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The aim of this study was to compare the kinetic response of middle cerebral artery blood velocity (MCAv) to moderate- and heavy-intensity cycling in adults, and explore the relationship between maximal oxygen uptake (V_ O2max) and MCAv kinetics. Seventeen healthy adults (23.8 ± 2.4 yr, 9 females) completed a ramp incremental test to exhaustion on a cycle ergometer to determine V_ O2max and the gas exchange threshold (GET). Across six separate visits, participants completed three 6-min transitions at a moderate intensity (90% GET) and three at a heavy intensity (40% of the difference between GET and V_ O2max). Bilateral MCAv was measured using transcranial Doppler (TCD) ultrasonography and analyzed using a monoexponential model with a time delay. The time constant (s) of the MCAv response was not different between moderate- and heavy-intensity cycling (25 ± 10 vs. 26 ± 8 s, P = 0.82), as was the time delay (29 ± 11 vs. 29 ± 10 s, P = 0.95). The amplitude of the exponential increase in MCAv from baseline was greater during heavy-intensity cycling (23.9 ± 10.0 cm·s-1, 34.1 ± 14.4%) compared with moderate-intensity cycling (12.7 ± 4.4 cm·s-1, 18.7 ± 7.5%; P < 0.01). Following the exponential increase, a greater fall in MCAv was observed during heavy-intensity exercise compared with moderate-intensity exercise (9.5 ± 6.9 vs. 2.8 ± 3.8 cm·s-1, P < 0.01). MCAv after 6 min of exercise remained elevated during heavy-intensity exercise compared with moderate-intensity exercise (85.2 ± 9.6 vs. 79.3 ± 7.7 cm·s-1, P < 0.01). V_ O2max was not correlated with MCAv s or amplitude (r = 0.11–0.26, P > 0.05). These data suggest that the intensity of constant-work rate exercise influences the amplitude, but not time-based, response parameters of MCAv in healthy adults, and found no relationship between cardiorespiratory fitness and MCAv kinetics. [ABSTRACT FROM AUTHOR]

Regulation of cerebral blood flow during exercise in youth is poorly understood. This study investigated the cerebrovascular and ventilatory responses to a ramp incremental cycle test to exhaustion in 14 children (means ± SD age: 9.4 ± 0.9 yr), 14 adolescents (12.4 ± 0.4 yr), and 19 adults (23.4 ± 2.5 yr). Middle cerebral artery blood velocity (MCAv), partial pressure of end-tidal CO2 (PETCO2), and ventilatory parameters were analyzed at baseline, gas exchange threshold (GET), respiratory compensation point (RCP), and exhaustion. The increase in minute ventilation relative to CO2 production during exercise was also calculated (V_E/VCO2 slope). Relative change from baseline (D%) in MCAv was lower in children, compared with adolescents and adults at GET [15 ± 10% vs. 26 ± 14%, and 24 ± 10%, respectively, P ≤ 0.03, effect size (d) = 0.9] and RCP (13 ± 11% vs. 24 ± 16% and 27 ± 15%, respectively, P ≤ 0.05, d 0.8). D%MCAv was similar in adults and adolescents at all intensities and similar in all groups at exhaustion. The magnitude of the VE/V CO2 slope was negatively associated with D%MCAv at GET and RCP across all participants (P ≤ 0.01, r = Δ0.37 to Δ0.48). D%PETCO2 was smaller in children and adolescents compared with adults at GET and RCP (P ≤ 0.05, d≥0.6). In children, D%PETCO2 and D%MCAv were not associated from baseline-GET (r = 0.14) and were moderately associated from RCP-exhaustion (r = 0.49). These relationships strengthened with increasing age and were stronger in adolescents (baseline-GET: r = 0.47, RCP-exhaustion: r = 0.62) and adults (baseline-GET: r = 0.66, RCP-exhaustion: r = 0.78). These findings provide the first evidence on the development of the regulatory role of PETCO2 on MCAv during exercise in children, adolescents, and adults. [ABSTRACT FROM AUTHOR]

Purpose: Cerebrovascular reactivity (CVR) is impaired in adolescents with cardiovascular disease risk factors. A breath-hold test is a noninvasive method of assessing CVR, yet there are no reliability data of this outcome in youth. This study aimed to assess the reliability of a breath-hold protocol to measure CVR in adolescents.Methods: Twenty-one 13 to 15 year old adolescents visited the laboratory on two separate occasions, to assess the within-test, within-day and between-day reliability of a breath-hold protocol, consisting of three breath-hold attempts. CVR was defined as the relative increase from baseline in middle cerebral artery mean blood velocity following a maximal breath-hold of up to 30 seconds, quantified via transcranial Doppler ultrasonography.Results: Mean breath-hold duration and CVR were never significantly correlated (r < .31, P > .08). The within-test coefficient of variation for CVR was 15.2%, with no significant differences across breath-holds (P = .88), so the three breath-hold attempts were averaged for subsequent analyses. The within- and between-day coefficients of variation for CVR were 10.8% and 15.3%, respectively.Conclusions: CVR assessed via a three breath-hold protocol can be reliably measured in adolescents, yielding similar within- and between-day reliability. Analyses revealed that breath-hold length and CVR were unrelated, indicating the commonly reported normalization of CVR to breath-hold duration (breath-hold index) may be unnecessary in youth. [ABSTRACT FROM AUTHOR]

Endothelial dysfunction of the internal carotid artery (ICA) is a risk factor for cerebrovascular diseases (such as stroke, dementia and Alzheimer's disease), but whether brachial FMD is associated with cerebral endothelium-dependent vasodilation is unknown. These novel findings merit further discussion with respect to: (1) the lack of association between baseline cerebrovascular and peripheral artery shear-mediated function and (2) the lack of association between ICA endothelium-dependent function and traditional CVR outcomes. CVR tests are often considered to reflect cerebrovascular endothelial function, but these results indicate that "typical" CVR outcomes (ICA dilation, ICA-CVR and MCA-CVR expressed as %/mmHg) do not reflect shear-mediated ICA function. [Extracted from the article]