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

Abstract

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. 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 cmH 2 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 cmH 2 O s) and women (12,986 ± 1846 cmH 2 O s; P > 0.05); however, women performed the PTL task at a higher relative 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.