Minimizing airflow turbulence in women lowers the work of breathing to levels similar to men

Abstract

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.

Sex, gender and the pulmonary physiology of exercise

“…• Males above the age of ∼14 years have proportionally greater airway luminal area of the large conducting airway (i.e., trachea to the third generation) than females [15,[17][18][19][20] • Males have larger absolute lung volumes and more alveoli than females [9, 21] • Females have "prismatic" geometry of the ribcage and lung while males have "pyramidal" ribcage and lung geometry [22][23][24] Functional differences • Higher W b and V ̇O2 RM for a given absolute V ̇E during exercise in females compared to males [10,13,53-27] Due to a higher resistive component of W b [25,[28][29][30] • Females have greater activation of "extra-diaphragmatic" inspiratory muscles for a relative or absolute V ̇E during exercise Noted in the scalene and sternocleidomastoid muscles [31,32] • Highly trained males are less likely to develop expiratory flow limitation during exercise than highly trained females [13,25] • Females have a blunted respiratory muscle metaboreflex [33][34][35][36][37] • EIAH can occur in untrained females but does not appear to occur in untrained males [38][39][40] • Older females have a higher perception of dyspnoea at absolute and relative exercise intensities than older males [10,11,41] EIAH: exercise-induced arterial hypoxaemia; V ̇E: minute ventilation; V ̇O2 RM : oxygen uptake of the respiratory muscles; W b : work of breathing.…”

“…The relationship between W b and V ̇E is curvilinear, whereby W b rises exponentially with increasing V ̇E. The point when W b rises out of proportion to V ̇E differs between individuals and likely reflects the transition to turbulent flow in the central airways [53]. For a given V ̇E >50-60 L•min −1 , females have a higher absolute W b than males [13,25,27,28,54], regardless of aerobic fitness.…”

Predictors of Expiratory Flow Limitation during Exercise in Healthy Males and Females

“…Frequency of EFL during exercise. Previous studies indicate that EFL occurs at peak exercise in 55%-63% of males and 44%-100% of females (2)(3)(4)(5)(6)(7)(8). Variability in the frequency of EFL between studies may be ascribed to several factors, including differences in the technique used to determine the presence of EFL, the ventilatory capacity and/or ventilatory demand across groups of individuals, and/or the exercise modality.…”

“…Data from n = 126 healthy adults (n = 60 males, n = 66 females) of varying fitness levels were included in the study. Data from n = 104 participants (n = 38 males, n = 66 males) who performed pulmonary function testing and an incremental exercise test as part of previous investigations (3)(4)(5)(6)26,27) were retrospectively analyzed, and n = 22 males were prospectively recruited. All participants provided written informed consent, and all experimental procedures were approved by the research ethics boards at The University of British Columbia and the University of Waterloo, both of which adhere to the Declaration of Helsinki, except for registration in a database.…”

“…xpiratory flow limitation (EFL) occurs when an individual reaches their maximum capacity to generate expired flow, which then ceases to increase even with greater expiratory effort (1). In young healthy individuals, EFL does not occur at rest or at most submaximal exercise intensities; however, at peak exercise, 55%-63% of males and 44%-100% of females develop some degree of EFL (2)(3)(4)(5)(6)(7)(8). During exercise, the presence of EFL increases respiratory muscle work (5), alters operational lung volumes (9), and has been linked to exercise-induced arterial hypoxemia (10,11), reduced exercise performance, and increased perception of dyspnea (12).…”