Implications of the metaboreflex on cerebral blood flow: physiological responses and assessment strategies

Abstract

Exercise is responsible for acutely modifying cerebral blood flow (CBF) during an exercise bout, and eliciting chronic increases to resting CBF in trained individuals. Research to date has focussed on the individual mechanisms such as neural metabolism, arterial gas concentration and blood pressure that are responsible for CBF control and are altered by exercise. Altering theses mechanisms at rest, and comparing changes in CBF has been how researchers study the CBF response to exercise. The purpose of this thesis was to provide a scoping review of an important system for the regulation of the CBF exercise response: the exercise pressor reflex (EPR). A total of 30 neurogenic, and 12 cerebrovascular studies were reviewed in a two-part scoping review. The review of cardiovascular responses to varying post exercise muscle ischemia protocols demonstrated a large variance between studies, and a methodological variety of exercise intensities and durations. The scoping review of the CBF research demonstrated a variety of CBF responses to EPR stimulation, where CBF increased, decreased, or remained unchanged. Decreases in arterial carbon dioxide concentration, caused by hyperventilation, were found to disguise the effect EPR stimulation on CBF in several of the investigations. When arterial carbon dioxide was experimentally clamped, increases in CBF appear to correspond with arterial blood pressure (BP) responses. Furthermore, microvascular changes in CBF occurred in some instances while macrovascular flow was unchanged, perhaps indicating a region effect of the EPR on CBF. Overall, it has been shown that the EPR affects CBF by manipulating carbon dixide, BP, and perhaps myogenic factors, depending on region. The influence of these factors requires further experimental research. Future research that manipulates carbon dioxide and BP levels while assessing cerebral artery diameter and blood flow velocity is recommended to address the inconsistencies discovered in the current review.