Blood Flow Asymmetry in Carotid Occlusive Disease

Patterns of anterior border zone (ABZ) and middle cerebral artery (MCA) cerebral blood flow (CBF) asymmetry were readily seen during both normocap nic room air (RA) and induced hypercapnic (CO₂) inhalation using fluoromethane and a multislice, high-resolution positron scanner. Wilcoxon two-sample rank testing showed symptomatic-over-nonsymptomatic CBF ratios for unilateral greater than 75 % carotid stenosis patients (n = 8) to be 1.05 ± 0.07 (p < 0.008 as compared with control of 0.97 ± 0.02) ABZ RA, 0.98 ± 0.11 ABZ Co₂, 0.98 ± 0.04 MCA RA, and 0.98 ± 0.06 MCA CO₂. Unilateral carotid occlusion pa tients (n = 8) had ratios of 0.90 ± 0.16 ABZ RA, 0.81 ± 0.19 (p < 0. 002) ABZ CO₂, 0.90 ± 0.12 and 0.89 ± 0.13 for MCA RA and CO₂, respectively (both p < 0.008 as compared with control 0.99). These preliminary results suggest an upgrade of autoregulation (ie, very high ratio) in the ABZ of high-grade stenosis patients during normocapnia. CBF was preferentially higher on the symptomatic side and then either did not increase or paradoxically fell in response to hypercapnia. In comparison, carotid occlu sion patients had low ABZ and MCA ratios during normocapnia, also unable to increase with hypercapnia. The fall in ratios from normocapnia to hypercap nia indicates that these areas, already subject to maximal vasodilation, fail to increase CBF or actually become hypoperfused following induced hypercapnia. These results aid in understanding the concept of “hemodynamic significance”.