Computational simulation of biomechanics in e-PTFE and venous Miller's cuffs: implications for intimal hyperplasia.

A computational distal end-to-side Miller's cuff anastomotic model was used to analyse the possible difference in intimal hyperplasia (IH) formed between e-PTFE and venous cuffs. A large strain FEA model was used to compute the strain after physiological loading and the deformed geometries used as wall boundaries for CFD analysis. Regression analysis was performed to investigate relationships between mechanical factors and prior IH. The results showed that the venous Miller's cuff anastomosis deformed twice as much as the e-PTFE cuff and that the expansion of both cuffs generated elevated strains in the artery floor while the fluid shear indices were qualitatively similar in each case. In the e-PTFE cuff, the strain and OSI correlated with IH in a proportional and equivalent manner; however, these regressions grossly over-estimated the predicted IH in the vein cuff. Thus, biomechanical effects may be important in synthetically cuffed anastomoses, but do not account for the reduced IH in venous cuffed anastomoses.

Linear relationship between electrical systole, mechanical systole, and heart rate.

The relationship between the duration of electrical systole (QT) and heart rate (HR) and the relationship between the QT interval and total electromechanical systole (QS2) were studied in the resting state in 200 patients (100 males and 100 females) without evidence of cardiovascular disease. A linear relationship was found between the QT and HR in males and females (males, QT = 521 msec - 2.0 HR, r = .91; females, QT = 511 msec - 1.8 HR, r = .90). In 20 male and 20 female subjects, the relationship between QT and QS2 was studied. The QT was slightly shorter but paralleled the QS2 (males QT = 529 msec - 2.1 HR, QS2 = 541 msec - 2.2 HR; females QT = 511 msec - 1.9 HR, QS2 = 540 msec - 2.0 HR). Thus, over the physiologic range of resting HR, a linear relationship exists between QT and HR. The QT interval is slightly shorter but parallels the QS2 in patients without heart disease. These linear relationships permit a direct comparison of the duration of electrical and mechanical systole.

Electrostatic endothelial cell seeding technique for small-diameter (<6 mm) vascular prostheses: feasibility testing.

Multiple studies have indicated the importance of surface charge in the adhesion of multiple cardiovascular cell lines including platelets and endothelial cells on the substrate materials (1,4,7-10,12-15). It is the purpose of this article to report a feasibility study conducted using an electrostatic endothelial cell seeding technique. The feasibility study was conducted using human umbilical vein endothelial cells (HUVEC), a static pool apparatus, a voltage source, and a parallel plate capacitor. The HUVEC concentration and seeding times were constant at 560,000 HUVEC/ml and 30 min, respectively. Scanning electron microscopy examination of the endothelial cell adhesion indicated that an induced temporary positive surface charge on e-PTFE graft material enhances the number and the maturation (flattening) of HUVECs adhered. The results indicated that the total number of endothelial cells adhered (70.9 mm2) was increased from 9198 +/- 1194 HUVECs on the control (no induced surface charge) e-PTFE to 22,482 +/- 4814 HUVECs (2.4 x control) on the maximum induced positive surface charge. The total number of cells in the flattened phase of adhesion increased from 837 +/- 275 to 6785 +/- 1012 HUVECs (8.1x) under identical conditions. Thus, the results of the feasibility study support the premise that electrostatic interaction is an important factor in both the endothelial cell adhesion and spreading processes and suggest that the electrostatic seeding technique may lead to an increased patency of small diameter (<6 mm) vascular prostheses.

Electrostatic endothelial cell transplantation within small-diameter (<6 mm) vascular prostheses: a prototype apparatus and procedure.

This article presents a novel, clinically relevant electrostatic endothelial cell transplantation (seeding/sodding) device (U.S. & Foreign Patent Protections Pending) for small-diameter (<6 mm) vascular prostheses. The prototype apparatus was designed and built to tissue engineer 4.0 mm, I.D. GORE-TEX (W.L. Gore & Associates, Inc.) standard wall graft segments varying in length from 4 to 12 cm. The prototype electrostatic endothelial cell transplantation apparatus is composed of an external and internal conductor, aluminum base, end supports, pillow blocks, filling apparatus, electric motor drive system, and a voltage source. The cylindrical capacitor arrangement of the device along with an electrical potential applied across the internal and external conductors creates the unique feature of this endothelial cell transplantation technique, an electric field within the cylindrical capacitor (within the graft lumen) which in turn induces a temporary positive surface charge on the graft (dielectric material) luminal surface. Multiple studies have shown that a positively charged substrate is more conducive to endothelial cell adhesion and morphological maturation (flattening) (1,2, 7,8,10,13-15). This induced positive surface charge dissipates immediately upon removal from the electrostatic endothelial cell transplantation device. Thus, after endothelial cell adhesion the graft luminal surface reverts back to its natural (nonthrombogenic) negative surface charge.

Determination of the prime electrostatic endothelial cell transplantation procedure for e-PTFE vascular prostheses.

The purpose of this study was to evaluate the extent of cellular adhesion (density and morphological maturation), cellular membrane damage, and cellular viability after an electrostatic transplantation of human umbilical vein endothelial cells (HUVECs) onto 6-cm segments of 4-mm I.D. e-PTFE (GORE-TEX) vascular prostheses using a prototype electrostatic endothelial cell transplantation device (EECTD). The electrostatic transplantation parameters evaluated were the apparatus-applied voltage and transplantation time. By our definition, the combination of applied voltage and transplantation time that met the a priori criteria of: 1) maximum transplanted cellular viability, 2) maximum transplantation density, 3) maximum morphological maturation (degree of cellular flattening), and 4) minimal cellular membrane damage would be the prime transplantation procedure. The results of the experimentation indicated that the prime conditions for HUVEC transplantation were obtained when +1.0 V was applied for a transplantation time of 16 min. These conditions achieved an average viable graft surface coverage of 97.4+/-1.6% with an average transplantation density of 73,540+/-8.514 HUVECs/cm2. Furthermore, the transplanted HUVECs were morphologically mature (flattened) with minimal apparent cellular membrane damage (lysis or pitting). The overall clinical significance of this study is that viable endothelial cell transplantation to synthetic vascular grafts can be accomplished at high cellular densities and morphological maturation in 16 min using the EECTD. With the promising in vitro transplantation results, the next logical investigations will include additional in vitro evaluations (cellular retention upon shear stress exposure and biochemical assays) followed by in vivo evaluations to examine thromboresistance and influence on intimal/anastomotic hyperplasia.

Audible interpretation of carotid Doppler signals. An improved technique to define carotid artery disease.

Direct interrogation of the common, internal, and external carotid arteries with a bidirectional Doppler detector was performed on 199 vessels in 101 patients. Audible interpretations of normal, disturbed (stenotic), or absent (occluded) flow signals were made by vascular technologists and the results were compared with independently assessed carotid arteriograms. The sensitivity in detecting severe (greater than or equal to 50%) stenosis or occlusion was 9.2%, with correct differentiation of these two conditions in 84% of vessels. Nonobstructive (less than 50%) stenoses were detected in 30% of cases. The specificity in identifying normal carotid arteries was 91%. The predictive values of normal, abnormal, and absent flow signals were 95%, 90%, and 90%, respectively. Direct carotid Doppler signal analysis is more accurate than indirect periorbital Doppler screening in defining carotid stenosis or occlusion.

Real-time Doppler spectrum analysis: predictive value in defining operable carotid artery disease.

A prospective study compared the sensitivity, specificity, and predictive value of indirect periorbital Doppler screening and direct carotid Doppler interrogation using real-time sound spectrum analysis to detect operable extracranial carotid occlusive disease. The results of noninvasive studies were compared with independently assessed contrast arteriograms of carotid arteries of patients studied for symptomatic cerebrovascular disease. Periorbital Doppler ultrasound was insensitive to carotid stenosis of less than 75% and did not distinguish operable stenosis from inoperable occlusion of the internal carotid artery. Carotid Doppler spectrum analysis was sensitive to 98% of hemodynamically significant disease and correctly discriminated 77% of carotid occlusions, 96% of stenoses larger than 50%, and 70% of stenoses of less than 50%. The predictive value for direct carotid interrogation in identifying carotid occlusion or operable stenosis was 94% and 86%, respectively. Real-time spectrum analysis of direct carotid Doppler signals substantially improves the diagnostic accuracy of noninvasive identification of operable carotid artery disease.

Prediction of amputation wound healing. Roles of Doppler ultrasound and digit photoplethysmography.

Segmental limb blood pressures (BPs) measured by Doppler ultrasound and digit photoplethysmography were used to define hemodynamic correlates of wound healing in 122 lower-extremity amputations for ischemia. Healing of digit or transmetatarsal amputations were influenced by the presence of diabetes and correlated better with digit pulsation and BP than with ankle pressure. Healing of below-knee amputation did not correlate with segmental limb BPs. In the absence of an absolute threshold to predict wound healing or failure, caution is urged in using hemodynamic indices to select the level of amputation of arterial insufficiency.

Evaluation of transplant renal artery blood flow by Doppler sound-spectrum analysis.

Doppler ultrasonography sound-spectrum analysis (SSA) was used to evaluate blood flow in the transplanted kidney and its renal artery. Seven patients with posttransplant hypertension and a bruit over the transplanted kidney were screened for renal artery stenosis (RAS). In five patients, RAS was diagnosed by SSA, and in two it was not. These findings were confirmed by subsequent angiography in all patients. Three patients studied after surgical correction of their RAS had improvement in their SSA patterns. Fourteen hypertensive patients with a cause other than RAS were evaluated by SSA. None of them had SSA findings suggestive of RAS. Doppler ultrasonography with SSA is an effective, noninvasive technique of monitoring transplant renal blood flow, especially in the screening of hypertensive transplant recipients for transplant RAS.

MR imaging of model fluid velocity profiles.

A projection MR technique for imaging the velocity profiles of moving fluids has been applied to various steady flow models designed to simulate a variety of flow conditions. From such profiles can be readily deduced peak velocities, volume flow rates, information concerning the degree of flow development, features such as flow separation, and estimates shear stresses at the vessel wall.

Flow dynamics in a stenosed carotid bifurcation model--Part II: Derived indices.

Basic velocity measurements from the previous study in this issue of modelled carotid artery bifurcation disease are post-processed to derive indices potentially useful for clinical diagnosis. Selected parameters are based upon ultrasound pulse Doppler velocity measurements made at sites +/- 0.625 radius at axial distances of 0, 1, 3, 5, and 10 diameters downstream of smooth, axially symmetric constrictions of 0, 20, 40, 60, and 80% diameter reduction. Indices based on single point velocity measurements include: (a) the center-line velocity index (CVI) at the throat of the constrictor which is sensitive to all degrees of constriction (p less than 0.05), and (b) various measures of velocity disturbance (VDI1-VD14) which show greatest sensitivity when measured at one-fourth diameter from the tube wall. Cross-sectional indices include: (a) the maximum slope index (MSI) which separates constrictions of less than 60% diameter reduction from those having greater than or equal to 60% diameter reduction (p less than 0.05), and (b) the cross-sectional profile index (CPI) which separates constrictions of less than or equal to 20% diameter reduction from those having greater than 20% diameter reduction (p less than 0.05). A field profile index (FPI) utilizes data from all available sites up to 5 diameters downstream and is able to separate all constrictor groups (p less than 0.05). Presence of constrictions produces characteristic flow patterns which can be quantified using indices based on the downstream velocity spectra. Specifically, locally increased velocity is very sensitive to degree of constriction and is best detected along the center line. Disturbance effects, while less pronounced, are complementary to changes in velocity magnitude and are first seen off-axis. Multiple point profile measures (cross sectional and full field) are also sensitive to degree of constriction and are best evaluated at 1 diameter and over 5 diameters, respectively, downstream of the constriction. All of the nondimensional indices offer the advantages of reduced probe and angle dependence and particular approaches may be implemented according to the data collection capability of the instrument used.

Doppler color flow images of iliofemoral graft end-to-side distal anastomotic models.

Hemodynamics within the distal anastomoses of iliofemoral bypass grafts were simulated using Plexiglas models (2.5 cm ID) within a pulsatile flow loop system (Re(mean) = 92, Re(peak) = 459 and alpha = 3.56). End-to-side distal anastomoses were constructed with angles of 30 degrees, 45 degrees and 60 degrees to bypass proximal artery segments with stenoses of 60% and 100% diameter reduction. Velocities were obtained over a two-dimensional field within the artery using an ultrasonic Doppler color flow imager operating at 5 MHz at positions from 1.5 diameters upstream to 3 diameters downstream of the anastomosis. Flow patterns downstream of an occlusion demonstrated definite skewing effects toward the outer wall with resultant flow separation along the inner wall. Presence of a partial (60% diameter reduction) arterial stenosis upstream of the anastomosis produced flow separation along both artery walls and a more symmetric profile downstream. Measurement of a separation area index (SAI) along each arterial wall demonstrated a minimum area exposed to low velocities for the 30 degrees anastomotic angle compared to the 45 degrees and 60 degrees anastomotic angles and for the occluded proximal artery cases compared to corresponding stenotic artery cases. The SAI values were minimal at peak systole compared to successive quarter cycle intervals. The findings of this study provide further information regarding the relationship between local fluid mechanics and predominant sites for intimal hyperplasia formation.

Flow dynamics in a stenosed carotid bifurcation model--Part I: Basic velocity measurements.

A model of the human carotid artery bifurcation has been constructed and tested under mean physiologic flow conditions (fluid viscosity = 0.035 poise; mean Re inlet = 400; unconstricted flow split = 70:30, internal carotid artery:external carotid artery). Smooth, axisymmetric constrictor plugs with 0, 20, 40, 60, and 80% diameter reduction are placed in the simulated proximal internal carotid artery to provide a range of flow conditions similar to those found clinically. Axial velocity measurements are made at sites +/- 0.625 radius within the lumen of the tube at distances of 0, 1, 3, 5 and 10 diameters downstream of the constrictor throat using H2 bubble markers and a 7.5 MHz ultrasound pulse Doppler. Measurement of mean, mode -3 dB down high and low and -9 dB down high and low velocities are made from selected fast-Fourier transform (FFT) spectra. The flow field downstream of the 0% diameter reduction is entirely laminar, exhibiting a peak skewed toward the flow divider along the entire 10 diameters downstream and having a consistently narrow bandwidth. Obstruction of the flow channel produces increased axial velocity at the constrictor (20%), an oscillatory jet extending approximately 5 diameters downstream (40%), a transitional jet extending approximately 5 diameters downstream (60%) and a turbulent jet extending approximately 3 diameters downstream (80%). Velocity bandwidth (both -3 dB and -9 dB) increases with degree of constriction and low velocity flow patterns exist between the center line and both tube walls. Presence of constriction produces characteristic downstream flow patterns which are distinctive for each degree of diameter reduction.

Quantification of Doppler color flow images from a stenosed carotid artery model.

Velocity fields at and downstream of graded, axisymmetric stenoses of 0%, 20%, 40%, 60% and 80% diameter reduction were obtained under pulsatile flow conditions simulating that in a carotid artery (Re(mean) = 379, Re(peak) = 1137 and alpha = 5.36) using an ultrasonic Doppler Color Flow Imager. Characteristic features of flow disturbances associated with degree of stenosis, such as jetting, flow separation and reversal and turbulence were quantified using nondimensional indices at peak systole, t0, and early diastole, t1. It was found that the Reverse Area Index (RAI), the Field Profile Index (FPI) and the Velocity Gradient Indices (VGIz and VGIr) were sensitive to these changes, especially at time t1. In particular, the mean values of RAI and VGIr were significantly different for all stenotic cases. These findings may provide a more quantitative and reproducible method of interpreting flow patterns in the region of stenosed carotid arteries.

Quantitative analysis of carotid artery Doppler spectral waveforms: diagnostic value of parameters.

Continuous-wave ultrasonic Doppler recordings made at the carotid bifurcation have been processed by spectrum analysis and off-line microprocessor software. Data reduction algorithms were defined and tested prior to incorporation in an integrated, hard-wired device. Subsequent analysis of waveforms in real-time provided measures of peak and mode frequencies and degree of spectral broadening. Correlation of results with 123 angiographically visualized internal carotid arteries showed that peak and mode frequencies generally increased with degree of stenosis while the systolic window decreased. Ratios of peak and mode frequencies in the proximal to the distal internal carotid artery and the systolic window at the distal internal carotid artery were retrospectively found to have diagnostic value with a combination of these three parameters providing the best overall accuracy.

Detection of renal artery stenosis: experimental and clinical analysis of velocity waveforms.

Mass screening of hypertensive patients by a noninvasive method could uncover the 10% of those cases where renal artery stenosis is the primary etiology. Treatment by transluminal angioplasty or surgery could replace a long-term medical regimen. To investigate an ultrasonic technique, normal velocity waveforms were obtained from the abdominal aorta, celiac artery and renal arteries in seven mongrel dogs using a 5 MHz, continuous-wave Doppler detector. Renal artery pressure gradients, volume flow rates and velocity recordings were subsequently made during induced proximal renal artery stenoses. The ratio of peak renal artery frequency to peak aortic frequency was 88% sensitive to stenoses of greater than 20 mm Hg pressure gradient, while the renal artery systolic frequency window was 79% sensitive to the same obstructions. Ninety-five hypertensive and vascular surgical patients were examined using a 3 MHz duplex scanner with 175 of the 190 (92%) renal arteries adequately detected (clear signal with high diastolic component). Analysis of velocity waveforms based on peak frequency, proximal to distal peak frequency changes, evidence of flow disturbances and associated bruit were compared to contrast arteriograms in 84 vessels. Of the 76 (90%) arteries adequately examined by duplex scanning, 59 of 61 (97%) with 0-59% diameter reduction, 10 of 12 (83%) with 60-99% diameter reduction and 1 of 3 (33%) occlusions were correctly identified. Velocity waveform analysis can accurately detect renal artery stenosis and may prove effective in mass screening of hypertensive patients for renovascular disease.

Potential role of velocity and wall tension in vein graft failure.

The factors contributing to early thrombosis or late intimal proliferation in arterialized vein grafts were studied in seven dogs. Autologous jugular veins replaced a segment of the cervical carotid artery bilaterally. Thirty minutes after graft installation and six months later, graft and proximal artery diameter, arterial pressure and blood flow were measured and wall tension, vascular resistance, flow velocity and energy loss between proximal artery and graft were computed. The diameter ratio of proximal artery to graft increased by 37% (p less than 0.05). Velocity of flow within the graft averaged 87% less than that in the proximal artery (p less than 0.05) and decreased by 58% after six months (p less than 0.05). Graft wall tension at both experimental periods was significantly higher than that of the proximal artery (p less than 0.05), increasing over six months by 91% (p less than 0.05). These results support contentions that low flow velocity contributes to early thrombosis of the graft and, that increased wall tension is significant factor in the pathogenesis of intimal thickening.

Effect of controlled local acetylsalicylic acid release on in vitro platelet adhesion to vascular grafts.

Thrombosis is the most serious acute problem for small diameter arterial bypass grafts. In this research, small diameter expanded polytetrafluoroethylene (e-PTFE) vascular grafts were coated with acetylsalicylic acid (ASA) loaded poly (d,l-lactide) (PLA) by a solvent casting method. The feasibility and efficacy of this approach were evaluated by ASA release studies and platelet adhesion tests. First, the ASA release kinetics were evaluated from the ASA/PLA coated vascular grafts in an in vitro steady flow loop model. ASA release was measured by a spectrophotometric technique. Finally, the efficacy of local ASA release to reduce in vitro canine platelet adhesion to grafts was determined with epifluorescent video microscopy and quantitative image analysis. The steady state release rates from the 5%, 10%, and 15% ASA/PLA coated grafts were 13.2 x 10(-5), 32.0 x 10(-5), and 41.5 x 10(-5) micrograms/cm2.sec, respectively. Platelet adhesion to 10% and 15% ASA/PLA coated grafts was reduced with respect to the control and 5% grafts for 10 days. Platelet adhesion to 5% ASA/PLA coated grafts was reduced with respect to controls at 2 and 10 days, but not initially.

The persistence of electrostatically seeded endothelial cells lining a small diameter expanded polytetrafluoroethylene vascular graft.

PURPOSE: The purpose of this study was to evaluate the persistence of electrostatically seeded endothelial cells (ECs) lining an expanded polytetrafluorethylene (e-PTFE) graft after one week exposure to in vivo circulation in a canine femoral artery bypass model. This was accomplished by visualizing the PKH 26 (red fluorescent) label placed in the EC membranes prior to the seeding procedure. Furthermore, this study was performed to confirm that the source of the ECs lining the graft were those from the initial inoculum. METHODS: This evaluation consisted of harvesting autologous, canine jugular vein ECs, PKH 26 labeling of the ECs, electrostatic EC seeding the e-PTFE grafts (4mm GORE-TEX, Length=6cm), implanting the grafts (femoral artery model) for one week, and explanting the grafts for light, fluorescent and scanning electron microscopy evaluations of the luminal surface. RESULTS: The unseeded grafts (controls) had a mean fluorescence surface coverage of 6.82 +/- 7.19%, while the EC seeded grafts had a mean of 90.3 +/- 14.3% which is significantly (p <0.001) different from the controls. Overall, the seeding time including the EC harvesting and PKH 26 labeling protocol was approximately 75 min. CONCLUSIONS: The electrostatically seeded ECs persisted after implantation of the graft as demonstrated by the PKH 26 labeling data. The fluorescent data also demonstrated that the neointima formed (EC luminal surface coverage) one week after implantation was in fact derived from the ECs initially seeded as determined by the abundance of the labeled ECs.

Non-invasive measurement of blood flow-rate in expanded polytetrafluoroethylene grafts.

Four 6 mm e-PTFE grafts were placed subcutaneously into four dogs between the carotid artery and jugular vein to model shunt flow through an access graft. Flow-rates were varied from 120 to 2000 ml/min (N = 42) by use of a cardiac stimulant (Aramine) and external constrictor. Measures of graft flow were made simultaneously by electromagnetic flowmeter (EMF) and Doppler ultrasound based on a known graft diameter, use of a flat-head probe providing a fixed beam-to-vessel angle and derivation of the mean Doppler-shift frequency from first moment computation of the frequency spectra. Correlation coefficients between the Doppler ultrasound and EMF recordings were 0.54, 0.96 and 0.92 at measurement sites just beyond the arterial/graft anastomosis, at the graft loop and just proximal to the graft/venous anastomosis, respectively. Availability of a simple and accurate non-invasive flowmeter for superficially placed e-PTFE grafts may be valuable for evaluating the systemic effects of vascular shunts, for monitoring the patency of access grafts and for assessing the function of bypass grafts for limb ischaemia.