Vascular and metabolic reserve were analyzed in probable Alzheimer's diseas
e (AD) and vascular dementia (VaD). Cerebral blood flow (CBF), cerebral blo
od volume (CBV), cerebral metabolic rate of oxygen (CMRO2), and oxygen extr
action fraction (OEF) were measured quantitatively with positron emission t
omography (PET). Vascular reactivity (VR) was also calculated by comparing
the CBF during 5% CO, inhalation with the CBF during normal breathing. Vasc
ular transit time (VTT) that was calculated as a ratio of CBV/CBF and VR re
flect vasodilating capacity of the small resistance vessels, whereas OEF de
signates metabolic (oxygen-extraction) reserve in threatening brain ischemi
a. Significant increase in OEF was seen in the parieto-temporal cortex and
both VTT and VR were preserved in AD patients. By constrast, there was no s
ignificant increase in OEF whereas VTT was prolonged and VR was markedly de
pressed in VaD patients. The increase of OEF and preserved VTT and VR seen
in AD patients indicate the possible participation of vascular factors in t
he pathogenesis of AD perhaps at the capillary level. (C) 2000 Published by
Elsevier Science Inc. All rights reserved.