Quantitative effects of respired helium and oxygen mixtures on gas flow using conventional oxygen masks.

A model lung system with a variable artificial constriction was used to quantify the changes in flow 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 flow through the orifice occurred at concentrations of helium up to 40%. A tracheal concentration of 40% helium could be obtained with all oxygen masks at fresh gas flow rates greater than 8 litres/minute. These findings will help to rationalise the administration of 79% helium/21% oxygen to patients with upper respiratory tract obstruction.