The gas-proportioning system on contemporary anesthesia machines is most likely to prevent delivery of a hypoxic gas mixture occurring as a result of:

Contemporary anesthesia machines are equipped with systems intended to prevent delivery of an hypoxic gas mixture due to an incorrect relationship in flowmeter settings on the oxygen and nitrous oxide flowmeters. Both Ohmeda and North American Drager anesthesia machines are constructed to ensure that a minimum of 25 percent oxygen is delivered to the common gas outlet.

Ohmeda machines use a mechanical (chain) connection between the flow control knobs for the nitrous oxide and oxygen flow meters that allows independent adjustments of the flowmeters to a minimum 3:1 nitrous oxide:oxygen ratio. At the threshold value, the mechanical connection results in oxygen and nitrous oxide flows changing in tandem: an increase in nitrous oxide flow results in an increase in oxygen flow or a decrease in the flow...

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