Effects of hypoxia on exercise-induced diaphragm fatigue in healthy males and females.

Following high-intensity, normoxic exercise there is evidence to show that healthy females, on average, exhibit less fatigue of the diaphragm relative to males. In the present study, we combined hypoxia with exercise to test the hypothesis that males and females would develop a similar degree of diaphragm fatigue following cycle exercise at the same relative exercise intensity. Healthy young participants (n = 10 male; n = 10 female) with a high aerobic capacity (120% predicted) performed two time-to-exhaustion (TTE; ~85% maximum) cycle tests on separate days breathing either a normoxic or hypoxic (FiO2  = 0.15) gas mixture. Fatigue of the diaphragm was assessed in response to cervical magnetic stimulation prior to, immediately post-exercise, 10-, 30-, and 60-min post-exercise. Males and females had similar TTE durations in normoxia (males: 690 ± 181 s; females: 852 ± 401 s) and hypoxia (males: 381 ± 160 s; females: 400 ± 176 s) (p > 0.05). Cycling time was significantly shorter in hypoxia versus normoxia in both males and females (p < 0.05) and did not differ on the basis of sex (p > 0.05). Following the hypoxic TTE tests, males and females experienced a similar degree of diaphragm fatigue compared to normoxia as shown by 20%-25% reductions in transdiaphragmatic twitch pressure. This occurred despite the fact that exercise time in hypoxia was substantially shorter relative to normoxia and the cumulative diaphragm work was lower. We also observed that females did not fully recover from diaphragm fatigue in hypoxia, whereas males did (p < 0.05). Sex differences in the rate of diaphragm contractility recovery following exercise in hypoxia might relate to sex-based differences in substrate utilization or diaphragm blood flow.

Sex differences in diaphragmatic fatigue: Effects of hypoxia during inspiratory loading.

KEY POINTS: Under normoxic conditions, both healthy female and male diaphragms fatigue at a similar degree when matched for absolute diaphragmatic work during inspiratory loading. We investigated whether similarities in diaphragm fatigability persist under acute hypoxic conditions. We found that, in acute hypoxia, fatigue of the diaphragm is greater in women compared to men, whereas the magnitude of fatigue in normoxia did not differ between sexes. When matched for maximal diaphragm strength, women and men had a similar pressor response to work-matched inspiratory loading, independent of oxygen availability. ABSTRACT: In normoxia, women and men display a comparable magnitude of diaphragmatic fatigue (DF) after work-matched inspiratory loading. Whether these sex similarities are maintained under acute hypoxic conditions is unknown. We investigated the influence of acute hypoxia during work-matched inspiratory pressure-threshold loading (PTL) on DF in healthy women (n = 8) and men (n = 8). Two 5 min isocapnic PTL tasks targeting a transdiaphragmatic pressure (Pdi ) of 92 cmH2 O in normoxia and hypoxia (8% O2 ) were performed on separate days (≥48 h). DF was quantified by twitch Pdi (Pdi,tw ) via cervical magnetic stimulation post-PTL. Women and men had similar maximal Pdi (Pdi,max ; women: 171 ± 16, men: 178 ± 20 cmH2 O) and relative target workload (women: 54 ± 5%, men: 53 ± 6% Pdi,max ). The absolute cumulative diaphragmatic work did not differ between sexes in normoxia (women: 12,653 ± 1796 cmH2 O s-1 , men: 13,717 ± 1231 cmH2 O s-1 ; P = 0.202) or hypoxia (women: 11,624 ± 1860 cmH2 O s-1 , men: 12 722 ± 1502 cmH2 O s-1 ; P = 0.189). In normoxia, the magnitude of reduction in Pdi,tw post-PTL was similar between sexes (women: -21.1 ± 8.4%, men: -22.5 ± 4.9 %; P = 0.193); however, a higher degree of DF was observed in women compared to men following PTL in acute hypoxia (women: -27.6 ± 7.7%, men: -23.4 ± 9.6%, P = 0.019). We conclude that the female diaphragm is more susceptible to fatigue after inspiratory loading under acute hypoxic conditions. This finding may be related to sex differences in diaphragm muscle metabolism, such as fibre type composition, contractile properties, substrate utilisation and blood perfusion.

Quantifying the mechanical work of breathing in men and women during rowing.

PURPOSE: To quantify the mechanical work of breathing (Wb) during an indoor rowing test in men and women. Additionally, to compare sex-based differences in the Wb and its components through a rowing test. METHODS: Fifteen collegiate rowers were recruited (8 women/7 men) and performed a 2000 m rowing test on a rowing ergometer. Esophageal pressure was measured during exercise via balloon catheterization, after which pressure-volume curves were used to calculate total, inspiratory resistive and elastic, and total expiratory Wb. RESULTS: Men had significantly higher values of instantaneous and cumulative total Wb at and beyond 37.5% (430.4 ± 42.5 vs. 282.1 ± 45.1 J min-1, P < 0.05) and 62.5% (1946.8 ± 150.9 vs. 1360.1 ± 197.2 J, P < 0.05) total exercise time, respectively. However, when compared at the same minute ventilation, women had higher values of total (at and above ~ 140 L min-1), inspiratory resistive (at and above ~ 120 L min-1), and inspiratory elastic (at and above ~ 135 L min-1) Wb, whereas men presented higher total expiratory Wb compared to women at any ventilation. CONCLUSION: Although female rowers present higher relative values of inspiratory resistive and elastic Wb, their male counterparts develop greater ventilatory efforts during a 2000 m rowing test, resulting in a larger total mechanical Wb. We interpret these findings to mean that the Wb reflects both anatomical (i.e., airways and lung sizes) and respiratory (i.e., minute ventilation) sex differences during rowing.

Diaphragm fatigue and inspiratory muscle metaboreflex in men and women matched for absolute diaphragmatic work during pressure-threshold loading.

KEY POINTS: The female diaphragm fatigues at a slower rate compared to that of males, with blunted cardiovascular consequences (i.e. inspiratory muscle metaboreflex). It is unclear if these findings are a function of relative or absolute diaphragmatic work. We asked if sex differences in diaphragm fatigue and the inspiratory muscle metaboreflex persisted during inspiratory loading performed at equal absolute intensities. We found that matching men and women for absolute diaphragmatic work resulted in an equal degree of diaphragm fatigue, despite women performing significantly greater work relative to body mass. Metabolite-induced reflex influences in sympathetic outflow originating from the diaphragm are attenuated in women, with potential implications for blood flow distribution during exercise. ABSTRACT: In response to inspiratory pressure-threshold loading (PTL), women have greater inspiratory muscle endurance time, slower rate of diaphragm fatigue development, and a blunted pressor response compared to men. It is unclear if these differences are due to discrepancies in absolute diaphragm force output. We tested the hypothesis that following inspirations performed at equal absolute intensities, females would develop a similar level of diaphragm fatigue and an attenuated cardiovascular response relative to men. Healthy young men (n = 8, age = 24 ± 3 years) and women (n = 8, age = 23 ± 3 years) performed PTL whilst targeting a transdiaphragmatic pressure (Pdi ) of 92 cmH2 O for 5 min. Diaphragm fatigue was assessed via twitch Pdi (Pdi,tw ) using cervical magnetic stimulation. Heart rate (HR) and mean arterial blood pressure were monitored continuously. During PTL, the absolute amount of diaphragm work was not different between men (13,399 ± 2019 cmH2 O s) and women (12,986 ± 1846 cmH2 O s; P > 0.05); however, women performed the PTL task at a higher relative P¯di /Pdi,max . Following inspiratory PTL, the magnitude of reduction in Pdi,tw was similar between men (-27.1 ± 7.2%) and women (-23.8 ± 13.8%; P > 0.05). There were significant increases in HR over time (P < 0.05), but this did not differ on the basis of sex (P > 0.05). Mean arterial blood pressure increased significantly over time in both men and women (P < 0.05); however, the rate of change was higher in men (6.24 ± 2.54 mmHg min-1 ) than in women (4.15 ± 2.52 mmHg min-1 ) (P < 0.05). We conclude that the female diaphragm is protected against severe fatigue when inspiratory work is excessive and as a result does not evoke overt sympathoexcitation.