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The safe practice of inhalation anesthesia requires control over the amount of volatile anesthetic delivered to the patient. This is facilitated by continuous monitoring and recording of the agents' concentration (“anestheticography”). Either the vaporizer or the circuit may be considered the “delivery system,” hence anestheticometry performed either in the fresh gas supply or in the inspiratory limb of the system is acceptable. Such monitoring practice may be expected to eliminate anesthesia-related death due to overdose of volatile agents. Besides the aspects of technical safety, anestheticometry is a useful adjunct to teaching and research. It supports a more widespread application of the minimal-flow and closed-circuit techniques, facilitates the understanding of inhalation anesthesia, and increases acceptance of low-flow systems. For minimal-flow anesthesia, anestheticometry should be considered mandatory because it provides the only means of detecting and correcting derangements of the anesthetic vapor concentration, often brought about by routine clinical maneuvers.
To demonstrate this, we recorded the course of the inspiratory and expiratory concentration of a volatile anesthetic (isoflurane) by infrared absorption and a trend recorder. Changing the carrier gas composition during high flow (fresh gas flow = 6.0 L∙ min-1) from 75 to 25 vol % nitrous oxide in oxygen resulted in a 10% increase of the inspiratory isoflurane concentration. Activating the oxygen bypass or exchanging the soda lime canisters was followed by a prolonged reduction of concentration, most pronounced with minimal flow (fresh gas flow = 0.5 L∙ min -1). Initiating emergence by closing the vaporizer during minimal flow led to a slow decrease in concentration, while at high flow the inspiratory isoflurane concentration rapidly decreased to subanesthetic levels. Integration of a charcoal filter into the inspiratory limb of the breathing circuit reduces the inspiratory concentration to undetectable levels within <3 min. Anesthe-ticography is a useful means of monitoring and documentation of inhalation anesthetic. Such monitoring should become part of ISO standards for reasons of patient safety, economy, teaching, and research.
monitoring, anesthetic vapors, concentration, anesthesia, inhalation, minimal flow technique, equipment, infrared analyzer, charcoal filter
Associate professor of anesthesiology, Johannes Gutenberg-University Medical School, Mainz,