Carbon dioxide absorption

Published on 07/02/2015 by admin

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Last modified 22/04/2025

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Carbon dioxide absorption

John M. VanErdewyk, MD

Scientists first began experimenting with substances capable of absorbing carbon dioxide (CO₂)in the early 1900s. Progress was made during World War I, when chemical warfare stimulated research to eliminate CO₂ from the closed breathing system of the gas mask. Today, CO₂ absorption is used daily to remove CO₂ from semiclosed or closed anesthetic circuits.

CO₂ absorbers

An ideal CO₂ absorber would have the following characteristics: efficiency, ease of handling, low resistance to airflow, low cost, lack of toxicity, and lack of reactivity when used with common anesthetics.

The amount of CO₂ that can be absorbed varies depending upon the absorbent. In practical use, the maximum amount is rarely achieved because of factors such as channeling of gas flow around the granules of absorbent. Channeling refers to the preferential passage of exhaled gases through the canister via the pathways of least resistance. Excessive channeling will bypass much of the granule bulk and decrease efficiency. Proper canister design—with screens and baffles plus proper packing—helps decrease channeling (Figure 8-1).

Figure 8-1 The dual-chamber absorber is now the most commonly used type of absorber for containing CO₂ absorbent. This device is permanently mounted, with a vertical gas-flow axis. This positioning eliminates dusting, channeling, and packing problems. Because condensation may collect at the bottom of the chamber, forming a caustic lye solution with the dust, it is important that the unit contains a drain valve. Each absorber design has unique opening, closing, and sealing components; however, most have a single-action clamp mechanism.

A dual-chamber canister is more efficient than a single-chambered canister, and an ideal water content of the absorbent is needed for optimal CO₂ absorption. For the greatest efficiency of absorption, the patient’s entire tidal volume should be accommodated within the void space of the container. Therefore, a properly packed canister should contain approximately one half of its volume in granules and one half as intergranular space.

The greater the surface area available for CO₂ absorption, the greater the absorptive ability. However, as granule size decreases (surface area increases), the resistance to airflow through the canister increases. A compromise has been reached in a granule size of 4 to 8 mesh, which allows good CO₂ absorption with an acceptable resistance to flow.

Benefits

CO₂ absorption is cost effective because it allows lower gas flows and use of less inhalation agent when using a semiclosed or closed circuit to deliver a general anesthetic. The use of lower gas flows decreases pollution in the operating room. In addition, the CO₂ absorber generates both heat and water, which help warm and humidify inspired gases.

Degradation of inhaled anesthetic agents

All CO₂ absorbers utilize Ca(OH)₂ as the primary component. Some absorbents contain various amounts of NaOH and KOH (e.g., sodalime) to serve as catalysts. Unfortunately inhalation anesthetic agents can interact with these catalysts to produce toxic byproducts (compound A, carbon monoxide [CO], methanol, and formaldehyde) and thermal injuries.

Compound A is a vinyl ether degradation product formed from the interaction of sevoflurane (only) with the bases KOH and NaOH (KOH > NaOH). Compound A is produced with either a moist or desiccated CO₂ absorber and is present in any rebreathing circuit utilizing sevoflurane and sodalime. Despite the initial concern of possible renal toxicity, extensive evaluation has found no clinically significant adverse effect on renal function in humans.

Thermal injuries and CO production

All modern inhalation anesthetic agents interacting with a desiccated CO₂ absorber containing KOH or NaOH cause an exothermic reaction and production of CO. CO production is increased with increasing temperature. Desflurane is more likely to result in CO production, as compared with isoflurane, with sevoflurane being the least likely. Other factors related to CO production include the dryness of absorbent and the concentration of the anesthetic agent.

Desiccation of the absorber can occur after a prolonged period of high fresh-gas flow rates, such as occurs when an anesthesia machine is inadvertently left on over a weekend. When an inhalation agent is introduced during the first case on Monday morning, the reaction can occur. Effects of CO can range from subclinical to severe, with carboxyhemoglobin concentrations greater than 30% described in case reports. Signs and symptoms of subclinical CO toxicity are nonspecific, or masked under anesthesia, making diagnosis difficult without a high degree of suspicion; the measurement of arterial blood-gas concentrations with a co-oximeter will document the concentration of carboxyhemoglobin.

Despite the lesser quantity of CO produced, sevoflurane has been proved to generate the most heat. Case reports have described explosions and fire in the CO₂ absorbers secondary to the interaction of sevoflurane with desiccated Baralyme. In experimental settings, sevoflurane has been shown to produce canister temperatures of 400°C, with associated smoldering, melting of plastic, explosion, and fire. Cases of severe heat associated with the use of sodalime have also been reported.

New absorbents

As a result of the reported dangerous reactions, Baralyme was withdrawn from the market in 2004. New CO₂ absorbents (e.g., Amsorb Plus, LoFloSorb, and Dragersorb 800 PLUS) without NaOH or KOH have been introduced. Composed of Ca(OH)₂ with small amounts of CaCl₂ and CaSO₄, they are not known to react with inhalation anesthetic agents or to produce compound A or CO. Thermal injuries should also be markedly reduced or eliminated.

An Anesthesia Patient Safety Foundation conference report recommends the use of a CO₂ absorbent that does not significantly degrade inhalation anesthetic agents or, if conventional CO₂ absorbents are used, that specific policies be instituted to prevent their desiccation.

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