximetry
- masked SpO₂ (CO binds Hgb similarly to O₂ at 660 nm wavelength)
- shut off FGF at end of cases
- monitor for dryness
- use Amsorb/lithium-hydroxide absorbents
Modern alternatives
Therefore no Compound A from sevoflurane, no CO from des/iso, no excessive exothermic concern.
Slightly less CO2 capacity than soda lime replaced more frequently.
Equivalent or better safety profile.
- very high CO2 capacity
- expensive — used in submarine + space applications
- not OR
CARBOLIME + DRÄGERSORB FREE + DRÄGERSORB 800+: variants with reduced or no KOH for safer profile.
Industry moving toward Amsorb-style Ca (OH)2-based absorbents over traditional soda lime in many institutions.
Replacement criteria + how to do it
- indicator turns purple in active flow (more than ~33% of canister)
- inspired CO2 rises above 0 on capnogram
- granules feel hot or wet (severe exhaustion)
- end-of-case routine if usage suggests near-end-of-life
- stop ventilator + fresh gas
- swap canister(s) (most machines have two stacked)
- restart system
- run high FGF flush x 30 sec to clear residual gases
- verify capnogram returns to normal phase I baseline
- recalibrate gas analyzers if needed
DOCUMENT replacement in the equipment log + anesthesia record.
Many institutions use color-coded labels showing date of last replacement.
Practical safety practices
FIRST CASE Monday morning: high suspicion of dry absorbent — verify color + replace if any concern, run high FGF for first 5-10 min as additional safety.
AVOID Baralyme entirely (off market in US since 2004 due to fire risk + worst CO/Compound A profile).
Quarterly inspection of canister seals + integrity.
Document absorbent type used in case record (relevant if patient develops postop renal issues — supports vs argues against compound A/agent toxicity).
⚠ Common pitfalls
- Using sevoflurane at low FGF (<2 L) with desiccated soda lime — Compound A → nephrotoxicity risk.
- Running desiccated absorbent over the weekend at high FGF — CO + heat from des/iso degradation.
- Trusting color change alone — once purple/blue, capacity is mostly gone; check ETCO₂ baseline.
- Forgetting to swap absorbent monthly even with low utilization — desiccation is the silent killer.
💎 Clinical pearls
- Amsorb is KOH/NaOH-free → no Compound A formation + no CO production. Default choice for low-flow.
- If the canister is unusually warm and the patient's ETCO₂ is climbing → exhausted absorbent.
- Always turn off FGF overnight to prevent desiccation; institutional policy varies.
- If you smell smoke or see brown discoloration → STOP and replace circuit + absorbent before continuing.
Recap
- Amsorb is KOH/NaOH-free → no Compound A formation + no CO production. Default choice for low-flow.
- If the canister is unusually warm and the patient's ETCO₂ is climbing → exhausted absorbent.
- Always turn off FGF overnight to prevent desiccation; institutional policy varies.
- If you smell smoke or see brown discoloration → STOP and replace circuit + absorbent before continuing.
Mark each section done to complete the module.
References
- · Stoelting Pharmacology 6e Ch 4
- · Dorsch + Dorsch Understanding Anesthesia Equipment 6e Ch 9
- · FDA Sevoflurane Package Insert