Computational estimation of the influence of the main body-to-iliac limb length ratio on the displacement forces acting on an aortic endograft. Theoretical application to Bolton Treovance® Abdominal Stent-Graft.

AIM: The influence of the relative iliac limb length of an endograft (EG) on the displacements forces (DF) predisposing to adverse effects are under-appreciated in the literature. Therefore, we conducted a computational study to estimate the magnitude of the DF acting over an entire reconstructed EG and its counterparts for a range of main body-to-iliac limb length (L1/L2) ratios. METHODS: A customary bifurcated 3D model was computationally created and meshed using the commercially available ANSYS ICEM (Ansys Inc., Canonsburg, PA, USA) software. Accordingly, Fluid Structure Interaction was used to estimate the DF. The total length of the EG was kept constant, while the L1/L2 ratio ranged from 0.3 to 1.5. RESULTS: The increase in L1/L2 slightly affected the DF on the EG (ranging from 3.8 to 4.1 N) and its bifurcation (4.0 to 4.6 N). However, the forces exerted at the iliac sites were strongly affected by the L1/L2 values (ranging from 0.9 to 2.2 N), showing a parabolic pattern with a minimum for 0.6 ratio. CONCLUSION: It is suggested that the hemodynamic effect of the relative limb lengths should not be considered negligible. A high main body-to-iliac limb length ratio seems to favor hemodynamically a low bifurcation but it attenuates the main body-iliac limbs modular stability. Further clinical studies should investigate the relevant value of these findings. The Bolton Treovance(®) device is presented as a representative, improved stent-graft design that takes into account these hemodynamic parameters in order to achieve a promising, improved clinical performance.

Simulation of systolic and diastolic left ventricular dysfunction in a mock circulation: the effect of arterial compliance.

Arterial compliance (AC) is expected to play a major role on cardiac efficacy by acute or long-term mechanisms. The aim of this study was to investigate the purely mechanical effect of AC on left ventricular (LV) performance, for different conditions of LV dysfunction (systolic versus diastolic). A hydraulic, Windkessel model of systemic circulation was used. LV function and aortic flow were simulated using a left ventricular assist device (LVAD). Two cases of LV dysfunction were simulated: Case A, systolic and Case B, diastolic dysfunction. In Case A, AC increased from 1.14 to 2.85 ml mm Hg(-1) leading to an increase in LVAD stroke volume up to 6%, while no significant effect was observed in Case B. LVAD systolic work was decreased by 4% in systolic and by 11% in diastolic LVAD dysfunction. The purely mechanical effect of AC changes on LVAD function was different between systolic and diastolic dysfunction. It might be expected that even an acute reduction in arterial stiffness could enhance LV performance by different means in systolic compared to diastolic dysfunction.

A new capillary viscometer for small samples of whole blood.

A new capillary viscometer is described in which a column of blood is discharged under a constant pressure, producing a variety of shear stresses during a single test. Measurement of the viscosity of Newtonian sucrose solutions showed good agreement between the viscosity determined from the new system and the expected values. The viscosity of whole blood was measured in a cone-and-plate viscometer at a wide range of shear rates and characterized using a power law model; good agreement was obtained between the capillary and rotational results at low and medium shear rates. High shear rate results could also be obtained by increasing the driving pressure. The new viscometer proved to be simple to use, utilized a small test volume and produced reliable results.

Assessment of coronary stenoses by Doppler wires: a validation study using in vitro modeling and computer simulations.

The present study evaluates the use of intracoronary velocity measurements by Doppler guidewires for assessing coronary obstructions. In vitro experiments were performed in a flow model using acrylic phantoms of coronary stenoses with different configurations (stenosis area: 56%, 75% and 89%; stenosis length: 1 and 5 mm; stenosis border: tapering or abrupt). Nonpulsatile laminar flow conditions of a test fluid were established at flow rates ranging from 0.5 to 2.0 mL/s to simulate baseline flow and flow after vasodilation. Peak Doppler velocity was measured proximal to, within and distal to the model stenoses. Computer simulations were employed to calculate radial flow profiles with and without a Doppler wire aligned with the vessel center. In 84 in vitro flow experiments, peak Doppler velocity correlated well with the average flow velocity as calculated from the actual flow rate and the vessel's cross-sectional area proximal to (r = 0.98, SEE = 1.4, p < 0.001) and within (r = 0.97, SEE = 16.4, p < 0.001) the stenosis. However, the ratio of calculated average velocity to Doppler-measured peak velocity was significantly different from 0.5, the expected value for a parabolic flow profile (0.76+/-0.08, 0.81+/-0.14; p < 0.001). Acceptable accuracy was found for the Doppler estimation of stenosis severity using the continuity equation (error: 0.9+/-1.2% and -4.6+/-3.5% for stenosis with a length of 5 mm and 1 mm, respectively). Doppler velocity reserve significantly underestimated the true flow reserve for the 56% and 75% stenoses (p < 0.01). Computer simulations demonstrated significant alterations of flow profiles by the wire, which explained the observed underestimation of the true flow reserve by the Doppler velocity reserve. Thus, Doppler guidewire measurements of intracoronary flow velocities are useful to assess the severity of coronary stenoses. However, the in vitro results and computer simulations indicate that guidewires alter the flow profile, so that Doppler velocity reserve may underestimate the true flow reserve.

A new capillary viscometer for whole blood viscosimetry.

A new capillary system was developed, incorporating infrared sensors, which allowed the determination of whole blood viscosity over a wide range of shear stresses. Flow conditions were defined by the geometry of the capillary and the sample pressure head. Whole blood was considered to be a power law fluid and a modified Mooney's formula was used for the calculation of the related invariants. The new viscometer proved to be very simple in use, requiring one run, had a short measuring time and utilised a small test sample volume. However it can be used for whole blood viscosity measurements only at medium and high shear stresses.

A computer model for the prediction of left epicardial coronary blood flow in normal, stenotic and bypassed coronary arteries, by single or sequential grafting.

This article describes a computer model for calculating left epicardial coronary blood pressure and flow waveforms of a right dominant coronary circulation. Using the geometry of 16 vascular branches and employing the one-dimensional Navier-Stokes equations the model allows for the prediction of blood pressure and flow patterns in normal and stenosed vessels. This model was also used to predict the haemodynamic changes observed after insertion of two single saphenous vein bypass grafts, as compared with the corresponding changes after insertion of a sequential (snake-like) saphenous graft. In normal vessels during systole and diastole, the pressure and the flow waveforms obtained showed patterns that correlate very well with the findings observed by other investigators using intracoronary flowmeter or Doppler velocimeter techniques. In coronary artery disease (90% stenoses in LAD and diagonal branch 1), the authors' main contribution is the reconfirmation of a previously described finding of systolic flow rises in stenotic segments. This finding seems to be an important compensatory mechanism, in contrast to normal coronary vessels, which maintain a mainly diastolic flow pattern. The introduction of single or sequential bypass grafts leads to pressure and flow restoration after graft revascularization. Besides this finding, the general concept of a diastolic flow restoration post-stenotically, in the previously decreased and systolic augmented flow areas, is also observed. The two revascularization methods were also compared with regard to their specific advantages, disadvantages and indications and were also extensively compared with several in vivo studies.

Mechanical blood traumatization by tubing and throttles in in vitro pump tests: experimental results and implications for hemolysis theory.

Blood has become essential as a test fluid to evaluate hemolysis and biocompatibility of blood pumps in vitro. The blood is usually pumped from a blood bag into a circuit against elevated pressure. A throttle or a length of tubing is used to produce the pressure head. Blood damage caused by the shear stress in these pressure-reducing devices should be minimal. It is not known whether the high but short-lasting shear stress in a throttle is more or less damaging to the blood than the low but long-lasting stress in tubing. In this study, throttles (width 11 mm, minimal height 0.9 mm, length 30 mm; shear stress = 136 N/m2 lasting for 3.23 ms); and tubing (inner diameter 9.5 mm, length 4.5 m, shear stress = 4.5 N/m2 lasting for 3.5 s) were compared at a flow of 5 L/min and a pressure drop of 150 mm Hg. Experiments (n = 10) with bovine blood were performed in two parallel setups using Bio-Medicus pumps BP80. Free hemoglobin in plasma (fHb) and thromboxane B2 (TXB2) were measured. After 6 h, the fHb increase was 31.9 +/- 19.1 mg% for the throttle setup and 32.3 +/- 16.2 for the tubing setup. The TXB2 release was 296 +/- 70 and 305 +/- 54 pg/0.1 ml respectively after 4 h. In summary, no significant differences between the two setups for either fHb or TXB2 could be detected. So the use of a throttle, which requires far less priming volume and a smaller blood-contacting surface while also offering a wider range of adjustment, seems preferable.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical simulation of shear stress on the walls of peripheral arteries.

In the last few years many attempts were made to line artificial vascular grafts with in vitro grown endothelial cell layers and thereby to minimize the risk of thromboembolism. However, adherence and resistance against shear stress forces were not tested under physiological pulsatile shear stress forces. In this paper, a mock-circulation apparatus is described, which simulates various forms of pulsatile shear stress, and which at the same time meets the requirements of cell cultivation. It can be sterilized and needs less than 700 ml of culture medium for priming. The generated flow profile can be adapted to a wide range of shear stress and also to different viscosities of used media. To take account of the different viscosities of culture medium and blood, a computerized calculation of the shear stress pattern was performed. Using the results of this computer model, the flow pattern was modified to obtain normal physiological shear stress when using culture medium. Results of pulse generation and simulation for the superficial femoral artery are presented.

Use of fibrin glue as a substrate for in vitro endothelialization of PTFE vascular grafts.

The shear stress resistance of cultured human endothelium was investigated on 6 mm polytetrafluoroethylene vascular grafts. Endothelial cell attachment was promoted by precoating the grafts with fibrin glue, which contained human fibronectin and inhibitors of fibrinolysis (aprotinin and tranexam acid). To evaluate the possible effect of fibrinolysis on cell detachment, seven grafts were lined with adult human saphenous vein endothelial cells (AHSVEC) and 11 with fibrinolytically almost inactive human umbilical vein endothelial cells (HUVEC). Endothelial cell seeding was performed in a microprocessor-controlled rotation device, allowing a low inoculum of 12 X 10(4) endothelial cells/cm2. Grafts were then cultivated for 9 days to enable the maturation of the cytoskeleton, before they were exposed to pulsatile shear stress for 48 hours. A mock circulation simulated the flow patterns and the wall shear forces of the femoral artery. After a 3-hour seeding process, 45% of AHSVEC and 43% of HUVEC were attached to the fibrin matrix, forming a confluent monolayer. After 24 hours of perfusion, a cell loss of 23% in AHSVEC- and of 42% in HUVEC-lined grafts was encountered. In spite of a further cell loss during the following 24 hours of perfusion, the majority of the graft surface was still covered by endothelial cells. Therefore we conclude that fibrin glue is a suitable substrate for the formation of a shear stress-resistant endothelial cell monolayer on polytetrafluoroethylene vascular grafts.

Pulsating blood flow in an initially stressed, anisotropic elastic tube: linear approximation of pressure waves.

A mathematical model for pulsating flow of a viscous incompressible fluid in an initially stressed elastic tube with anisotropic structure is developed and an analysis of the propagation of pressure waves is presented. The theoretical model can be applied to the study of blood flow in arteries, and its solution leads to a dispersion equation relating wave number with frequency and the rheological parameters of the wall. Determination of fluid velocities and displacement components is obtained.

The inverse Womersley problem for pulsatile flow in straight rigid tubes.

In this study a numerical solution for the problem of pulsating flow in rigid tubes is described. The method applies to the case of known flow rate waveform, as opposed to Womersley solution where the pressure gradient was the known quantity. The solution provides the pressure gradient and wall shear stress waveforms as well as the instantaneous velocity profiles. Results show that the method can be used to study the blood flow characteristics in large arteries.