Evidence for temperature‐mediated regional increases in cerebral blood flow during exercise

Abstract

Key points Aerobic exercise elicits increases in cerebral blood flow (CBF), as well as core body temperature; however, the isolated influence of temperature on CBF regulation during exercise has not been investigated The present study assessed CBF regulation and neurovascular coupling during submaximal cycling exercise and temperature‐matched passive heat stress during isocapnia (i.e. end‐tidal PnormalCO2 was held constant) Submaximal cycling exercise and temperature‐matched passive heat stress provoked ∼16% increases in vertebral artery blood flow, independent of changes in end‐tidal PnormalCO2 and blood pressure External carotid artery blood flow increased by ∼43% during both exercise and passive heat stress, with no change in internal carotid artery blood flow Neurovascular coupling (i.e. the relationship between local increases in cerebral metabolism and appropriately matched increases in regional cerebral blood flow) is preserved during both exercise and temperature‐matched passive heat stress Abstract Acute moderate‐intensity exercise increases core temperature (Tc; +0.7‐0.8°C); however, such exercise increases cerebral blood flow (CBF; +10‐20%) mediated via small elevations in arterial PnormalCO2 and metabolism. The present study aimed to isolate the role of Tc from PnormalCO2 on CBF regulation during submaximal exercise. Healthy adults (n = 11; 10 males/one female; 26 ± 4 years) participated in two interventions each separated by ≥48 h: (i) 60 min of semi‐recumbent cycling (EX; 50% workload max) and (ii) 75 min of passive heat stress (HS; 49°C water‐perfused suit) to match the exercise‐induced increases in Tc (EX: Δ0.75 ± 0.33°C vs. HS: Δ0.77 ± 0.33°C, P = 0.855). Blood flow (Q) in the internal and external carotid arteries (ICA and ECA, respectively) and vertebral artery (VA) (Duplex ultrasound) was measured. End‐tidal PnormalCO2 and PnormalO2 were effectively clamped to resting values within each condition. The QICA was unchanged with EX and HS interventions (P = 0.665), consistent with the unchanged end‐tidal PnormalCO2 (P = 0.327); whereas, QVA was higher throughout both EX and HS (EX: Δ16 ± 21% vs. HS: Δ16 ± 23%, time effect: P = 0.006) with no between condition differences (P = 0.785). These increases in QVA contributed to higher global CBF throughout both EX and HS (EX: Δ12 ± 20% vs. HS: Δ14 ± 14%, time effect: P = 0.029; condition effect: P = 0.869). The QECA increased throughout both EX and HS (EX: Δ42 ± 58% vs. HS: Δ53 ± 28%, time effect: P < 0.001; condition effect: P = 0.628). Including blood pressure as a covariate did not alter these CBF findings (all P > 0.05). Overall, these data provide new evidence for temperature‐mediated elevations in posterior CBF during exercise that are independent of changes in PnormalCO2 and blood pressure.

Influence of the mode of heating on cerebral blood flow, non‐invasive intracranial pressure and thermal tolerance in humans

“…2020; Caldwell et al . 2020) show little evidence of a decrease in CBF with passive increases in T c ranging from 0.7 to 2°C when decreases in were mild or absent (Gibbons et al . 2019; Bain et al .…”

“…2020; Caldwell et al . 2020). However, the absence of a decrease in global CBF with heating is not universal (Nybo et al .…”

Integrated respiratory chemoreflex‐mediated regulation of cerebral blood flow in hypoxia: Implications for oxygen delivery and acute mountain sickness

“…Volumetric blood flow in the ICA, ECA and VA were determined by two separate investigators concurrently using duplex ultrasound (Vivid-i; GE Healthcare, Tokyo, Japan) equipped with a 10-13 MHz linear transducer. We and others have demonstrated that this approach is technically feasible and free of any potential confounds caused by probe positioning or competitive ultrasound beam interference (Caldwell et al, 2020;Hirasawa et al, 2016;Ogoh, Nakahara et al, 2014Sato et al, 2011). The ICA and ECA measurements were taken 1.0−1.5 cm distal to the carotid bifurcation on the right side of the neck, with the subject's chin slightly elevated.…”

Is all heat equal? Implications for the stimulus for adaptation in the brain

“…As previously shown by Caldwell et al . (2020), the isocapnic cerebral blood flow (CBF) response to passive heat and thermal strain‐matched exercise is similar, both in magnitude and in topography of distribution. This begs the question, at least in terms of cerebral blood flow regulation: is all heat equal?…”

“…Although the isocapnic CBF response is comparable between thermally‐matched exercise and passive heating (Caldwell et al . 2020), the prevailing poikilocapnic CO 2 concentration will dictate the brain's environment, and most probably influence the adaptive stimulus. We suspect that there is a fine line between a beneficial adaptive stimulus and an acute cerebral insult, characterized by increased blood–brain barrier permeability, inflammation, oxidative stress and elevated intracranial pressure.…”