Thesis Defence: Investigating Sex Differences in Diaphragm Blood Flow during Exercise Hyperpnea in Humans

Madison Coyle, supervised by Dr. Glen Foster, will defend their thesis titled “Investigating Sex Differences in Diaphragm Blood Flow during Exercise Hyperpnea in Humans” in partial fulfillment of the requirements for the degree of Master of Science in Health and Exercise Sciences.

An abstract for Madison Coyle’s thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.

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Abstract

During whole-body progressive exercise, V̇E increases to support the elevated metabolic demands, increasing work of breathing (WOB) and oxygen cost of respiration (V̇O2RM). At maximal exercise, the respiratory muscles can account for up to 15% of whole-body V̇O2, requiring substantial blood flow. Females generally have a higher WOB and V̇O2RM than males because of smaller airways and lungs, which may necessitate greater diaphragm blood flow (Q̇di), but this has not been studied in humans. Additionally, Q̇di and WOB relationships show high inter-individual variability, but the role of respiratory muscle efficiency (EfficiencyRM) has not been examined. We hypothesized that (1) females would require greater increases in Q̇DIA as WOB increases during voluntary hyperpnea and (2) individual variability in the Q̇di-WOB relationship will be partly explained by inter-individual variability in EfficiencyRM. Healthy, young (27±5 years) females (n=9) and males (n=8) first completed an incremental maximal cycle exercise test to determine spontaneous breathing patterns. On another day, participants mimicked exercise breathing patterns targeting 45%, 60%, 75%, and 100% of maximal exercise ventilation (V̇Emax). Each trial lasted three minutes, and the inspired gas composition was controlled to maintain exercise blood gases. Esophageal and gastric balloon-tipped catheters quantified WOB, mixed expired gas analysis estimated V̇O2RM, and EfficiencyRM was calculated as the WOB divided by the metabolic energy expenditure from the V̇O2RM. Contrast-enhanced ultrasound measured Q̇di at the end of each trial. Females had a higher WOB and V̇O2RM than males at V̇E of 78 l min-1 (P=0.0467) and 67 l min-1 (P=0.0482), respectively. Females had higher Q̇di for a given WOB (interaction: +0.001 ± 0.0004 s-1 per J min-1, P=0.0203), partitioned inspiratory work (interaction: +0.001 ± 0.0006 s-1 per J min-1, P=0.0363), and V̇E than males (exponential coefficienct: 0.03 ± 0.003 versus 0.02 ± 0.003, P=0.0347), but a similar MFRdi-V̇O2RM relationship (interaction: +0.0003 ± 0.0005, P=0.581) between sexes. EfficiencyRM had a negative relationship with individual Q̇di-WOB relationships (-0.0021 ± 0.0006 s-1 per J min-1%-1, P=0.0476). Our findings indicate (1) for a given WOB, females have greater diaphragmatic perfusion compared with males during hyperpnea and (2) EfficiencyRM contributes to the inter-individual variability in the MFRdi-WOB relationship.