CO₂ response for the brain stem artery blood flow velocity in man

We examined changes in the blood flow velocity of brain stem artery (BSA) and middle cerebral artery (MCA) in response to hypercapnic, normocapnic and hypocapnic hyperventilation in seven awake subjects with a transcranial Doppler to determine if there are differences in blood flow control in regional brain perfused by these respective arteries, and to separate the effects of CO₂ and ventilation itself on blood flow velocity during CO₂ loading. During hypercapnic hyperventilation, BSA flow velocity increased linearly with an increase in end-tidal partial pressure of CO₂ (PET(CO₂)) During hypocapnic hyperventilation, BSA flow velocity decreased linearly with decrease in PET(CO₂) but did not change during normocapnic hyperventilation. The mean CO₂ reactivity of BSA was 2.8%/mmHg. The responses of MCA to these hyperventilations and CO₂ reactivity were similar to those of BSA. These findings suggest that CO₂ rather than ventilation per se is the important stimulus to changes in brain blood flow velocity and that the CO₂ responses of brain arteries are not affected by differences in vascular beds.