K. Wayne Johnston

Pulsatile flow through constricted tubes: an experimental investigation using photochromic tracer methods

A photochromic tracer method has been used to record pulsatile flow velocity profiles simultaneously at three axial locations along a flow channel. Two major advantages of this multiple-trace method are that it enables velocity data to be acquired in an efficient non-invasive manner and that it provides a detailed description of the spatial relationship of the flow field. The latter is found to be particularly useful in the investigation of transitional type flows; for example, in describing coherent flow structures. Studies of the flow patterns in tubes with mild to moderate degrees of vessel constriction were performed using a 2.9 Hz sinusoidal flow superimposed on a steady flow (frequency parameter of 7.5; mean and modulation Reynolds numbers of 575 and 360, respectively). With mild constrictions (

Mesenteric arterial bypass grafts: Early and late results and suggested surgical approach for chronic and acute mesenteric ischemia

BACKGROUND The purposes of this study were to determine the early and late results of placement of arterial bypass grafts in the treatment of chronic and acute intestinal ischemia and to ascertain whether multiple grafts provide better late results than a single graft. METHODS Records of 34 patients who underwent mesenteric vascular graft placement were retrospectively reviewed. RESULTS All 21 patients with chronic ischemia had a history of intestinal angina and weight loss. Food fear was reported by 33% of patients; also, diarrhea in 57%, constipation in 29%, acalculous cholecystitis in 19%, ischemic gastritis or peptic ulcer in 19%, and elevation of liver enzymes in 22% were reported. Angiogram showed more than 50% stenosis or occlusion of the superior mesenteric artery (SMA) in 100% of patients, celiac artery in 90%, and inferior mesenteric artery in 90%. Although not described previously, a reduction in collateral flow from the internal iliac arteries was caused by severe pelvic disease in 56% of patients. There were no in-hospital deaths. The rate of survival at 1 year was 100%; at 2 years it was 93% +/- 6%, at 3 years 86% +/- 9%, at 5 years 79% +/- 11%, and at 10 years 50% +/- 15%. During follow-up, graft thrombosis occurred in three patients. Of the patients who underwent only a single SMA or celiac bypass, two of five died of bowel infarction; only one of 16 patients who underwent both celiac and SMA bypass had to undergo a repeat surgical procedure because of graft occlusion. Three of 16 retrograde bypasses thrombosed, compared with zero of five prograde bypasses. In nine patients who underwent placement of mesenteric bypass grafts because of acute ischemia caused by acute mesenteric thrombosis, the early mortality rate was 22%; the two deaths were the result of bowel ischemia. The cumulative survival rate was 78% +/- 14% at 1 month, 65% +/- 17% at 1 year, and 52% +/- 16% at 5 years. One of the two late deaths was due to graft thrombosis and bowel infarction. Three of four patients who underwent concomitant mesenteric bypass at the time of aneurysm repair or aortobifemoral bypass survived the surgical procedure. CONCLUSIONS When chronic and acute mesenteric ischemia are diagnosed and treated with a bypass graft, the early and late results are good. Complete revascularization of the SMA and celiac artery or pelvis or both and prograde bypass may reduce the risk of late bowel ischemia.

The value of late computed tomographic scanning in identification of vascular abnormalities after abdominal aortic aneurysm repair

PURPOSE The purpose of this study was to determine the prevalence of late arterial abnormalities after aortic aneurysm repair and thus to suggest a routine for postoperative radiologic follow-up examination and to establish reference criteria for endovascular repair. METHODS Computed tomographic (CT) scan follow-up examination was obtained at 8 to 9 years after abdominal aortic aneurysm (AAA) repair on a cohort of patients enrolled in the Canadian Aneurysm Study. The original registry consisted of 680 patients who underwent repair of nonruptured AAA. When the request for CT scan follow-up examination was sent in 1994, 251 patients were alive and potentially available for CT scan follow-up examination and 94 patients agreed to undergo abdominal and thoracic CT scanning procedures. Each scan was interpreted independently by two vascular radiologists. RESULTS For analysis, the aorta was divided into five defined segments and an aneurysm was defined as a more than 50% enlargement from the expected normal value as defined in the reporting standards for aneurysms. With this strict definition, 64.9% of patients had aneurysmal dilatation and the abnormality was considered as a possible indication for surgical repair in 13.8%. Of the 39 patients who underwent initial repair with a tube graft, 12 (30.8%) were found to have an iliac aneurysm and six of these aneurysms (15.4%) were of possible surgical significance. Graft dilatation was observed from the time of operation (median graft size of 18 mm) to a median size of 22 mm as measured by means of CT scanning at follow-up examination. Fluid or thrombus was seen around the graft in 28% of the cases, and bowel was believed to be intimately associated with the graft in 7%. CONCLUSION Late follow-up CT scans after AAA repair often show vascular abnormalities. Most of these abnormalities are not clinically significant, but, in 13.8% of patients, the thoracic or abdominal aortic segment was aneurysmal and, in 15.4% of patients who underwent tube graft placement, one of the iliac arteries was significantly abnormal to warrant consideration for surgical repair. On the basis of these findings, a routine CT follow-up examination after 5 years is recommended. This study provides a population-based study for comparison with the results of endovascular repair.

Sources of error in maximum velocity estimation using linear phased-array Doppler systems with steady flow.

Using linear-array Doppler ultrasound (US) transducers, the measured maximum velocity may be in error and lead to incorrect clinical diagnosis. This study investigates the existence and cause of maximum velocity estimation errors for steady flow of a blood-mimicking fluid in a tissue-mimicking phantom. A specially designed system was used that enabled fine control of flow rate, transducer positioning and transducer angle relative to the flow phantom. Doppler machine settings (transducer aperture size, focal depth, beam-steering, gain) were varied to investigate a wide range of clinical applications. To estimate the maximum velocity, a new signal-to-noise ratio (SNR) independent method was developed to calculate the maximum frequency from an ensemble averaged Doppler power spectrum. This enabled the impact of each factor on the total Doppler error to be determined. When using the new maximum frequency estimator, it was found that the effect of transducer focal depth, intratransducer, intramachine, intermachine (that was tested) and beam-steering did not significantly contribute to maximum velocity estimation errors. Instead, it was the dependence of the maximum velocity on the Doppler angle that made, by far, the greatest contribution to the estimation error. Because our maximum frequency estimator took into account the effect of intrinsic spectral broadening, the degree of overestimation error was not as great as that previously published. Thus, the effects of Doppler angle and intrinsic spectral broadening are the chief sources of Doppler US error and should be the focus of future efforts to improve the accuracy.

A viscoelastic model of arterial wall motion in pulsatile flow: implications for Doppler ultrasound clutter assessment

The existing computational model studies of pulsatile blood flow in arteries have assumed either rigid wall characteristics or elastic arterial wall behavior with wall movement limited to the radial direction. Recent in vivo studies have identified significant viscoelastic wall properties and longitudinal wall displacements over the cardiac cycle. Determining the nature of these movements is important for predicting the effects of ultrasound clutter in Doppler ultrasound measurements. It is also important for developing an improved understanding of the physiology of vessel wall motion. We present an analytically-based computational model based on the Womersley equations for pulsatile blood flow within elastic and viscoelastic arteries. By comparison with published in vivo data of the human common carotid artery as well as uncertainty and sensitivity analyses, it is found that the predicted waveforms are in reasonable quantitative agreement. Either a pressure, pressure gradient or volumetric flow rate waveform over a single cardiac cycle is used as an input. Outputs include the pressure, pressure gradient, radial and longitudinal fluid velocities and arterial wall displacements, volumetric flow rate and average longitudinal velocity. It is concluded that longitudinal wall displacements comparable to the radial displacements can be present and should be considered when studying the effects of tissue movement on Doppler ultrasound clutter.

The relationship between wall shear stress distributions and intimal thickening in the human abdominal aorta

PurposeThe goal of this work was to determine wall shear stress (WSS) patterns in the human abdominal aorta and to compare these patterns to measurements of intimal thickness (IT) from autopsy samples.MethodsThe WSS was experimentally measured using the laser photochromic dye tracer technique in an anatomically faithful in vitro model based on CT scans of the abdominal aorta in a healthy 35-year-old subject. IT was quantified as a function of circumferential and axial position using light microscopy in ten human autopsy specimens.ResultsThe histomorphometric analysis suggests that IT increases with age and that the distribution of intimal thickening changes with age. The lowest WSS in the flow model was found on the posterior wall inferior to the inferior mesenteric artery, and coincided with the region of most prominent IT in the autopsy samples. Local geometrical features in the flow model, such as the expansion at the inferior mesenteric artery (common in younger individuals), strongly influenced WSS patterns. The WSS was found to correlate negatively with IT (r2 = 0.3099; P = 0.0047).ConclusionLow WSS in the abdominal aorta is co-localized with IT and may be related to atherogenesis. Also, rates of IT in the abdominal aorta are possibly influenced by age-related geometrical changes.

In Vitro Hemodynamic Evaluation of a Simon Nitinol Vena Cava Filter: Possible Explanation of IVC Occlusion

PURPOSE To evaluate the local hemodynamics in the region of the Simon nitinol filter (SNF), used to prevent pulmonary emboli by capturing clot and promoting lysis. MATERIALS AND METHODS The hemodynamics of the Simon nitinol inferior vena cava (IVC) filter were evaluated under steady flow (Re = 600) in a 20-mm-diameter IVC model. The photochromic dye tracer technique was used to estimate the velocity and wall shear stress. These flow features were determined for the unoccluded and partially occluded (clot volume = 1,500 mm(3)) states of the SNF along its center plane. RESULTS A region of low velocities developed around the central axis of the filter extending from the leading edge of the central strut to the filter tip. This phenomenon was created by the strong redirection of flow toward the periphery of the filter. With the presence of the clot, these effects were enhanced, causing flow separation and recirculation. In addition, the shear stress on the hip of the clot was about 30 times that of the upstream value, and turbulence developed in the near-downstream region. CONCLUSIONS The extended region of almost-stagnant flow near the midsection of the umbrella region could lead to organization of thrombus and fibrin mesh network development. The presence of a simulated clot led to a significant increase in the size of the stagnant, thrombus-prone region as well as turbulence, which, overall, may contribute to caval occlusion.

Two-dimensional velocity reconstruction strategies for color flow doppler ultrasound images

A method for calculating the two-dimensional (2-D) velocity vector flow field from color flow Doppler ultrasound images obtained from two or three steering angles has been recently demonstrated. Various strategies for calculating 2-D vectors from noise-corrupted images obtained at multiple angles are described and compared in the present work. This was achieved by using some simple computer flow simulation models to generate color flow images for various beam steering angles. Velocity vectors were calculated for the entire image, and the vectors are displayed at selected points by superimposing them on a magnitude image. It is shown that such displays help improve the qualitative understanding of images obtained from vessels with complex flow geometry. The bias and variance of four reconstruction methods are compared for stimulated laminar flow in a tube. Based on the simulation results, it was found that the dominant factor that affects the reconstruction accuracy is the angle between the observation directions, and that a simple two component method generally gives as good a performance as the more complex alternatives that were studied.

Hemodynamics of a side-to-end proximal arterial anastomosis model

PURPOSE The purpose of this article was to analyze the fluid mechanical effects of a side-to-end proximal anastomosis and to compare the results with those from our earlier study on the end-to-side distal anastomosis. METHODS The photochromic tracer technique was used to determine the instantaneous wall shear stress and to visualize the overall flow field under pulsatile flow conditions. The flow consisted of a sinusoid plus a steady component with mean and modulation Reynolds numbers of 355 and 565, respectively, and a Womersley number of 7.9. RESULTS At the toe and heel of the junction, very high and positive wall shear stresses were seen together with substantial nonperiodic fluctuations. The peak wall shear stress was about four times higher at the toe and about seven times higher at the heel than the maximum values observed at about four tube diameters upstream from the junction. On the bed of the host vessel, nonperiodic fluctuations were also observed, but the shear stresses were mainly negative with magnitudes comparable to those seen upstream. With leakages of 11% and 28% of the mean flow through the blocked end of the host vessel, the shear stress pattern seemed to be significantly affected only at the toe for the higher leakage. Further, when the mean Reynolds number was reduced to 320, the magnitudes of the variations in the wall shear stress were reduced proportionately, except at the heel, where the reduction was much larger than expected. CONCLUSIONS It appears that the preferential development of intimal hyperplasia at the distal end-to-side anastomosis may be promoted by low wall shear stress at the toe and heel, and probably by high shear stresses or shear stress gradients on the bed.

Histology and morphology of 59 internal thoracic artery grafts and their distal anastomoses

BACKGROUND The left internal thoracic artery (LITA) is accepted as a superior graft for the left coronary system because of its better long-term patency rate than saphenous grafts. The postsurgical histomorphometric changes at the distal anastomosis of LITA grafts are not well documented. METHODS The cellular changes within the intima of 59 LITA grafts were analyzed by light microscopy. RESULTS Grafts implanted 1 week or less (n = 34) showed no postsurgical tissue proliferation. Of the 7 grafts implanted 1 to 8 weeks, only the suture sites exhibited intimal thickening (6 of 7 grafts, 0.08 +/- 0.07 mm). The remaining grafts (n = 18), aged 2 months to 10 years, showed significant intimal thickening at the suture sites (0.39 +/- 0.17 mm) and on the hood (0.29 +/- 0.25 mm), with variable thickening on the floor (10 of 18 left anterior descending coronary arteries, 0.11 +/- 0.12 mm). The graft body showed insignificant intimal changes (10 of 18, 0.03 +/- 0.04 mm), with mild focal atherosclerotic lesions in 2 of 18 late LITA grafts. CONCLUSIONS Left internal thoracic artery grafts develop fibromuscular intimal hyperplasia primarily around the anastomosis. The response on the hood appears to be a hemodynamic response, secondary to that of the suture sites.