Contribution of central hemodynamics and end-tidal CO2 to cerebrovascular dynamics during aerobic exercise

During aerobic exercise, central hemodynamics and CO2 partial pressure are central to middle cerebral artery blood velocity (MCAv) response. Still, the extent of their contribution is unknown. The purpose of this study was to characterize and utilize statistical modeling to determine the contribution of heart rate (HR), mean arterial pressure (MAP), and end-tidal CO2 (PETCO2) dynamics to MCAv dynamics. Three randomized exercise bouts were completed on a recumbent stepper at 30-40% (Low), 45-55% (Mod1), and 60-70% (Mod2) of estimated HRmax. A 90-s resting period was followed by 6-min of continuous exercise within the estimated HR ranges. HR, MAP, PETCO2, and MCAv exercise dynamics were modeled with a monoexponential curve. From this modeling, the baseline (BL), time delay (TD), time constant ({tau}), and steady-state (SS) responses were determined. Backward AIC linear regression models determined contributing dynamics. Seventeen healthy adults completed all exercise bouts (28 {+/-} 6 yrs, 8 females). The time from initiation of exercise to an exponential increase in HR (HRTD) was significantly longer for Low than Mod2 (p=0.047). The time constant for the rise in HR (HR{tau}) was significantly shorter for Low than Mod1 and Mod2 (p=0.01). The absolute change in HR from baseline to steady-state (HRSS) was significantly lower for Low than Mod1 and Mod2 (p<0.001), and Mod1 was significantly lower than Mod2 (p<0.001). MAPSS was significantly lower for Low than Mod1 (p=0.01) and Mod2 (p<0.001). Exercise intensity, HRTD, and MAPTD accounted for 17% of variation for MCAvTD (p=0.01). HRTD, PETCO2TD, and MCAvTD accounted for 21% of variation MCAv{tau} (p<0.01). MCAv{tau}, MAPSS, and PETCO2SS accounted for 60% of variation for MCAvSS (p<0.001). Throughout the MCAv dynamic response pathway central hemodynamics and end-tidal CO2 do not account for most MCAv response until the steady-state phase. Thus, other physiological factors should be considered with assessing cerebrovascular function during aerobic exercise.