Carbon Dioxide Absorbers

Introduction

Overview

• Circle systems were designed to reduce waste by allowing expired gas to be rebreathed. This requires removing CO₂, which is achieved using a CO₂ absorber.
• In an anesthetic circuit, CO₂ absorbers are usually placed between the reservoir bag and the FGF inlet (Figure 1).

Composition

• Traditional CO₂ absorbents consisted of calcium hydroxide combined with strong bases such as sodium hydroxide and potassium hydroxide (soda lime).¹
• These formulations were found to produce toxic degradation products upon exposure to volatile anesthetics: compound A with sevoflurane and CO with desflurane.^1,2
• Modern absorbents have been reformulated to minimize the presence of strong bases, containing little to no sodium hydroxide (typically < 2%) and no potassium hydroxide, thereby eliminating concerns about compound A and CO production.¹
• For example, Amsorb® consists of calcium hydroxide with calcium chloride as a humectant, plus calcium sulfate and polyvinylpyrrolidine as setting agents to improve hardness and porosity. It contains no sodium or potassium hydroxide.²
• When exposed to sevoflurane, desflurane, isoflurane, or enflurane, this formulation produces negligible amounts of compound A and CO, even when dehydrated.⁸

Mechanism

• CO₂ reacts with water to form carbonic acid. Carbonic acid subsequently undergoes a neutralization reaction with metal hydroxides present in the absorbent, producing water, carbonate or bicarbonate salts, and heat (Figure 2).
• Historically, absorbents consisted of calcium hydroxide combined with strong bases to enhance the speed of the chemical reaction mentioned above.¹
• Following the introduction of sevoflurane and desflurane, toxic byproducts, including compound A (from sevoflurane) and CO (from desflurane, enflurane, and isoflurane), were identified as products of anesthetic degradation in the presence of a strong base.
• Elimination of KOH and reduction of NaOH concentration to less than 2% significantly reduce or eliminate compound A and CO production.¹

Clinical Design

Canister Design

• Single-chamber canisters are the predominant design in modern anesthesia workstations, with capacities typically ranging from 750 mL to 1,350 mL.
• Exhaled gas enters at the bottom of the canister and flows upward through the absorbent granules, with CO₂ absorption occurring progressively from bottom to top.
• The relative efficacy of CO₂ absorption varies across the single-chamber canister, with the highest absorption occurring in the lower portion, where fresh exhaled gas first contacts the absorbent.⁹
• Channeling occurs when gas preferentially flows through paths of least resistance rather than uniformly through the absorbent bed, reducing contact time and absorption efficiency.¹
• Factors contributing to channeling include improper granule packing, settling during transport, and vibration during use.
• Granule size optimization (typically 4-8 mesh) balances surface area for absorption against resistance to airflow and channeling risk.¹
• Transparent canister walls allow continuous visual monitoring of absorbent color change and exhaustion.

Absorbents

• CO₂ absorbents vary in chemical composition, safety profile, and efficiency (Table 1).
• Factors affecting efficiency include granule size, surface area, moisture content, and chemical composition.
• An ideal absorbent is characterized by high CO₂ absorptive efficiency, low airflow resistance, minimal toxicity, lack of reactivity with volatile anesthetics, and low cost.
• Soda Lime
  • Soda lime contains calcium hydroxide with added sodium hydroxide and potassium hydroxide.
  • These strong bases increase the risk of anesthetic degradation and toxic byproduct formation when the absorbent becomes desiccated.
  • A pH-sensitive color indicator, ethyl violet, is incorporated into soda lime.
  • As CO₂ accumulates and pH decreases, the indicator changes color from white to purple, signaling exhaustion of absorptive capacity.⁵
• Amsorb®
  • Amsorb® is a modern calcium hydroxide-based absorbent composed primarily of calcium hydroxide with calcium chloride (humectant), calcium sulfate, polyvinylpyrrolidone, and water.³
  • Strong bases are absent, minimizing CO and Compound A formation, which makes Amsorb® preferable for low-flow anesthesia and prolonged cases.³

Performance Characteristics

• Toxic byproduct formation
  • With dehydrated traditional soda lime, exposure to desflurane, enflurane, and isoflurane produces CO concentrations of 580-620 ppm.³
  • In contrast, Amsorb® produced minimal to no detectable CO and only small concentrations of compound A regardless of hydration status.³
• Absorption capacity
  • Absorption capacity varies substantially by formulation, but modern absorbents can absorb at least 35% of their weight in CO₂.⁷
  • Amsorb® has approximately half the CO² absorption capacity of traditional soda lime (5.5-7.6 L/100g versus 10.7-14.8 L/100g).⁸
• Longevity
  • In clinical low-flow anesthesia (1 L/min), Amsorb®’s longevity was 213-218 minutes compared to 445-538 minutes for soda lime formulations, which represents 40-50% of the duration.²
  • At higher FGFs (2 L/min), Amsorb® lasted 127 minutes versus 267-321 minutes for soda lime products.²
  • This translates to requiring 2-2.5 times more frequent canister changes with Amsorb® compared to traditional soda lime.
• Total cost
  • Cost-effectiveness must consider not only the purchase price per canister but also the total cost of ownership, including replacement frequency, volatile anesthetic savings from low-flow capability, and operational costs.
  • Even with Amsorb®’s higher cost and lower capacity, the total cost (absorbent + volatile agent) decreases as FGF decreases, making ultra-low-flow anesthesia economically favorable.⁷

Anesthetic Considerations

• Desiccation of CO₂ absorbents increases the risk of toxic byproduct formation with volatile anesthetics and represents the most significant safety concern with CO₂ absorbers.²
  • When absorbents become desiccated through prolonged FGF exposure (particularly at high flow rates of 10 L/min for 24-48 hours), they can produce dangerous levels of CO and other toxic byproducts when exposed to volatile anesthetics.^8-9
  • With traditional soda lime, desiccated absorbent exposed to desflurane can generate circuit CO concentrations of 8,800-13,600 ppm, resulting in carboxyhemoglobin levels exceeding 70%.⁸
  • All measures should be taken to prevent accidental drying out of the absorbent, including turning off FGF when the machine is not in use and replacing absorbents that have been exposed to prolonged gas flow without patient use.⁹
  • Rehydration of desiccated absorbent is an effective means of reducing CO formation and the risk of intraoperative CO poisoning.²
• Absorbents should be changed based on the inspired CO₂ concentration rather than arbitrary time intervals or color change alone.¹
  • Color change may occur prematurely (leading to wasteful early replacement) or may fail to appear despite absorbent exhaustion (creating patient safety risk).
  • One study found that the time required for 1 kg of fresh soda lime to reach 0.7 kPa of inspired CO₂ ranged from 1,270 to 1,978 minutes, depending on formulation, despite similar color-change patterns.
  • Once inspired CO₂ reaches 3 mm Hg (0.4 kPa) for at least 3 consecutive minutes, the absorbent typically becomes exhausted within 95 minutes in most canisters.⁴
  • In research settings, a threshold of FiCO₂ ≥ 0.5 (approximately 4 mmHg) sustained for 5 minutes is commonly used as the operational definition of absorbent exhaustion.⁵
  • In clinical practice, monitoring for any sustained rise in inspired CO₂ above baseline (typically 0 mmHg) signals impending absorbent failure and is the most reliable indicator of absorbent exhaustion.⁶
  • Absorbent management should include primarily continuous inspired CO₂ monitoring, visual assessment of color change as a secondary indicator, and mathematical models for predictive planning.³
• Modern absorbent formulations without strong bases enable safer low-flow anesthesia, thereby reducing volatile anesthetic consumption and greenhouse gas emissions in the operating room.
  • Healthcare contributes 4.6-4.7% of total greenhouse gas emissions, with anesthesia representing a disproportionately large component of this footprint.¹
  • Traditional soda lime produces compound A and CO, particularly when desiccated, thereby limiting the use of minimal FGFs.^2-4
  • Newer formulations, such as Amsorb®, produce negligible amounts of compound A and CO, enabling low-flow techniques that substantially reduce volatile anesthetic consumption.^3-4
  • Low-flow anesthesia (FGFs ≤ 1L/min) reduces volatile anesthetic consumption by approximately 60-75%, directly decreasing the amount of potent greenhouse gases released into the atmosphere.⁶
  • Volatile anesthetics are potent greenhouse gases with varying atmospheric lifetimes: sevoflurane (1-5 years), isoflurane (3-6 years), desflurane (9-21 years), and nitrous oxide (114 years).¹⁰
  • Desflurane has a global warming potential approximately 2,500 times that of CO₂, while sevoflurane’s is approximately 130-195 times that of CO₂.
  • Desflurane accounts for 74-80% of anesthesia-related greenhouse gas emissions despite representing a smaller proportion of use.¹⁰
  • By enabling ultra-low-flow anesthesia without the risk of toxic byproduct formation, modern absorbents allow clinicians to implement environmentally sustainable practices that reduce operating room emissions by 60-75%.¹⁰
• Modern practice emphasizes reducing FGF to approach closed-circuit conditions while monitoring inspired CO₂ levels.¹
  • This approach minimizes both anesthetic agent waste and environmental impact while maintaining patient safety.