Smaller airways increase resistance and the propensity towards turbulent airflow, both of which are thought to be mechanisms behind greater resistive and total work of breathing (Wb) in females. Previous research examining the effect of airway size on the Wb between the sexes is limited by the inability to experimentally manipulate airway size. Heliox (21% oxygen, balance helium) is less dense than room air which reduces turbulent airflow and airway resistance. The purpose of our study was to utilize heliox inspiration in females to provide a physiologically similar stimulus as increasing airway size. We hypothesized that when breathing heliox, females would have a similar Wb to males breathing room air. Eighteen healthy young subjects (n=9 females) completed two maximal exercise tests on a cycle ergometer over two days. For one test subjects breathed room air and the other heliox. The Wb was assessed with an esophageal balloon catheter. During the room air trial, when ventilations were >65 l·min-1, females had a significantly greater Wb compared to males (p<0.05). The greater Wb in females was due to greater resistance to turbulent flow. For both sexes, breathing heliox resulted in increased expiratory flow (+132±18% of room air), an elimination of expiratory flow limitation, and a reduction in the Wb (69±12% of room air) (all p<0.05). When breathing heliox, the females Wb was not different than the males breathing room air. Our findings support the idea that the smaller conducting airways in females are responsible for a greater total and resistive Wb.
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