Cerebral endothelial function and pulsatile metaboreflex hemodynamics in adults
Date
2024
Authors
Taylor, Luc
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Abstract
Few studies measuring cerebral blood flow (CBF) during metabolic afferent stimulation consider the mechanistic implications of endothelial function and compliance of vessels supplying the brain. The central question of this thesis study was whether pulsatile cerebral hemodynamic damping (DFi) plays a role in how the afferent muscle metaboreflex, a component of the exercise pressor reflex (EPR), impacts brain hemodynamics during exercise. Using ultrasound imaging techniques on 28 subjects (11 biological females, 17 biological males; 23.6±4.2 years of age; BMI of 24.0±3.2 kg/m^2), measures of DFi from the extracranial internal carotid artery (ICA) to the intracranial middle cerebral artery (MCA) were assessed during post-exercise muscle ischemia (PEMI) and transient hypercapnia (9% CO2). Endothelial function was indexed by the %Δ in ICA vessel diameter (ICAd) during transient hypercapnia using a Douglas bag technique. DFi did not change during a paced-breathing metaboreflex stimulation; stimulation of the blood pressure-dependent metaboreflex with a 30% maximal voluntary contraction (MVC) dynamic handgrip protocol that preceded post-exercise muscle ischemia (PEMI) was effective (p = 0.005). A 6.77±3.97% increase in ICAd was observed during transient hypercapnia (p = 0.021). Cerebral hemodynamic buffering is maintained in healthy young male and female adults. Transient hypercapnia via Douglas bag provides an easy stimulus to increase PetCO2 and shear stress to provoke ICA vasodilation. While no sex differences were observed in this study, future research could benefit from an increased sampling power to explore the impact of sex hormone levels. Additionally, considering healthy individuals appear to buffer cerebral hemodynamics, further mechanistic understanding is required to assess how aging and chronic disease-state populations lead to an impaired DFi.
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Keywords
cerebral blood flow, brain hemodynamics, exercise, endothelial function, hypercapnia, ultrasound imaging