Assessment of subjective and hemodynamic tolerance of different high‐ and low‐flux dialysis membranes in patients undergoing chronic intermittent hemodialysis: A randomized controlled trial

Clinical experience and experimental data suggest that intradialytic hemodynamic profiles could be influenced by the characteristics of the dialysis membranes. Even within the worldwide used polysulfone family, intolerance to specific membranes was occasionally evoked. The aim of this study was to compare hemodynamically some of the commonly used polysulfone dialyzers in Switzerland. We performed an open‐label, randomized, cross‐over trial, including 25 hemodialysis patients. Four polysulfone dialyzers, A (Revaclear high‐flux, Gambro, Stockholm, Sweden), B (Helixone high‐flux, Fresenius), C (Xevonta high‐flux, BBraun, Melsungen, Germany), and D (Helixone low‐flux, Fresenius, Bad Homburg vor der Höhe, Germany), were compared. The hemodynamic profile was assessed and patients were asked to provide tolerance feedback. The mean score (±SD) subjectively assigned to dialysis quality on a 1–10 scale was A 8.4 ± 1.3, B 8.6 ± 1.3, C 8.5 ± 1.6, D 8.5 ± 1.5. Kt/V was A 1.58 ± 0.30, B 1.67 ± 0.33, C 1.62 ± 0.32, D 1.45 ± 0.31. The low‐ compared with the high‐flux membranes, correlated to higher systolic (128.1 ± 13.1 vs. 125.6 ± 12.1 mmHg, P < 0.01) and diastolic (76.8 ± 8.7 vs. 75.3 ± 9.0 mmHg; P < 0.05) pressures, higher peripheral resistance (1.44 ± 0.19 vs. 1.40 ± 0.18 s × mmHg/mL; P < 0.05) and lower cardiac output (3.76 ± 0.62 vs. 3.82 ± 0.59 L/min; P < 0.05). Hypotension events (decrease in systolic blood pressure by >20 mmHg) were 70 with A, 87 with B, 73 with C, and 75 with D (P < 0.01 B vs. A, 0.05 B vs. C and 0.07 B vs. D). The low‐flux membrane correlated to higher blood pressure levels compared with the high‐flux ones. The Helixone high‐flux membrane ensured the best efficiency. Unfortunately, the very same dialyzer correlated to a higher incidence of hypotensive episodes.     

Computer assessment of hemodynamic severity of coronary artery stenosis from angiograms

A computer-assisted analysis of coronary obstructions from cineangiograms is presented, which includes both the geometric and hemodynamic evaluation of coronary stenosis severity. Single frame images are digitized into a 512 X 512 X 8 bit array after a cine-to-video conversion. Automatic edge tracking is performed using a combination of derivative and threshold methods. Vessel borders from two orthogonal views of the arterial segment are used to create a three-dimensional reconstruction of the stenosis, which serves as a basis for calculation of absolute and relative geometric dimensions, stenotic resistance and trans-stenotic pressure gradient for various given flow rates. A preliminary performance evaluation of this method was made by analysis of x-ray phantoms representing stenoses of known dimensions, which were filmed under quasi-clinical conditions. The results are discussed with respect to their accuracy and reproducibility.     

Cerebral arterial inflow assessment with F-FDG PET: Methodology and feasibility

Positron emission tomography (PET) with ¹⁸fluorodeoxyglucose (¹⁸F-FDG) is increasingly used in neurology. The measurement of cerebral arterial inflow (QA) using ¹⁸F-FDG complements the information provided by standard brain PET imaging. Here, injections were performed after the beginning of dynamic acquisitions and the time to arrival (t0) of activity in the gantry's field of view was computed. We performed a phantom study using a branched tube (internal diameter: 4 mm) and a ¹⁸F-FDG solution injected at 240 mL/min. Data processing consisted of (i) reconstruction of the first 3 s after t0, (ii) vascular signal enhancement and (iii) clustering. This method was then applied in four subjects. We measured the volumes of the tubes or vascular trees and calculated the corresponding flows. In the phantom, the flow was calculated to be 244.2 mL/min. In each subject, our QA value was compared with that obtained by quantitative cine-phase contrast magnetic resonance imaging; the mean QA value of 581.4 ± 217.5 mL/min calculated with ¹⁸F-FDG PET was consistent with the mean value of 593.3 ± 205.8 mL/min calculated with quantitative cine-phase contrast magnetic resonance imaging. Our ¹⁸F-FDG PET method constitutes a novel, fully automatic means of measuring QA.     

Characteristics of transient blood flow in MHVs with different maximum opening angles using fluid-structure interaction method

A numerical analysis has been performed to investigate the characteristics of two-dimensional transient blood flows interacting with the leaflet motion in a bileaflet mechanical heart valve with different maximum opening angles, located in the aortic position. Here, for one cycle of heartbeat the analysis has been carried out in the light of fluid-structure interaction since the blood flow and the leaflet motion are coupled with each other. Blood has been assumed to be a Newtonian and non-Newtonian fluid, where the Carreau model has been used for the simulation of non-Newtonian fluid. Physiologic ventricular and aortic pressure waveforms have been used as flow boundary conditions at the ventricle and the aorta. A finite volume computational fluid dynamics code and a finite element structure dynamics code have been used concurrently to solve the flow and the structure equations, respectively, where the two equations are strongly coupled. Flow fields, leaflet behavior, and shear stresses with time have been obtained. Also the discharge and the regurgitation flow rates have been calculated. The maximum shear stress, an important issue for valve hemodynamic analysis, has been found in the vicinity of the contact point where a leaflet contacts with housing in the final stage of the closing phase.     

Extraction of the first bolus passage in dynamic susceptibility contrast perfusion measurements

Object  The processing of dynamic susceptibility contrast perfusion measurements requires an extraction of the first bolus passage of the injected contrast agent. State-of-the-art methods employ the fit of a gamma variate function to the measured data. The use of a gamma variate function is motivated by its shape similarity to the expected relaxation rate time-course during the first bolus passage. However, the quality of this result is strongly influenced by the amount of overlap of the first and second bolus passage. In this work we present an alternative, data-driven method for the extraction of the first bolus passage from a measured relaxation time-course. Materials and methods  By using prior knowledge of the injection function, the measured time-courses can be transformed to time-courses that would occur at a shorter injection duration where the two bolus passages have less overlap. This time-course is found by Tikhonov regularized deconvolution of the measured time-courses with an injection function that bases on the measurement protocol. A minimum search yields the cut-off point at which the first bolus can be extrapolated to zero. The gamma variate fit is performed using Powells algorithm. The proposed approach is compared to the gamma variate fit approach using simulations and an exemplary dataset from one healthy volunteer. Results  The new method performs comparably stable as the gamma variate function fit approach in simulations. Both methods are superior to a simple exponential extrapolation approach. Applied to volunteer data, the new method performs much faster than the gamma variate fit approach. The results obtained from both methods correspond well. Conclusion  The new method offers a conceptual understanding of the first bolus passage and yields similar results to the gamma variate function fit approach but performs much faster.     

Flow of a newtonian fluid in a blood vessel with permeable wall—A theoretical model

A perturbation solution of the flow of a homogeneous Newtonian fluid through a tube of finite length with a permeable wall is analysed through a theoretical model in view of its applications in hemodynamics. The flow is characterized by three important parameters: G, the characteristic Reynolds number associated with the pressure outside the tube, β the ratio of radius to length of the tube, and ε a filtration coefficient. ε is assumed to be small so that the validity of the perturbation method is ensured. Perturbation solutions obtained up to O(ε²) are discussed.     

Hemodynamics in normal cerebral arteries: qualitative comparison of 4D phase-contrast magnetic resonance and image-based computational fluid dynamics

Detailed knowledge of the hemodynamic conditions in normal cerebral arteries is important for a better understanding of the underlying mechanisms leading to the initiation and progression of cerebrovascular diseases. Information about the baseline values of hemodynamic variables such as wall shear stresses is necessary for comparison to pathological conditions such as in cerebral aneurysms or arterial stenoses. The purpose of this study was to compare the blood-flow patterns in cerebral arteries of normal subjects determined by 4D phase-contrast magnetic resonance and image-based computational fluid-dynamics techniques in order to assess their consistency and to highlight their differences. The goal was not to validate (or disprove) any of the two methodologies but rather to identify regions where disagreements are to be expected and to provide guidance when interpreting the data produced by each technique.     

Fluid–structure interaction simulation of aortic blood flow

The numerical tools to simulate blood flow in the cardiovascular system are constantly developing due to the great clinical interest and to scientific advances in mathematical models and computational power. The present work aims to address and validate new algorithms to efficiently predict the hemodynamics in large arteries. These algorithms rely on finite elements simulation of the fluid–structure interaction between blood flow and arterial wall deformation of a healthy aorta. Different sets of boundary conditions are devised and tested. The mean velocity and pressure time evolution is plotted on different sections of the aorta and the wall shear stress distribution is computed. The results are compared with those obtained with a rigid wall simulation. Pulse wave velocity is computed and compared with the values available from the literature. The flow boundary conditions used for the outlets are obtained using the solution of a one-dimensional model. The results of the simulations are in agreement with the physiological data in terms of wall shear stress, wall displacement, pressure waveforms and velocities.     

The reservoir pressure concept: the 3-element windkessel model revisited? Application to the Asklepios population study

Traditionally the arterial system is either modeled as a lumped-parameter windkessel or a wave system. Recently, a hybrid model has been proposed in which the arterial system is considered to be a reservoir allowing for superimposed wave phenomena. This approach was applied to non-invasively obtained carotid pressure waveforms from 2019 subjects from the Asklepios population to investigate the contribution of reservoir pressure (PP _res,WS) to carotid pulse pressure (PP _car) with age and gender. Additionally, reservoir pressures were compared to the reservoir pressure (PP _res,WK) obtained from a 3-element windkessel model. PP _res,WK and PP _res,WS were determined by applying a 3-element windkessel model and the wave separation model to scaled carotid artery tonometry readings. The evolution of PP _car, PP _res,WK and PP _res,WS was examined for men and women after stratification into age quartiles. PP _car increased with age regardless of sex, but was more pronounced in women, with significant (P < 0.001) age–gender interaction. PP _res increases with age (P < 0.001), regardless of the model used for its determination, but more significantly for women. In men it only increases markedly in the oldest age group. Overall, the reservoir pressure concept showed large similarities to the classical 3-element windkessel model, especially in subjects characterized by a high reflection magnitude and high “windkesselness” of their arterial system. When applied to the Asklepios population, both models show the increase of pulse pressure with age to be largely due to increasing reservoir pressures.