Currently available CO₂ absorbents consist of soda lime (sodium hydroxide lime), barium hydroxide lime, potassium-hydroxide-free lime, calcium hydroxide lime, and non-caustic lime. These absorbents all contain hydroxide bases as their active component. The main reactant in these absorbents (except for in barium hydroxide lime) is calcium hydroxide. Since it is a slow reactant, other constituents are needed to allow the conversion to occur at a sufficient rate in vivo.

Soda Lime Reaction:

The CO₂ absorbent is typically stored in two containers that operate in sequence on the expiratory limb of the anesthetic circuit. They lie distal to the ventilator relief valve and the adjusting pressure limiting (APL) valve. Hence, exhaled gas is preferentially routed to the scavenging system before it is cleared of CO₂. The lower the fresh gas flow, the more rapid the consumption of absorbent.

Because the flow of expired gas is unidirectional, most of the reaction occurs in the proximal chamber. As the reactant is consumed, the pH changes and the indicator in the white granules turns blue. Generally, the first
chamber shows substantial change in color before the second chamber shows any change in color. The second chamber begins to change in color when the amount of available reactant becomes insufficient to eliminate fully the CO₂ at lower flows. It is then necessary to change the reactant or to use higher fresh gas flows.

Caustic water and residue can build up at the base of the absorber and along the distal tubing. To avoid inadvertent obstruction to gas flow, draining the fluid periodically and following routine maintenance recommendations, as provided by the anesthesia machine manufacturer, are necessary.

Channeling is a condition that occurs when the gas flow through the absorbent is diverted through areas of low resistance, causing a ‘channel’ to be formed. Gas flowing through a channel does not undergo the reaction clearing CO₂—hence, absorbents that form channels are less effective. More ominously, absorbent that is exhausted or in which channeling has occurred may permit rebreathing of CO₂ through the circuit, with subsequent patient morbidity.

The issue of channeling is related to that of absorbent pellet size. Air flow through a mass of pellets varies inversely with pellet size; that is, larger pellets generate less resistance through the circuit. At the same time, large pellets expose less total surface area to air flow, decreasing the total surface area available to participate in the reaction that removes CO₂. The most commonly available pellet size for CO₂ absorbents is 4 to 8 mesh. “Mesh” indicates the number of openings per linear inch in a sieve used to measure the pellet particles.

Not all CO₂ rebreathing is related to consumption of CO₂ absorbent. Inspiratory and expiratory valves stuck in the open position can cause rebreathing. Kinked anesthesia hoses can also play a role.