Shear-dependent thickening of the human arterial intima

Hemodynamic data were obtained by laser Doppler anemometry at 163 sites in 10 flow-through casts of minimally diseased human aortic bifurcations, and the intimal thickness at each of these sites in the original vessels was measured (mean = 208 micron, range = 10-967 micron). Analysis of the results suggests that the intimal thickness at sites exposed to high shear stresses increases quickly to a modest value, growing slowly thereafter, while the thickness at sites exposed to low shears rises more slowly but, after time, reaches higher values. Thus the intima of younger vessels is thicker where shear is high, and that of older vessels is thicker where shear is low. This behavior is rationalized by a parsimonious model in which a substance from the lumen enters the wall and is catabolized or otherwise removed. The intimal permeability and removal rate both increase as the shear stress is raised. Intimal thickness is related to the amount of the substance in the wall. This model fits the experimental data with an overall standard error of estimate of 105 micron. Although the model is an extreme simplification of the actual thickening process, it shows that the observed results can be the consequence of competing shear-dependent processes.