Critical stenosis of the internal carotid artery.

To better define the hemodynamic significance of internal carotid artery stenosis, intraoperative electromagnetic blood flow measurements were taken in 47 patients before and after carotid endarterectomy. When the ratio of pre- to postendarterectomy blood flew (Qpre/Qpost = Y) is compared to the fractional percent of internal carotid artery stenosis (X), the linear regression lines for the 25 arteries with Qpre/Qpost less than 0.70 are Y = -2.67 x +2.59 and Y = -10.10 x +10.06, respectively, for diameter and area stenosis. The intercepts of the regression lines with (a) Qpre/Qpost = 1 (normal blood flow) are 60% diameter and 90% are stenosis, (b) Qpre/Qpost = 0.60 (40% reduction in blood flow) are 75% diameter and 94% are stenosis and (c) with Qpre/Qpost = 0.36 (64% reduction in blood flow) are 84% diameter and 96% area stenosis. These data indicate that a critical or hemodynamically significant internal carotid artery stenosis may be of a higher grade than previously appreciated and that a stenosis greater than 75% diameter or 94% area may be necessary to symptomatically reduce cerebral blood flow.