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

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 = 243 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-di,P-tw) using cervical magnetic stimulation. Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat throughout PTL via photo-plethysmography, and low-frequency systolic pressure (LFSBP; 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 (similar to 23% reduction in P-di,P-tw; P = 0.33). However, time to task failure was significantly longer in women than in men (2711 vs. 1611 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 LFSBP (23 +/- 11 vs. 34 +/- 8 mmHg(2); 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.