Thank you @Akimbo; during general anesthesia, a semi circuit system is typically used. Gas given to the patient typically consists of oxygen mixed with air and the anesthetic vapor. To preserve resources, the air is recirculated and similar to a rebreather, the carbon dioxide is scrubbed out. The carbon dioxide concentration is monitored closely (both when it enters the lungs and when it is exhaled). The absorber of carbon dioxide undergoes a chemical reaction when it picks up the exhaled carbon dioxide. The color changes when the absorber is exhausted. A good byproduct of this chemical reaction is generation of heat which albeit low helps prevent some heat loss in the anesthetized patient.The US Navy used a brand of Barium Hydroxide as an CO2 absorbent before the mid-1970s. It changed from a pink color to purple when it converted into Barium Carbonate. I'm not recommending it as a superior absorbent but I wonder if the feature is available on some modern absorbents.
It seems like it would as useful on an anesthesiology machine as a rebreather.
Ping @rsingler and @Compressor
Hopefully the questions are answered appropriately.
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