Cerebral blood flow (CBF) and the response to hypercapnia (cerebral reactivity) have been measured in 41 patients with unilateral or bilateral internal carotid artery occlusion in an attempt to identify those with limited collateral reserve. Normocapnic CBF was within normal limits in the majority of subjects. The response to hypercapnia varied from normal to absent, with impaired reactivity becoming increasingly likely when more than one artery was diseased. In 19 patients with unilateral carotid occlusion, hemisphere reactivity was well preserved in the majority, but was significantly lower on the side of the occlusion (mean 2.9%/mm Hg) compared to the normal side (mean 3.4%/mm Hg). Reactivity on the side of the occlusion was further reduced in 15 patients with occlusion and contralateral internal carotid artery stenosis (mean 1.7%/mm Hg) and was even lower in seven patients with bilateral occlusion (mean 1.1%/mm Hg). There was no difference in reactivity between asymptomatic hemispheres in the 41 patients (mean 2.7%/mm Hg) and hemispheres in which a previous stroke had occurred (mean 2.8%/mm Hg). In contrast the response in hemispheres subject to continuing transient ischaemic attacks was significantly impaired (mean 1.6%/mm Hg), suggesting that the cerebral symptoms in some of these patients may have had a haemodynamic origin more often than suspected from the clinical history.
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