Reactivity of the middle cerebral artery to carbon dioxide

Transcranial Doppler ultrasound (TCD) is used for the assessment of cerebral blood flow velocity (CBFV) at the middle cerebral artery (MCA) with the assumption that diameter of the artery does not change. Thus, CBFV is equivalent to cerebral blood flow (CBF). The purpose of this thesis was determine if the MCA dilates during hypercapnia (HC) and/or constricts during hypocapnia (HO) in healthy young and older adults using 3T magnetic resonance imaging (MRI). We also determined how these changes in MCA cross-sectional area (CSA) influence estimates of CBF and cerebrovascular reactivity (CVR) from TCD in young and older adults. Lastly, we compared whether changes in MCA CSA mimic those at the internal carotid artery (ICA) as assessed with duplex ultrasound during HC and HO. For all studies, HC was induced with 6% carbon dioxide and HO with hyperventilation at 30 breaths per minute, each for five minutes. T2-weighted sagittal images of the MCA were performed with MRI and collection of an image took approximately one minute. When assessing the peak response there was a significant increase in MCA CSA during HC and a decrease during HO. Using these MCA CSA values to calculate CBF resulted in a greater percent change during each protocol compared to CBFV. Changes in MCA CSA were also examined every minute over the five minute periods of HC and HO and significant increases were seen within the first minute of HC while decreases during HO were not evident until minute four. No changes in ICA CSA occurred during HC or HO. Using CBF rather than CBFV to calculate CVR resulted in a greater CVR for each protocol. Finally, when the response to HC was compared between young and older adults the increase in MCA CSA was reduced in older adults compared to young. Cerebrovascular conductance was also reduced in older adults compared to young during HC, while CVR was not different. In summary, the diameter of the MCA changes during manipulations of carbon dioxideand CBFV underestimates CBF and CVR. Also, CVR may not be the best metric to compare the vasodilatory response to HC between groups