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Our aim was to study the relationship between cerebral blood flow (CBF) responses to induced hypotension and to CO2 inhalation in patients with occlusive disease of the carotid or middle cerebral arteries. In 13 patients (8 men, 5 women) aged 31–73 years (mean ± 1 SD = 63.2 ± 10.6), regional CBF values during the resting state (CBFrest), 7 % CO2 inhalation (CBFhypercapnia), and hypotension induced by 10–20 μg/kg/min intravenous trimethaphan (CBFhypotension) were measured using positron-emission tomography (PET) with H2 15O. The % CBF change during induced hypotension (% CBFhypotension) was defined as (CBFhypotension− CBFrest)/CBFrest multiplied by 100. The % CBF change during CO2 inhalation (% CBFhypercapnia) was defined as (CBFhypercapnia− CBFrest)/CBFrest/mm Hg arterial partial pressure of CO2× 100. We defined symptomatic hemispheres as those with a stenotic or occlusive lesion with neurological symptoms or signs and asymptomatic hemispheres as those which had a similar lesion and/or were influenced by the collateral flow pattern without neurological symptoms. In the territory of the occlusive lesion, % CBFhypotension ( r = 0.793, P < 0.002) in the symptomatic hemispheres. In the brain regions in which trimethaphan did not induce a reduction in CBF, % CBFhypercapnia ( r = 0.793, P < 0.002) in the symptomatic hemispheres. In the brain regions in which trimethaphan did not induce a reduction in CBF, % CBFhypercapnia was 6.13 ± 1.79. In those in which % CBFhypotension ranged from 0 to −5, from −5 to −10, and more than −10 %, % CBFhypercapnia was 4.05 ± 1.99, 3.21 ± 1.17, and 1.73 ± 1.61, respectively, with significant differences between each pair of groups. In the asymptomatic hemispheres, % CBFhypotension ( r = 0.979, P < 0.0001). Failure to maintain CBF during induced hypotension was associated with diminished cerebrovascular vasoreactivity to hypercapnia in patients with arterial disease. This may indicate that failure of autoregulation can be assessed by the CBF response to both induced hypotension and COhypercapnia ( r = 0.979, P < 0.0001). Failure to maintain CBF during induced hypotension was associated with diminished cerebrovascular vasoreactivity to hypercapnia in patients with arterial disease. This may indicate that failure of autoregulation can be assessed by the CBF response to both induced hypotension and CO2 inhalation. From the technical point of view, estimation of the CO2 response may be useful for assessing failure of autoregulation. [ABSTRACT FROM AUTHOR]