QUANTITATIVE MORPHOLOGIC STUDY OF INTIMAL THICKENING AT THE HUMAN CAROTID BIFURCATION .2. THE COMPENSATORY ENLARGEMENT RESPONSE AND THE ROLE OF THE INTIMA IN TENSILE SUPPORT

Arteries enlarge where intimal plaques form, tending to preserve lumen cross sectional area but causing an increase in mural tangential tension due to the increase in radius. To characterize the compensatory enlargement process at the carotid bifurcation and to evaluate the possible contribution of intima thickening to mural tensile support during the enlarging process, we assessed the relationships among intimal thickening, artery size and estimated tensile stress at 9 sequential axial levels in 42 human carotid bifurcations obtained during post-mortem examinations of 36 adults with no clinical or anatomical evidence of cerebrovascular disease. Right and left bifurcations were available for 6 patients. The arteries were fixed under conditions of controlled pressure distention and histologic sections were prepared at 0.5 cm axial intervals. We determined vessel radius (r), intima thickness (IT), media thickness (MT), intima area (IA), lumen area (LuA) and the area encompassed by the internal elastic lamina (IELA), i.e. the lumen area if there were no intimal thickening. Although IT, IA and r were greatest in the proximal sinus region, there was a highly significant linear relationship between IA and IELA at each axial level; correlation coefficients ranged from 0.64 to 0.97 with P < 0.001 at each level. Stenosis (IA/IELA x 100) ranged from 10.8 +/- 8.0% at the common carotid level immediately proximal to the bifurcation angle to 22.3 +/- 17.9% at the level immediately distal to the angle, but LuA remained nearly constant at each level regardless of IA. When wall tensile stress (TS) was computed using only media thickness (MT), values ranged from 8.2 +/- 2.5 to 17 +/- 7.0 x 10(5) dyn/cm(2) (1 dyn = 10(-5) N) with the maximum occurring at the proximal sinus. When, however, total wall thickness (IT + MT) was used, TS was similar at all levels with a mean of 6.5 +/- 2.5 x 10(5) dyn/cm(2), (range, 5.5 +/- 1.8 to 7.3 +/- 2.7 x 10(5) dyn/cm(2)) with no significant differences among the 9 levels, including those with little or no intimal thickening. We conclude that during stages of minimal or moderate intimal thickening and plaque formation at the carotid bifurcation, artery size (IELA) at each level increases with IA, thereby preserving LuA. The sum of intima and media thickness corresponds to the increase in radius such that estimated tangential mural tensile stress remains normal at each level.