Jeffrey M Feldman, Jan Hendrickx, R Ross Kennedy. Carbon Dioxide Absorption During Inhalation Anesthesia: A Modern Practice. Anesth Analg 2021 Apr 1;132(4):993-1002. PMID: 32947290
One of Ron Litman’s close friends and colleagues at the Children’s Hospital of Philadelphia, Jeff Feldman, has been at the forefront of thinking about and researching how modern anesthesia machines and anesthesia ventilators work. In particular, Jeff has been at the forefront of thinking about fresh gas flows (FGFs), how to prime circuits at the beginning of an anesthetic, how to flush out residual vapor anesthetics from the circuit at the end of an anesthetic, how to minimize waste gas (and their effects on the environment), and now with this article, the carbon dioxide (CO2) absorber when using a circle system.
The CO2 absorber (and circle system) is ubiquitous in modern practice (I think the last time I used a Mapleson circuit to deliver anesthesia was when I was using a precordial stethoscope, or several decades ago!). When working with students, it always amazed me how little they knew about how the CO2 absorber worked and how low the fresh gas flows could go safely (closed-circuit). Indeed, there is an enormous amount of misinformation about this. As Jeff points out repeatedly in this article, sevoflurane and desflurane do NOT degrade into compound A and CO respectively at fresh gas flows less than 2 L/min. Further, there is no evidence that compound A produces nephrotoxicity in humans and modern absorbent chemistry (discussed in length in the article) has virtually eliminated carbon monoxide production in the CO2 absorber when desflurane is used. Indeed, “Modern absorbent formulations that limit or eliminate strong base have made it possible to safely reduce FGF to closed-circuit conditions without concern for toxic byproducts.” The common practice of changing the CO2 absorber every Monday is nonsense. “Modern anesthesia workstations that provide capnography, and the ability to change CO2 absorbents in the middle of an anesthetic, support a clinical practice of changing absorbent based on the appearance of inspired CO2 (rather than the purple color indicator) so that the absorbent does not enter the waste stream until it has been maximally utilized.”
He concludes: “Waste and environmental pollution associated with inhaled anesthetic delivery can be minimized by reducing FGF to closed-circuit conditions (basically 500 mL/min or less!), by selecting absorbent products that are safe and effective at low FGFs, and adopting clinical practices that ensure absorbent is used to the greatest extent possible before it is discarded”.
This is a great review of a ubiquitous part of our anesthesia machines. It should be required reading for all trainees (and maybe old timers like myself as well).
Myron Yaster MD
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