Velocity distribution and intimal proliferation in autologous vein grafts in dogs.

Abstract

SUMMARY Autologous femoral veins grafted between the external iliac arteries in 18 dogs provided a model for studying the hemodynamics and histopathology of vein graft bypasses. The angle of proximal anastomosis was varied by groups (<90°, 90°, >90°) to produce a wide range of flow conditions within the grafts. Four months after implantation, point velocity measurements of blood flow and histological examination of the superior and inferior walls were made at proximal, middle, and distal locations in each graft. Hot-film velocity measurements revealed outwardly skewed velocity profiles in the proximal locations in all grafts, and flow visualization models showed secondary fluid motions and separation zones at those regions. Velocity profiles in the middle and distal regions of the grafts were more symmetrical and showed no flow separation. Histological examination of wall sections along the graft length showed that intimal proliferation occurred focally and ranged from 1 to 100 fita in thickness. No signficant correlation between graft angle and degree of intimal proliferation was found. However, there was a weak inverse correlation between the apparent fluid shear rate and the corresponding degree of intimal proliferation, with the greatest proliferation occurring in the regions experiencing the lowest shearing forces. Regions of low shear rate should be avoided by maintaining adequate flow rates through the grafts and thus minimising losses of patency due to both thrombus formation and intimal proliferation.

Influence of hemodynamic forces on vascular endothelial function. In vitro studies of shear stress and pinocytosis in bovine aortic cells.

“…Increased endothelial permeability has been measured near vessel branches and bifurcations (24), regions that are exposed to both high and low shear stresses (25). Disruption of endothelial cell alignment has been observed in areas where flow separation is expected to occur (26,27), and such areas appear to be more susceptible to endothelial damage, intimal cell proliferation, and the development of athersclerotic lesions (28). Sites of highest shear stress are located near the flow dividers which tend to be spared endothelial damage and lipid deposition.…”

Immediate and Long-Term Evolution of the Blood-Venous Graft Interface

“…By the 2nd week after implantation we saw a regular proliferation of the mediointimal layers of the venous graft, which is a con stant feature [3,4,8,14,23]. Here there is evidence for the presence in the medial and the ncointimal layer of the graft of a unique cell which has been previously suggested [4] to possess both fibroblastic and myocytic characteristics and is able to synthetize both collagen and elastin.…”

Design, fabrication, and in vitro evaluation of an in vivo ultrasonic Doppler wall shear rate measuring device

“…Although the LDA performs better under in vitro conditions, it is excluded from in vivo applications because of its requirement for a transparent medium. Other techniques that have been utilized for in vivo measurement of shear rate include: electromagnetic flowmeter (EMF) [SI, hot film anemometry [6], and singlecrystal PUDV [17], [18], [20], [21] methods. The EMFderived technique consists of using the mean flow rate together with the Poiseuille flow assumption to estimate the wall shear rates.…”

“…H ticular have been widely associated with development of atherosclerosis [ 11451 and intimal hyperplasia [6]-[ 111. Electrochemical techniques have been applied in studies using in vitro models to make direct shear measurements [12], [13].…”

“…More exact measurements have been obtained using hot-film anemometry (HFA) [6] and ultrasound pulse Doppler (PUDV) [17], [I81 velocity recordings to compute shear. From recent work carried out in the laboratory [19], it has been shown that shear rate measurements can be made under pulsatile conditions to within * 20% accuracy using four velocity points (three free stream measurements together with the wall no-slip condition) and either a second-or third-order polynomial curve fit.…”