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We examined flow characteristics and recirculation of a novel dialysis catheter with helically contoured lumens, which optimize the vectors of blood entering and leaving the device. Computational flow dynamics were used to measure shear stress, residence time (RT), Platelet Lysis Index (PLI), and recirculation of the study catheter, termed the VectorFlow catheter, compared with the Palindrome catheter. Bench and animal models were used to measure recirculation compared with other dialysis catheter designs. The VectorFlow catheter was associated with an 18% reduction in mean shear stress compared with the Palindrome catheter, at 13.6 and 16.6 Pa, respectively. Low PLI was seen with the VectorFlow (PLI = 0.019) and Palindrome (PLI = 0.015) catheters. The VectorFlow catheter was associated with a % RT >0.030 seconds of 1.2%, compared to 0.9% for the Palindrome. Calculated recirculation of the VectorFlow, Palindrome, and step-tip catheter designs in reversed position were 1.2 × 10(-4) %, 0.039%, and 13.6%, respectively. In vitro, the VectorFlow catheter had no detectable recirculation (0%) compared to 7.3-9.5% with the Palindrome at flow rates of 400-600 ml/minute. In vivo, the VectorFlow catheter had no detectable recirculation, compared to 6.7-12% for the Palindrome. The VectorFlow catheter was associated with an absence of detectable recirculation with favorable flow dynamics.

Extended nocturnal home hemodialysis has gained renewed interest. However, no removal data for single/double needle (lumen) (SL and DL, respectively) or for low/high blood flow in extended dialysis are available. Therefore, we studied dialysis adequacy in different nocturnal home hemodialysis strategies. Coupling a kinetic with a dialyzer model, we calculated a reduction ratio from pre- to post-dialysis (RR) and total solute removal (TSR) of urea, methylguanidine (MG), β2-microglobulin, and phosphate. Simulations were done for dialysis with blood flow Qb350 ml/min (DL-4h), extended DL high flow with Qb350 (DL-HF-8h) and low flow with Qb175 (DL-LF-8h), and SL with Qb273 (SL-8h). Compared to DL-4h, TSR was 28–59% larger for DL-HF-8h. TSR was most increased for β2-microglobulin (18%) with DL-LF-8h, and for MG (35%) with SL-8h. Furthermore, RRs were equal (DL-LF-8h), higher (SL-8h), and even more increased (DL-HF-8h) for all studied solutes. In the home setting, DL-LF-8h and SL-8h are safe and promising strategies.

Introduction: Catheter dysfunction is a concern when using double-lumen catheters in hemodialysis (HD). Reversing the connection mode results in higher blood flows, but also enhanced recirculation. We evaluated total solute removal (TSR) of different uremic retention solutes during a complete HD session, once with reversed (RL) and once with correctly connected lines (CL). Methods: Genius dialysis was performed in 22 HD patients at maximum blood flow (Q(B)), once with CL and once with RL. TSR was determined for urea, creatinine, phosphate and beta(2)-microglobulin (beta M-2). Using a kinetic model, we simulated TSR and reduction ratio (RR) for urea for different percentages of access recirculation and different Q(B) during CL vs. RL. Results: RR and TSR of the tested solutes were not different in clinical practice between CL and RL. Mathematically, urea RR did not differ with CL or RL, but TSR decreased by 4.5%-23.3% when changing from CL to RL for a recirculation of 5%-25%, respectively. For a recirculation of 5%-25%, Q(B) in RL should be increased by 6.7% and 52.0%, 8.5% and 72.0%, and 10.0% and 115.2%, respectively, for a blood flow in CL mode of 150, 200 or 250 ml/min. Conclusions: Connecting patients to double-lumen dialysis catheters in RL does not impair TSR in clinical practice. Mathematically, TSR during RL was dependent upon the obtained Q(B) and degree of recirculation. A nomogram indicating the increase in Q(B) needed in RL to obtain equal TSR as in the CL mode, at different degrees of recirculation, is provided.

Purpose An arteriovenous fistula (AVF) is the first choice vascular access for hemodialysis. The AVF pathway can be seen as consisting of seven segments: proximal artery, distal artery, arterial collaterals, proximal vein, distal vein, venous collaterals, and the anastomosis. While most studies describe access complications without considering the impact of the anastomosis (7th segment), the present mathematical study investigated the hemodynamic impact of anastomosis size and angle on pressure drop and flow distribution. Methods A side-to-end AVF model was developed, consisting of an anastomosis with a given cross-sectional area (substudy 1) and angle (substudy 2). Starting from two reference cases (one for each substudy) with fixed flow distribution, pressure drop over the anastomosis was calculated for an arterial inflow in the range 600 to 1200 ml/min. The same reference cases, subsequently with fixed pressure boundary conditions, were further used to assess flow distribution over the proximal vein and distal artery. Results Pressure drop decreased with a larger anastomosis cross-sectional area and an angle wider than 43°, while it was almost stable for smaller angles. Although proximal arterial inflow increased for larger anastomosis areas, the overall flow distribution shifted almost totally to the proximal vein. When the anastomosis angle exceeded 58°, the proximal arterial inflow was not sufficient to deliver enough flow, leading to distal arterial flow reversal. Conclusion Despite the underestimation of the hemodynamic impact of the anastomosis size and angle in the literature, this study showed major influences on the pressure drop over the anastomosis and, with it, on flow distribution towards the arterial and venous outflow.

We compared the multigate and single gate pulsed wave Doppler (PWD) approach to assess the hemodynamics in an arteriovenous fistula for hemodialysis purposes. In particular, we evaluated volume flow and wall shear rate measurements based on PWD because: (i) volume flow measurements are typically performed to assess the maturation of the arteriovenous fistula (AVF), with low postoperative fistula flow being a prognostic marker for failure of the AVF; (ii) wall shear stress may play an important role in vascular remodeling processes of the fistula. As in-vivo validation of fistula flow measurements is cumbersome, we performed simulations integrating computational fluid dynamics with an ultrasound (US) simulator. Flow in the arm was calculated, based on a patient-specific model of the arm vasculature pre and post AVF creation. Next, raw ultrasound signals were simulated and processed using a multigate and single gate Doppler approach in the radial artery, both for the preop and postop setting. Results showed that the extreme postop flow conditions (high velocity magnitudes and complex flow profiles) hampered accurate assessment of both wall shear rate and volume flow, while reasonable estimates were obtained preop (lower velocity magnitudes and laminar flow).

In this work we investigated the possibility to predict the pressure drops across a patient-specific arterio-venous fistula (AVF) by means of an open-source hemodynamics solver aimed at convection-dominated incompressible flows. To account for the very high flow rates that develop in AVFs we considered a wide range of steady input flow conditions (corresponding to Reynolds numbers 100, 200, 550, 1000, 1500, and 2000), and compared with experiments for over 200 flow rates, up to Reynolds 2000. Three meshes for the numerical model, based on a micro-CT acquisition of the in vitro silicon model, were generated, in order to perform a h-refinement study and assess the mesh density allowing to correctly estimate the losses across the anastomosis. For the sake of validation, in addition to pressure assessment, the velocity solutions for Re 550 and 1000 were compared with particle image velocimetry (PIV) acquisitions. Once the solver was validated we also simulated pulsatile input flow conditions to investigate the role of pulsatility in predicting pressure drops. When the finer grid is considered almost all the experimental values for the pressure drop vs. flow measurements are within the standard deviation range of the numerical pressure drops. For the PIV validation, a good agreement is observed between in vitro data and numerical results. The ability to simulate unstable convection-dominated flows in complex 3D geometries is demonstrated and more insight is obtained about the non-common physiological flow conditions induced by fistula creation.

Purpose: Maturation of an arterio-venous fistula (AVF) frequently fails, with low postoperative fistula flow as a prognostic marker for this event. As pulsed wave Doppler (PWD) is commonly used to assess volume flow, we studied the accuracy of this measurement in the setting of a radio-cephalic AVF. Methods: As in-vivo validation of fistula flow measurements is cumbersome, we performed simulations, integrating computational fluid dynamics with an ultrasound (US) simulator. Flow in the arm was calculated, based on a patient-specific model of the arm vasculature pre and post AVF creation. Next, raw ultrasound signals were simulated, from which the Doppler spectra were calculated in both a proximal (brachial) and a distal (radial) location. Results: The velocity component in the direction of the US beam, in a centred, small, sample volume, can be captured accurately using PWD spectrum mean-tracking. However, deriving flow rate from these measurements is prone to errors: (i) the angle-correction which is influenced by the radial velocity components in the complex flow field; (ii) the largest error is introduced due to a lack of knowledge on the spatial flow profile.

Double lumen (DL) central venous catheters (CVC) often suffer from thrombosis, fibrin sheet formation, and/or suction towards the vessel wall, resulting in insufficient blood flow during hemodialysis. Reversing the catheter connection often restores blood flows, but will lead to higher recirculation. Single lumen (SL) CVCs have often fewer flow problems, but they inherently have some degree of recirculation. To assist bedside clinical decision making on optimal catheter application, we investigated mathematically the differences in dialysis adequacy using different modes of access with CVCs. A mathematical model was developed to calculate reduction ratio (RR) and total solute removal (TSR) of urea, methylguanidine (MG), beta-2-microglobulin (β2M), and phosphate (P) during different dialysis scenarios: 4-h dialysis with a well-functioning DL CVC (DL-normal, blood flow QB 350 ml/min), dysfunctional DL CVC (DL-low flow, QB 250), reversed DL CVC (DL-reversed, QB 350, recirculation R = 10%) and 12 Fr SL CVC (effective QB273). With DL-normal as reference, urea RR was decreased by 3.5% (DL-reversed), 13.0% (SL), and 15.6% (DL-low flow), while urea TSR was decreased by 3.3% (DL-reversed), 13.2% (SL), and 13.5% (DL-low flow). The same trend was found for MG and P. However, β2M RR decreased only 1.5% with SL CVC although TSR decrease was 17.2%, while RR decreased 21.1% with DL-low flow although TSR decrease was only 4.9%. In the case of dysfunctional DL CVCs, reversing the catheter connection and restoring the blood flow did not impair TSR, with 10% recirculation. The SL CVC showed suboptimal TSR results that were similar to those of the dysfunctional DL CVC.

We examined flow characteristics and recirculation of a novel dialysis catheter with helically contoured lumens, which optimize the vectors of blood entering and leaving the device. Computational flow dynamics were used to measure shear stress, residence time (RT), Platelet Lysis Index (PLI), and recirculation of the study catheter, termed the VectorFlow catheter, compared with the Palindrome catheter. Bench and animal models were used to measure recirculation compared with other dialysis catheter designs. The VectorFlow catheter was associated with an 18% reduction in mean shear stress compared with the Palindrome catheter, at 13.6 and 16.6 Pa, respectively. Low PLI was seen with the VectorFlow (PLI = 0.019) and Palindrome (PLI = 0.015) catheters. The VectorFlow catheter was associated with a % RT0.030 seconds of 1.2%, compared to 0.9% for the Palindrome. Calculated recirculation of the VectorFlow, Palindrome, and step-tip catheter designs in reversed position were 1.2 × 10(-4) %, 0.039%, and 13.6%, respectively. In vitro, the VectorFlow catheter had no detectable recirculation (0%) compared to 7.3-9.5% with the Palindrome at flow rates of 400-600 ml/minute. In vivo, the VectorFlow catheter had no detectable recirculation, compared to 6.7-12% for the Palindrome. The VectorFlow catheter was associated with an absence of detectable recirculation with favorable flow dynamics.

Today, hemodialysis is a common therapy to treat people with severe chronic kidney disease. This therapy strongly relies upon the vascular access that connects the patient’s circulation to the artificial kidney and which is obtained by surgically creating an arteriovenous fistula in the arm. However, due to the high flows involved at the venous side and elevated venous pressures, the functioning of venous valves in the arm is significantly disturbed, which too often bring about serious dysfunctions or complications in the patient [1–2]. To this end, research is done to improve the outcome of vascular access in patients on hemodialysis therapy by means of computational modeling [3]. One crucial challenge, however, is experimental validation of these computer models, preferably by using Particle Image Velocimetry (PIV) for simulations of flow fields. Yet, the task of modeling the venous valve is daunting because this valve functions at very low physiological pressure differences. Moreover, PIV requires an experimental model to be fully transparent. In this study, we propose an innovative design of a PIV-compatible venous valve model which has the ability to function at minimal pressure differences and which is able to generate valuable PIV data.Copyright © 2011 by ASME

Image-based computational modeling offers a virtual access to spatially and temporally high resolution flow and structural mechanical data in vivo. Due to inter-subject morphological variability, mesh generation represents a critical step in modeling the patient-specific geometry and is usually performed using unstructured tetrahedral meshing algorithms. Although hexahedral structured meshes are known to provide higher accuracy and reduce the computational costs both for Finite Element Analysis and Computational Fluid Dynamics, their application in computational cardiovascular studies is challenging due to the complex 3D and branching topology of vascular territories. In this study, we propose a robust procedure for structured mesh generation, tailoring the mesh structure to the subject-specific vessel topology. The proposed methodology is based on centerline-based synthetic descriptors (i.e. centerlines, radii and centerlines' normals) which are used to solve the meshing problem following a bottom-up approach. First, topologically equivalent block-structures are placed inside and outside the lumen domain. Then, a projection operation is performed, returning a parametric volume mesh which fits the original triangulated model with sub-micrometric accuracy. Additionally, a three-layered arterial wall (resembling the intima, media and adventitia) is artificially generated, with the possibility of setting variable thickness (e.g. proximal-to-distal tapering) and material anisotropy (e.g. position-dependent collagen-fibers' orientation). This new meshing procedure, implemented using open-source software packages only, is demonstrated on two challenging human cases, being an aortic arch and an abdominal aortic aneurysm. High-quality meshes are generated in both cases, according to shape-quality metrics. By increasing the computation accuracy, the developed meshing tool has the potential to further add "confidence" to the use of computational methods in vascular applications.

Hemodialysis vascular access (VA) stenosis remains a frequent complication. However, early detection is challenging and costly. The aim of this in vitro study was to assess a new detection method based on pulse pressure analysis at the hemodialysis needle. A silicon model of a radiocephalic arteriovenous fistula was built in a mock loop. Pressure profiles were measured at the arterial hemodialysis needle and in the proximal feeding artery. Stenoses (50 and 25% diameter reduction) were created proximal to the anastomosis (proximal artery) and distal to the arterial needle (distal vein and proximal vein). The pulse pressure (PP) at the needle was divided by the PP at the feeding artery to obtain a dimensionless ratio, %PP. Experiments were conducted at different blood flow (500-1200 mL/min) and heart rates (60-90 beats/min) to test this new index over a wide range of hemodynamic conditions. In the control model (no stenosis), %PP was 20.26 +/- 4.55. A proximal artery 50% stenosis significantly decreased %PP to 7.69 +/- 2.08 (P < 0.0001), while the presence of 50% stenosis in the distal (36.20 +/- 2.12) and proximal (32.38 +/- 2.17) vein led to significantly higher values of %PP (P < 0.0001). For stenosis of 25% diameter reduction in the proximal artery, the %PP decreased to 15.45 +/- 2.13 (P = 0.0022) and the %PP increased with a 25% stenosis in the distal vein to 26.71 +/- 3.01 (P = 0.0003) and in the proximal vein to 26.53 +/- 2.67 (P = 0.0004). This in vitro study shows that the analysis of the PP at the dialysis needle is useful for early detection and localization of hemodialysis VA stenosis, independent of heart rate and flow level.

This study assesses malresorptive hydrocephalus treatment by ventriculosinus (VS) shunting with the shunt in antegrade or retrograde position. First, an experimental model of the cerebral ventricles, the arachnoid villi, the cortical veins and the superior sagittal sinus (SSS) was built. For this purpose, the compliance of a human cortical vein was measured ex vivo and then modelled by means of Penrose tubes. The dimensions of the superior sagittal sinus were also determined in vivo by measurements on MRI-scans of 21 patients. Secondly, with the experimental model, a numerical model of the cortical veins and the superior sagittal sinus was validated. The experimental and numerical pressure difference between the intracranial pressure and the static sinus pressure was small (0–20 Pa) and corresponded with the theoretically expected values. No overdrainage was found in either antegrade or retrograde position of the shunt. Blood reflow was only found while mimicking lumbar puncture or changes in position with the experimental model (fast lowering the intracranial pressure respectively fast increasing the sinus pressure). Both model results confirm the potential of ventriculosinus shunting as therapy for malresorptive hydrocephalus patients. The ventriculosinus shunt thus proves to be a promising technique.Copyright © 2007 by ASME

An arteriovenous fistula (AVF) is a surgically created connection between an artery and vein in the arm, made for connecting hemodialysis patients to an artificial kidney [Tordoir, 2003]. Previously, a pulse wave propagation model was developed [Huberts, 2011] that has potential in supporting decision-making in AVF surgery by predicting mean postoperative flow. The model was able to select the same AVF configuration as an experienced vascular surgeon (> 1000 AVFs created) in 9/10 patients. Although the model might already be of clinical use, model improvements can be made since the predicted flow differed in 4/10 patients with flow measured after surgery. Differences can result either from inaccurate postoperative flow measurements, inaccurate model input, neglecting adaptation and/or an incomplete physical description of the pulse wave propagation after AVF surgery. To examine if the physical description is complete the model should be validated with an experimental setup. Moreover, the experimental setup gives the possibility to validate the pressure and flow waveforms which are required when adaptation laws will be incorporated in the model. Therefore, the aim of this study was to validate the pulse wave propagation model with an experimental setup mimicking AVF surgery.