carbon dioxide absorption
The hallmark of a circle anesthesia circuit is rebreathing. Of course, a circle circuit is not a true circle, but it is a continuous path recycling the gases and anesthetic vapors you have chosen by your flowmeters and vaporizers. The only things not part of the recycling process are what comes in from the fresh gas inlet and what goes out to the scavenger system. How much is recycled depends on your fresh gas flow (FGF; described in more detail in another section).
The three important things in a circle system are the two unidirectional valves (one inspiratory, one expiratory) and the carbon dioxide absorber. At low or normal FGF rates, lack of a means to get rid of CO2 would quickly lead to rebreathing and hypercarbia, just like breathing into a paper bag. This chapter will describe how the carbon dioxide absorption system in an anesthesia machine works.
A BRIEF HISTORY
Similar to many technical innovations, carbon dioxide absorption capability is a result of exploration and military advances. In this case, mining, underwater exploration, and submarine development were the catalysts for developing a reliable way to “scrub” CO2 out of a closed environment. Various means of rebreathing have been described since the 1700s, and by the late 19th century, soda lime was in use as an absorbent. The Davis Escape Set was invented and produced at the turn of the 20th century as a means of rescue from disabled submarines. It was a self-contained vest and mask system containing oxygen and barium hydroxide. By 1912, Draeger (yes, that Draeger) was mass producing rebreathers for German navy divers. In 1923, the pioneering anesthesiologist Dr. Ralph Waters incorporated a CO2 absorption system into an anesthesia circuit, beginning the practice of low-flow anesthesia and reuse of agent with the patient rebreathing his own scrubbed exhalations. Since then, circle anesthesia systems and rebreathing have become the most commonly used means of delivery of inhalational anesthetics.
The typical anesthesia machine absorber is a clear plastic canister or collection of canisters of varying sizes but at least of the same volume as a patient’s typical tidal volume. The canister is full of some type of alkaline absorbent granules. The canisters are incorporated into the ventilatory apparatus of the anesthesia machine between the expiratory and inspiratory limbs of a circle circuit. The setup is located as to be visible to the anesthesia provider because visual inspection of its status and operation is important. At expiration, that expired volume travels through the absorber canister and is exposed to the absorbent granules, where the CO2 is converted to carbonic acid and then to carbonate, water, and heat. So, in actuality it is a simple conversion of a base to an acid.
In some designs, the expired gas enters the absorber canister from the top and travels to the bottom before being piped back up into the rest of the ventilatory apparatus. In other designs, the expired gas travels down the middle of the canister in a center tube and then flows upward through the granules (Figure 9-1). This means that in some designs, the purple, exhausted granules will be on the top of the canisters or at the bottom of the canister (Figures 9-2 and 9-3).
Figure 9-2 Datex-Ohmeda stacked canisters showing depleted (purple) granules on top; expired gas travels down through canisters and then back up to the circuit.