Margot R. Roach

The Hemodynamic Importance of the Geometry of Bifurcations in the Circle of Willis (Glass Model Studies)

The critical Reynolds number, Rec, at which turbulence developed in glass model bifurcations was measured with an Evans blue indicator for bifurcations with a branch/trunk area ratio of unity, and bifurcation angles of 45°, 90°, 135°, and 180°. The Rec dropped from 2,500 in a straight tube to 1,200 in the 180° bifurcation. Further drops occured with pulsatile flow (if the mean flow rate was used to calculate the velocity). Three sizes of aneurysms at the apex of the 90° bifurcation lowered the Rec at small bifurcations, and less in the 180° ones. The curves for steady and pulsatile flow crossed at 135°. We did qualitative, but not quantitative, assessments of axial stream impingement on the apex of the bifurcation in the site of aneurysm formation, and of boundary layer separation and vortex shedding at the lateral angles. Both appeared to vary with the angle of the bifurcation and the Reynolds number. We also studied flow profiles in glass models of anterior cerebral-anterior communicating artery bifurcations and the posterior communicating artery origin from the internal carotid. The relevance of these studies to localization of intimal cushions, aneurysms, and atherosclerosis was discussed.

An Experimental Study of the Production and Time Course of Poststenotic Dilatation in the Femoral and Carotid Arteries of Adult Dogs

Three degrees of stenosis were produced in the femoral and carotid arteries of twenty-six adult dogs. Minimum stenosis caused neither proximal nor distal dilatation. Moderate stenosis, defined by the presence of a distal thrill and bruit, always produced post-stenotic dilatation. Severe stenosis, with a marked decrease or absence of distal. pulsation caused temporary proximal dilatation. The arteries were painted with a radi-opaque paste which did not change their elastic properties and allowed daily measurements of arterial diameter. Radiograms showed that if turbulence was present, the artery usually started to dilate within twenty-four hours, reached a steady state within ten days, and then remained essentially unchanged for up to ten months. The dilatation was fusiform, and usually extended for 1.75 to 2.0 cm distal to the end of the band. Frequency analysis of the murmurs showed a wide frequency spectrum with no resonant points and no components over 1500 cycles/ sec. This type of sound is more likely to be produced by turbulence than by cavitation or vortex formation. Dye injection in a model showed a random dye distribution starting a short distance distal to the constriction. Analysis of pressure-volume diagrams from these arteries35 suggested that the dilatation was due to a weakness of the arterial wall in the poststenotic segment.

Effect of Low Frequency Vibration on the Arterial Wall

Poststenotic dilation is accompanied by alteration of the elastic properties of the arterial wall in the region where a murmur occurs. The low frequency vibrations contained in a murmur may cause the changes observed. Isolated human external iliac arteries were distended with 100 mm Hg pressure and vibrated with specific frequencies from 30 to 400 Hz. When dilation occurred, the radius increased at a rate of 7.1±3.4 SD % per day. However, for each artery, dilation tended to occur with one frequency in preference to others. The amplitude, as long as sufficient to produce vibration of the wall, was not important in determining this tendency. The older the artery the higher the frequency necessary to produce dilation. Young vessels, under 45 years, responded best to frequencies <100 Hz, and vessels 45−60 years old responded to frequencies 100−200 Hz and older vessels, >60 years, dilated with frequencies >200 Hz. The elastic diagram of the vessel after dilation showed the major change to be in the elastin component of the arterial wall rather than the collagen. These findings correspond to those seen in a vessel displaying poststenotic dilation.

Quantitative method for the evaluation of athero-sclerotic lesions

Abstract A simple accurate quantitative method for the evaluation of atherosclerotic plaques surrounding aortic orifices is presented. The usefulness of this method is illustrated by the detailed analysis of an atherosclerotic rabbit aorta. The lesions were found to be distal to the major orifices of the descending aorta with slight shifts where flow disturbances occurred. Lesions around the coronary arteries were different as they completely surrounded the orifice. The method involves projecting photographs of the lesions onto polar coordinates, and then obtaining measurements from these that can be put on rectangular coordinates.

The role of radial elastic properties in the development of aortic dissections

PURPOSE The response of the upper and lower thoracic aorta to radial tensile stresses was compared with the response to circumferential and longitudinal stresses to understand the role of tensile stress in the tearing phase of an aortic dissection. METHODS Square tissue samples (1.6 by 1.6 cm) were cut from the upper and lower segments of six porcine thoracic aortas and were elongated in the radial direction with a tensile testing machine. The radial extensibility of the thoracic aorta was compared with adjacent tissue samples that were tested in tension in the circumferential and longitudinal directions based on Youngs modulus (ie, the ratio of tensile stress to strain). RESULTS The elastic properties of the thoracic aorta in the radial direction were markedly different from both the circumferential and longitudinal properties. The average Youngs modulus (calculated immediately before failing) was significantly lower in the radial direction for both the upper and lower thoracic segments (61.4 +/- 4.3 kPa, SEM) than the Youngs modulus of corresponding segments in the circumferential and longitudinal directions that were not tested to failure (151.1 +/- 8.6 kPa and 112.7 +/- 9.2 kPa, respectively; P <. 05). Sections 7 micrometer thick were collected from four samples obtained from one upper thoracic aorta that were strained at 0, 1.0, 2.5, and 4.0 and then stained either with Movats pentachrome or with hematoxylin and eosin. Histological analysis of the samples stressed in the radial direction revealed that smooth muscle cells were torn loose from their attachments to each other and to adjacent elastin. CONCLUSION Although the aorta normally functions under radial compressive stresses associated with lumen blood pressure, these results show that the aorta tears radially at a much lower value of stress than would have been predicted from previous studies that have reported longitudinal and circumferential Youngs modulus. This could explain why dissections propagate readily once the initial tear occurs.

The strength of the aortic media and its role in the propagation of aortic dissection

The elastic properties of the thoracic aorta are well known, but this is the first study of the inherent strength of the tunica media. The latter is crucial to understand how dissecting aneurysms occur. Pressure-volume (P-V) measurements were recorded as a dilute suspension of India ink was infused into the tunica media using a constant flow pump (0.21 or 0.88 ml min-1) attached to a 20 G needle inserted into the media. Tests done on 31 opened pig upper descending thoracic aortas showed the peak pressure to tear the media averaged 77.2 +/- 1.5 kPa (579 mm Hg). The initial slope of the P-V curve revealed the average distensibility of the media of 3.02 +/- 0.28 (MPa)-1. The work per unit area of tissue required to propagate a tear in the aorta was 15.9 +/- 0.9 mJ cm-2. These values were independent of the tear depth at the 95% confidence interval.

Quantitative measurements of early atherosclerotic lesions on rabbit aortae from vascular casts

We have measured the size and location of fatty atherosclerotic lesions in the cholesterol fed rabbit by scanning electron microscopic analysis of vascular casts. The aortic periorificial lesions examined in this study have been divided into proximal (upstream), distal (downstream) and lateral (to the sides of the branch). Lateral lesions developed after 1 week on diet, followed by distal lesions at 2 weeks, and occasional proximal lesions at 4 weeks. The area covered by the lesions shows a strong linear relationship (P less than 0.001) to the cross-sectional area of the branch after 2 weeks on the cholesterol diet.

The effect of the duration of cholesterol feeding on the development of sudanophilic lesions in the rabbit aorta

Polar coordinate mapping was used to determine the rate of growth of individual sudanophilic lesions on the aortic wall around several major branches of the aortae of cholesterol fed rabbits. Four groups, with 6 8-month old male albino white rabbits in each, were used in the study. One group served as a control and the remaining 3 were fed a diet of 2% cholesterol and 6% heated corn oil mixed with ground rabbit pellets for 4, 8, and 10 weeks each. Animals were sacrificed, the aortae removed, stained with Sudan III, pinned at in vivo dimensions, and mapped by the polar coordinate method. No sudanophilic lesions were observed in the control animals. In the experimental groups, the early lesions, except the coronaries, were almost entirely distal to the orifices, and maintained roughly the same contour while spreading around the orifice. The coronary lesions completely encircled the orifices as described previously. As lesions progressed, they became elevated and often granular, so that the lesions themselves may have affected flow profiles around the orifices. Lesions around adjacent orifices were fused in 48% of the cases after 10 weeks on the diet, as opposed to 2% after 4 weeks on the diet. More prolonged experiments were not possible with this diet as the animals developed jaundice and diarrhea. Hemodynamically, these results suggest that early sudanophilic lesions in cholesterol-fed rabbits develop on the aortic wall in areas of high shear stress.

The effect of tear depth on the propagation of aortic dissections in isolated porcine thoracic aorta.

An in vitro model was used to determine the influence of tear depth on the propagation pressure of aortic dissections. Saline was injected into the media of segments of 20 porcine thoracic aortas to create blebs. A circumferential slit was made on the intimal side of each bleb, connecting the true lumen to the false lumen. Each aorta was then pressurized under no-flow conditions until propagation in either the anterograde or retrograde direction occurred. Histological sections of each principal propagating edge were used to determine depth of tear, measured as the ratio of elastin layers in the intimal flap to the elastin layers in the intact wall. Propagation occurred for tear depths ranging from 0.44 to 0.89, with dissections closest to the adventitia (with tear depths near 1) requiring the lowest pressures. Propagation pressure (P) depends on the number of elastin layers (L) in the outer wall of a dissection, P = 0.44 L + 25(kPa), r2 = 0.465, p = 0.003 and also on tear depth (d): P = -58 d + 81(kPa), r2 = 0.547, p < 0.001. Various in vivo factors are discussed which may affect these experimentally determined relationships.

A new probability mapping method to describe the development of atherosclerotic lesions in cholesterol-fed rabbits.

A new probability mapping method was developed to quantify the size and location of lesions near aortic orifices. The precise location of any part of the lesion could be compared between rabbits. Colour photographs of lesions were projected onto a digitizing tablet, and coded as lesion or non-lesion. Next the orifices were warped onto a standard orifice, and then the lesion mapped to maintain the original length and angular location of the lesion from the edge of the orifice. This method, unlike the previously used polar mapping method, excludes neither absent lesions nor ones which surround more than one orifice. In contrast to other probability mapping methods it warps the orifice rather than the artery wall containing the lesion, and so is easier to use for correlation with histological studies. The eventual aim is to use the probability maps as a tool to estimate the age of various positions of the lesion and to identify areas for histological sampling. The method was used to describe the distribution of lesions in 21 rabbits fed a diet with cholesterol levels declining from 0.5% during the first week, to 0.25% during the next two weeks, to 0.125% for weeks 3-10, to 0.1% for weeks 11-24. This feeding protocol produces fatty lesions which are transformed into fibro-fatty and fibrous lesions with time.