A computational exploration of helical arterio-venous graft designs.

Although arterio-venous grafts (AVGs) are the second best option as long-term vascular access for hemodialysis, they suffer from complications caused by intimal hyperplasia, mainly located in vessel regions of low and oscillating wall shear stress. However, certain flow patterns in the bulk may reduce these unfavorable hemodynamic conditions. We therefore studied, with computational fluid dynamics (CFD), the impact of a helical AVG design on the occurrence of (un)favorable hemodynamic conditions at the venous anastomosis. Six CFD-models of an AVG in closed-loop configuration were constructed: one conventional straight graft, and five helical designed grafts with a pitch of 105 mm down to 35 mm. At the venous anastomosis, disturbed shear was assessed by quantifying the area with unfavorable conditions, and by analyzing averaged values in a case-specific patch. The bulk hemodynamics were assessed by analyzing the kinetic helicity in and the pressure drop over the graft. The most helical design scores best, being instrumental to suppress disturbed shear in the venous segment. There is, however, no trivial relationship between the number of helix turns of the graft and disturbed shear in the venous segment, when a realistic closed-loop AVG model is investigated. Bulk flow investigation showed a marked increase of helicity intensity in, and a moderate pressure drop over the AVG by introducing a lower pitch. At the venous anastomosis, unfavorable hemodynamic conditions can be reduced by introducing a helical design. However, due to the complex flow conditions, the optimal helical design for an AVG cannot be derived without studying case by case.

Experimental validation of a pulse wave propagation model for predicting hemodynamics after vascular access surgery.

Hemodialysis patients require a vascular access that is, preferably, surgically created by connecting an artery and vein in the arm, i.e. an arteriovenous fistula (AVF). The site for AVF creation is chosen by the surgeon based on preoperative diagnostics, but AVFs are still compromised by flow-associated complications. Previously, it was shown that a computational 1D-model is able to describe pressure and flow after AVF surgery. However, predicted flows differed from measurements in 4/10 patients. Differences can be attributed to inaccuracies in Doppler measurements and input data, to neglecting physiological mechanisms or to an incomplete physical description of the pulse wave propagation after AVF surgery. The physical description can be checked by validating against an experimental setup consisting of silicone tubes mimicking the aorta and arm vasculature both before and after AVF surgery, which is the aim of the current study. In such an analysis, the output uncertainty resulting from measurement uncertainty in model input should be quantified. The computational model was fed by geometrical and mechanical properties collected from the setup. Pressure and flow waveforms were simulated and compared with experimental waveforms. The precision of the simulations was determined by performing a Monte Carlo study. It was concluded that the computational model was able to simulate mean pressures and flows accurately, whereas simulated waveforms were less attenuated than experimental ones, likely resulting from neglecting viscoelasticity. Furthermore, it was found that in the analysis output uncertainties, resulting from input uncertainties, cannot be neglected and should thus be considered.

Pressure monitoring during neuroendoscopy: new insights.

BACKGROUND: Significant increases in intracranial pressure (ICP) may occur during neuroendoscopic procedures. To detect and prevent serious and sustained increases, ICP should be monitored. At present, controversy exists on the optimal location of the monitoring sensor. Therefore, we conducted an in vitro study to estimate the pressure gradients between the ventricle, the 'gold standard' site, and the rinsing inlet and outlet. METHODS: A head model and a standard endoscope were used. Rinsing was enforced by using a pressurized infusion bag. Using clinically relevant flow rates, pressure was measured at the rinsing inlet and outlet, in the ventricle, and at the distal end of the rinsing channel using a tip sensor or a capillary tube. RESULTS: At a flow of 61 ml min(-1), the steady-state pressures measured at the rinsing inlet, in the ventricle, and at the rinsing outlet were 38, 26, and 12 mm Hg, respectively. At 135 ml min(-1), these increased to 136, 89, and 42 mm Hg. Transendoscopic pressure measurements were always within 1 mm Hg of the ventricular pressure. CONCLUSIONS: During endoscopy, measurements at the rinsing inlet overestimated the ventricular pressure by ∼50 mm Hg during heavy rinsing, whereas measurements at the rinsing outlet underestimated the pressure by ∼50 mm Hg. An electronic tip sensor or a pressure capillary tube placed at the distal end of the lumen of the rinsing channel of the endoscope did not interfere with rinsing flow and produced measurements that were equal to ventricular pressures.

Experimental and numerical modelling of the ventriculosinus shunt (El-Shafei shunt).

This study assesses malresorptive hydrocephalus treatment by ventriculosinus shunting with the shunt in the antegrade or retrograde position. First, an experimental model of the cerebral ventricles, the arachnoid villi, the cortical veins, and the superior sagittal sinus was built. For this purpose, the compliance of a human cortical vein was measured and then modelled by means of Penrose tubes. The dimensions of the superior sagittal sinus were determined in vivo by measurements on magnetic resonance imaging scans of 21 patients. Second, a numerical model of the cortical veins and the superior sagittal sinus was built. The numerical results were validated with the results from the experimental model. The experimental and numerical pressure difference between the intracranial pressure and the static sinus pressure was small (0-20 Pa) and corresponded to the theoretically expected values. No overdrainage was found in either the antegrade or the retrograde position of the shunt. Blood reflow was only found while mimicking lumbar puncture or changes in position with the experimental model (lowering the intracranial pressure or increasing the sinus pressure rapidly). Optimal results can be obtained with the shunt positioned in the most downstream half of the superior sagittal sinus. The experimental and numerical results confirm the potential of ventriculosinus shunting as therapy for malresorptive hydrocephalus patients. The ventriculosinus shunt thus proves to be a promising technique.