While heliox gas (a premixed 70:30 helium/oxygen mixture) was delivered to the nebulizer at a fixed rotameter FR of 10 L/min; through a second gas source, heliox gas (a premixed 70:30 helium/oxygen mixture) was delivered to the open side arm of the TC. The flow to the nebulizer was not allowed to exceed 10 L /min during all experimental settings. When an attempt was made to deliver heliox gas at a higher FR to the nebulizer, the back pressure resulted in a disconnection of the tubing from the nebulizer at an FR of 13 L/min. The flow from the second gas source to the TC was incrementally adjusted until no RAE was observed. A quadralizer (model 224A; Raytech; North Vancouver, BC, Canada) was incorporated into the inspiratory limb of the circuit between the ventilator and the inspiratory filter, which allowed continuous monitoring of the concentration of four gases (oxygen, nitrogen, carbon dioxide, and helium). The helium concentration determined with the quadralizer was a directly measured value using the principles of the nondispersive infrared absorption, paramagnetic, and thermal conductivity properties of the gas. On all days, prior to the initiation of the experiments, the quadralizer was calibrated with room air (21% oxygen), 100% oxygen, a 50:50 heliox mixture, a 60:40 heliox mixture, a 70:30 heliox mixture, a 80:20 heliox mixture, and 100% helium. The quadralizer readings stabilized in less than 1 min, and the final readings for all experimental settings were obtained 5 min poststabilization. An oxygen analyzer (MaxO2; Maxtec; Salt Lake City, UT) was placed in series with a quadralizer to assess the oxygen concentration. Since a quadralizer is not readily available in most clinical settings, we decided to use a drop in the oxygen concentration (using an oxygen analyzer) to < 30% as an indicator of RAE. Both an oxygen analyzer and a quadralizer were used in our study to see whether there was a correlation between the helium readings calculated mathematically from the oxygen analyzer readings and those measured by the quadralizer.