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 (P_aCO₂) during the transient phase of exercise transitions when modeling MCAv kinetics, despite P_aCO₂ having known effects on cerebrovascular tone. The purpose of our study was to determine the independent effects of mean arterial pressure (MAP) and estimated P_aCO₂ (eP_aCO₂) 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 eP_aCO₂ 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 cmꞏs^-1) to 90% of ventilatory threshold (68.8±15.1 cmꞏs^-1). MAP_MCA (Δ = 14±8 mmHg, p<0.001) and eP_aCO₂ (Δ = 2.7±1.8 mmHg, p<0.001) also increased with exercise intensity. Autoregressive moving average analysis isolated the independent effects of dynamic changes in MAP_MCA and eP_aCO₂ on MCAv, with low prediction error (mean absolute error = 1.12±0.25 cmꞏs^-1). Calculated steady-states of the ARMA step responses were 0.13±0.15 cmꞏs^-1ꞏmmHg^-1 for Δmean MCAv/ΔMAP_MCA and 1.95±0.83 cmꞏs^-1ꞏmmHg^-1 for Δmean MCAv/ΔeP_aCO₂. These data demonstrate that the combination of dynamic changes MAP and eP_aCO₂ largely explain the MCAv response during transitions in exercise intensity.