Abstract
In the occlusion of internal carotid artery (ICA), collateral blood flow is supplied either via the circle of Willis (large vessel collaterals) or through small vessel anastmoses such as cortical anastmotic channels or the anastmoses between external carotid arterial system and ophthalmic artery (small vessel collaterals). Efficiency difference between these two types of collaterals was studied in 40 patients with ICA occlusion using 133Xe inhalation rCBF measurements.
Materials and Methods : Forty patients with chronic cerebral infarction due to ICA occlusion were subjected to this study. There were 35 males and 5 females with ages ranging from 39 to 76 years. On the basis of four-vessel angiography, these patients were divided into two groups, one receiving collateral flow mainly from the circle of Willis (n=27) and the other with collaterlas mainly through small vessel anastmoses (n=13). Each group was further divided into two subgroups according to the CT findings, one with large cerebral infarcts and the other with small infarcts. The regional cerebral blood flow (rCBF) was measured by 133Xe inhalation methods, and cerebrovascular resistence in occluded hemisphere was estimated from rCBF values and mean blood pressure. In 11 cases, rCBF measurement was repeated 2 weeks to 5 months afterward, when blood pressure decreased spontaneously or due to administration of hypotensive agents.
Results : At the time of first rCBF measurement, mean blood pressure in cases of small vessel collaterals was generally higher than in those of large vessel collaterals. The mean hemispheric rCBF in occluded hemisphere was significantly reduced in patients with large infarcts compared to those with small infarcts, whether they had large vessel collaterals or small vessel collaterals. The rCBF in cases of large vessel collaterals and in those of small vessel collaterals was the same, if the size of infarcts was the same. Patients with small vessel collaterals had, however, significantly higher cerebrovascular resistence compared to those with large vessel collaterals, even if compared in groups with same size of infarcts. When blood pressure decreased spontaneously or after administration of hypotensive agents, rCBF showed little changes in patients with large vessel collaterals. On the other hand, marked rCBF reduction occurred in 3 of 6 patients with small vessel collaterals.
Comments : In the present study, cerebrovascular resistence in cases of small vessel collaterals was shown to be higher than that in cases of large vessel collaterals. This might be partly attributed to differences of blood pressure level, which virtually existed between two groups at the time of rCBF measurement. However, blood pressure difference does not seem to be the main reason for these results, since rCBF in cases of small vessel collaterals tended to decrease following blood pressure reduction, suggesting that autoregulatory response was impaired in this group. In small vessel collaterals, blood flow must pass through resistent vessels twice, while in large vessel collaterals only once. It is considered that high cerebrovascular resistence in cases of small vessel collaterals is mainly attributed to such an inefficiency in this type of collateral circulation. In order to supply sufficient blood flow to occluded hemisphere through small vessel collaterals, considerably high blood pressure may be required.
