DIFFERENCES IN COMPENSATORY VESSEL ENLARGEMENT, NOT INTIMAL FORMATION, ACCOUNT FOR RESTENOSIS AFTER ANGIOPLASTY IN THE HYPERCHOLESTEROLEMIC RABBIT MODEL

Background In de novo human atherosclerosis, compensatory vessel enlargement limits the effect of intimal plaque formation on lumen narrowing. We hypothesized that arterial remodeling may also play an important role in determining the chronic lumen size after angioplasty and tested this hypothesis using the hypercholesterolemic rabbit iliac artery angioplasty model. Methods and Results Morphometric analysis of histological cross-sectional areas of vessels from animals killed immediately after angioplasty (acute group, n=11) were compared with the same areas from animals killed 4 weeks after the procedure (chronic group, n=37), when restenosis occurs in this model. The area circumscribed by the internal elastic lamina (IEL) increased by 20% from acute to 4 week follow-up after angioplasty (acute group, 2.36+/-0.45 mm(2); chronic group, 2.84+/-0.89 mm(2)). Over the same time period, intimal area increased by 0.82 mm(2). Despite this increase in intimal area, lumen area decreased by only 0.34 mm(2) because of the compensatory enlargement of the IEL area. In the chronic group, polynomial regression analysis revealed a quadratic relation between intimal area and lumen area (R(2)=.35, P<.001). A lumen area of 0.45 mm(2) (the nadir of the quadratic relation) was used to divide the chronic group into two subgroups: restenotic (n=21; lumen area, <0.45 mm(2)) and nonrestenotic (n=16; lumen area, >0.45 mm(2)). By definition, there was a significant difference in lumen area between the two subgroups (0.15+/-0.15 mm(2) for restenotic; 0.73+/-0.18 mm(2) for nonrestenotic). Surprisingly, the intimal areas in the two subgroups were virtually identical (2.41+/-0.92 mm(2) for restenotic, 2.49+/-0.69 mm(2) for nonrestenotic, P=NS). The difference in the lumen area between restenotic and nonrestenotic vessels was a result of the significantly greater IEL area in the nonrestenotic subgroup (3.22+/-0.83 mm(2) for nonrestenotic, 2.56+/-0.84 mm(2) for restenotic, P<.05). In both restenotic and nonrestenotic vessels, the IEL area increased with increases in intimal area. In the restenotic arteries, the slope of this correlation was <1, showing inadequate compensatory enlargement for the intimal plaque. In the nonrestenotic vessels, the slope was >1, limiting the effect of intimal plaque on luminal narrowing. Conclusions These data indicate that the iliac artery in an atherosclerotic rabbit model compensates for intimal formation after angioplasty by vessel enlargement. Furthermore, the degree of vessel enlargement is more important than intimal area in determining the chronic lumen size.