SummaryA model lung system with a variable art$cial constriction was used to quantgy the changes in$ow produced by the addition of helium to the gas mixture in the system. The tracheal concentration of helium obtained when administering 79% helium/21% oxygen to the system via conventional, disposable face masks was determined using a resuscitation dummy. The greatest proportion of the increase in fzow through the orifice occurred at concentrations of helium up to 40%. A tracheal concenrration of 4@/, helium could be obtained with all oxygen masks at fresh gas Jow rates grealer than 8 litres/minute. These Jindings will help to rationalise the administration of 79% helium/2I% oxygen to patients with upper respiratory tract obstruction.
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