Competing influences of arterial pressure and carbon dioxide on the dynamic cerebrovascular response to step transitions in exercise intensity.

Abstract

Recent investigations of middle cerebral artery blood velocity (MCAv) kinetics at the onset of exercise have not accounted for potential dynamic changes in arterial partial pressure of carbon dioxide ([Formula: see text]) during the transient phase of exercise transitions when modeling MCAv kinetics, despite [Formula: see text] having known effects on cerebrovascular tone. The purpose of our study was to determine the independent effects of mean arterial pressure (MAP) and estimated [Formula: see text] ([Formula: see text]) on mean MCAv during repeated moderate-intensity exercise transitions. We hypothesized that cerebral autoregulation would minimize the effect of sustained exercise-induced changes in MAP on mean MCAv and that dynamic changes in [Formula: see text] would contribute to changes in mean MCAv. Eighteen young healthy adults (7 women, age: 28 ± 5 yr) performed three exercise transitions from 25 W to 90% of the ventilatory threshold in sequence with 5-min stages. Mean MCAv increased (P < 0.001) from 25 W (60.5 ± 14.0 cm·s^-1) to 90% of the ventilatory threshold (68.8 ± 15.1 cm·s^-1). MAP at the level of the middle cerebral artery (MAP_MCA) (Δ = 14 ± 8 mmHg, P < 0.001) and [Formula: see text] (Δ = 2.7 ± 1.8 mmHg, P < 0.001) also increased with exercise intensity. Autoregressive moving average (ARMA) analysis isolated the independent effects of dynamic changes in MAP_MCA and [Formula: see text] on MCAv, with low prediction error (mean absolute error = 1.12 ± 0.25 cm·s^-1). Calculated steady states of the ARMA step responses were 0.13 ± 0.15 cm·s^-1·mmHg^-1 for Δmean MCAv/ΔMAP_MCA and 1.95 ± 0.83 cm·s^-1·mmHg^-1 for Δmean MCAv/Δ[Formula: see text]. These data demonstrate that the combination of dynamic changes in MAP and [Formula: see text] largely explains the MCAv response during transitions in exercise intensity.NEW & NOTEWORTHY Time-series analysis of moderate-intensity exercise transitions suggested that cerebral autoregulation buffered the effect of sustained changes in mean arterial pressure on middle cerebral artery blood velocity (MCAv) and that changes in estimated arterial partial pressure of carbon dioxide ([Formula: see text]) contributed to the dynamic changes in MCAv during exercise transitions. Therefore, changes in [Formula: see text] at the onset of exercise are central to modeling dynamic MCAv responses and understanding the benefits of exercise on cerebral blood flow.