UBC Theses and Dissertations
Sex differences in diaphragmatic fatigue Welch, Joseph Frank
Purpose: The purpose of the thesis was to: 1) establish the reliability of cervical magnetic stimulation and chest wall surface EMG in the assessment of the diaphragmatic compound muscle action potential (CMAP) in healthy men and women (Study #1, Chapter 3), and 2) explore sex-based differences in the mechanisms and consequences of diaphragmatic fatigue (DF), specifically, 2a) the cardiovascular response to inspiratory resistance (Study #2, Chapter 4), and 2b) the effect of DF on subsequent exercise performance (Study #3, Chapter 5). Methods: Diaphragmatic fatigue was assessed in healthy men and women by measuring transdiaphragmatic twitch pressure using cervical magnetic stimulation. Surface electrodes were placed on the left and right hemi-diaphragm. Inspiratory pressure-threshold loading (PTL) was used to induce DF at rest, whilst a host of cardiovascular variables were measured (including: heart rate [HR], mean arterial [MAP] and low-frequency systolic blood pressure variability [LFSBP]). A time-to-exhaustion cycle test was performed with and without the induction of DF. Results: All CMAP characteristics demonstrated high reproducibility within and between experimental sessions. At PTL task failure, the degree of DF was not different between sexes (~23%); however, time to task failure was longer in women than men (27 vs. 16 min). Furthermore, women exhibited less of an increase in HR (13 vs. 19 bpm) and MAP (10 vs. 14 mmHg), and significantly lower LFSBP (23 vs. 34 mmHg²) during PTL compared to men. Prior-induced DF negatively and equally affected subsequent exercise performance in men and women (~15%). Conclusions: Cervical magnetic stimulation is a reliable means to evaluate phrenic nerve conduction in healthy men and women. The female diaphragm is highly fatigue resistant, leading to an attenuation of the inspiratory muscle metaboreflex (i.e. cardiovascular consequences of DF). Yet, DF impairs exercise independent of sex.
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