Sex differences in diaphragmatic fatigue: the cardiovascular response to inspiratory resistance

Abstract

Diaphragmatic fatigue (DF) elicits reflexive increases in sympathetic vasomotor outflow (i.e. metaboreflex). There is some evidence suggesting women may be more resistant to DF compared to men, and therefore may experience an attenuated inspiratory muscle metaboreflex. To this end, we sought to examine the cardiovascular response to inspiratory resistance in healthy young men (n = 9, age = 24 ± 3 years) and women (n = 9, age = 24 ± 3 years). Subjects performed isocapnic inspiratory pressure-threshold loading (PTL, 60% maximal inspiratory mouth pressure) to task failure. Diaphragmatic fatigue was assessed by measuring transdiaphragmatic twitch pressure (P ) using cervical magnetic stimulation. Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat throughout PTL via photoplethysmography, and low-frequency systolic pressure (LF ; a surrogate for sympathetic vasomotor tone) calculated from arterial waveforms using power spectrum analysis. At PTL task failure, the degree of DF was similar between sexes (∼23% reduction in P ; P = 0.33). However, time to task failure was significantly longer in women than in men (27 ± 11 vs. 16 ± 11 min, respectively; P = 0.02). Women exhibited less of an increase in HR (13 ± 8 vs. 19 ± 12 bpm; P = 0.02) and MAP (10 ± 8 vs. 14 ± 9 mmHg; P = 0.01), and significantly lower LF (23 ± 11 vs. 34 ± 8 mmHg ; P = 0.04) during PTL compared to men. An attenuation of the inspiratory muscle metaboreflex may influence limb and respiratory muscle haemodynamics with implications for exercise performance.

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

“…More recently, we (Welch et al . ) and others (Smith et al . ) have demonstrated that women have an attenuated cardiovascular response to high levels of inspiratory muscle work.…”

“…The target pressure selection was based upon previous work (Welch et al . ), where men and women performed inspiratory PTL to task failure. The group mean P di during the task was 98 and 86 cmH 2 O for men and women, respectively.…”

“…; Welch et al . ), and therefore only a brief description follows. A handheld circular coil powered by a magnetic stimulator (200‐2, Magstim; Whitland, UK) was used to stimulate the phrenic nerve roots at the level of the third–seventh cervical vertebrae and the optimal site of stimulation was identified for all subsequent stimuli.…”

“…) or under conditions of external inspiratory loading (Bellemare & Grassino, ; Welch et al . ). Both human and experimental animal studies show that fatiguing diaphragmatic work is associated with a sympathetically mediated metaboreflex.…”

“…; Welch et al . ), including differences in the absolute energy requirements and work produced, may confound our understanding of sex differences in diaphragm fatigue and the reflexive cardiovascular effects. We therefore tested the hypothesis that following an inspiratory loading task matched for absolute P di , females would develop a similar level of diaphragm fatigue and exhibit an attenuated sympathetic response of the cardiovascular system relative to men.…”