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The effect of transmural pressure (TMP) (intraluminal minus extraluminal pressure) on flow was measured for 26 isolated major cerebral arteries from human autopsies. The maximum flow rate through an artery was determined by the perfusion pressure (PP), the maximum vessel caliber, and the presence and magnitude of external resistances. Assuming a diastolic PP of 85 mm Hg, intracranial pressure increases above 33±2 SEM mm Hg (45±3 cm H 2 O) resulted in a reduction of flow through these arteries. For atherosclerotic arteries, flow was reduced by 50% at a TMP of 12±1 mm Hg; for nonatherosclerotic arteries, the critical TMP for 50% flow reduction was 20±1 mm Hg. Flow ceased at a TMP of +3 mm Hg for small (cerebellar) arteries to -7 mm Hg for large (posterior cerebral) arteries. For grossly atherosclerotic arteries, this closing pressure was as low as -15 mm Hg. When two arteries were cannulated and perfused in parallel, preferential flow reductions of up to 50% were noted in one of the arteries with no flow change in the other at the same TMP. This preferential narrowing depended on the relative sizes and degree of atherosclerosis of the two arteries. Wall thickness to lumen diameter ratios were obtained for all arteries and their relevance to the possibility of active closure was discussed.
Farrar et al. (Sat,) studied this question.