Middle cerebral artery blood velocity and end-tidal carbon dioxide responses to moderate intensity cycling in children, adolescents and adults

Abstract

This study investigated the middle cerebral artery blood velocity (MCAv) response to constant work-rate moderate-intensity cycling exercise in 21 children (9.3±0.8 years), 17 adolescents (12.3±0.4 years) and 20 young adults (23.6±2.4 years). Participants completed an incremental ramp test to exhaustion on a cycle ergometer, to determine maximal oxygen uptake and gas exchange threshold (GET), before completing three 6-minute transitions at a moderate-intensity (90% GET), on separate visits. On each visit, bilateral MCAv was measured by transcranial Doppler ultrasonography and breath-by-breath end-tidal carbon dioxide (PETCO2) via a metabolic cart. Data were ensemble-averaged for each participant and analysed using a mono-exponential model. Absolute MCAv was significantly higher throughout exercise in children and adolescents, compared to adults (P<0.001). Children had a significantly lower relative increase in MCAv from baseline (~12%) compared to adolescents (~20%) and adults (~18%, P<0.040). All adolescents and adults had a mono-exponential rise in MCAv and PETCO2, but this was observed in only eight children. Children and adolescents had a significantly faster MCAv time constant (τ , 12±6 and 14±8 s, respectively) compared to adults (27±9 s, P<0.001). MCAv τ was positively associated with faster PETCO2 τ in adolescents (r=0.70, P=0.002) but not children (r=-0.20, P=0.640).Time- and amplitude-based response parameters of MCAv kinetics were significantly associated with PETCO2 kinetics in adults (r=0.50 to 0.74, P≤0.025), but not in children (r=-0.19 to -0.48, P>0.227). These findings suggest that the transition from childhood to adulthood impacts the MCAv response to exercise, and the relationships between PETCO2 and MCAv kinetics during exercise.