[Show abstract][Hide abstract] ABSTRACT: Implantable left ventricular assist devices (LVADs) became the therapy of choice in treating end-stage heart failure. Although survival improved substantially and is similar in currently clinically implanted LVADs HeartMate II (HM II) and HeartWare HVAD, complications related to blood trauma are frequently observed. The aim of this study was to compare these two pumps regarding their potential blood trauma employing computational fluid dynamics. High-resolution structured grids were generated for the pumps. Newtonian flow was calculated, solving Reynolds-averaged Navier–Stokes equations with a sliding mesh approach and a k-ω shear stress transport turbulence model for the operating point of 4.5 L/min and 80 mm Hg. The pumps were compared in terms of volumes subjected to certain viscous shear stress thresholds, below which no trauma was assumed (von Willebrand factor cleavage: 9 Pa, platelet activation: 50 Pa, and hemolysis: 150 Pa), and associated residence times. Additionally, a hemolysis index was calculated based on a Eulerian transport approach. Twenty-two percent of larger volumes above 9 Pa were observed in the HVAD; above 50 Pa and 150 Pa the differences between the two pumps were marginal. Residence times were higher in the HVAD for all thresholds. The hemolysis index was almost equal for the HM II and HVAD. Besides the gap regions in both pumps, the inlet regions of the rotor and diffuser blades have a high hemolysis production in the HM II, whereas in the HVAD, the volute tongue is an additional site for hemolysis production. Thus, in this study, the comparison of the HM II and the HVAD using numerical methods indicated an overall similar tendency to blood trauma in both pumps. However, influences of turbulent shear stresses were not considered and effects of the pivot bearing in the HM II were not taken into account. Further in vitro investigations are required.
No preview · Article · Aug 2015 · Artificial Organs
[Show abstract][Hide abstract] ABSTRACT: When using artificial surfaces that come into contact with the bloodstream, it is important to consider the undesirable consequences of thrombus formation and embolization. Although great progress has been made by creating new surfaces and antithrombotic coatings or evaluating flow conditions, unexpected platelet adhesion and aggregation can lead to the sudden formation of an adverse thrombus. Our experiments in a stagnation point flow chamber with citrate-anticoagulated whole blood and ADP-stimulated platelets mimicked the situations of implanted artificial organs, e.g., mechanical circulatory support devices, or extravascular circulation. With video microscopy, real-time platelet characteristics were observed at shear rate levels between 50 and 500 s−1 on glass, von Willebrand factor, and polyurethane surfaces for at least 5 min after the first contact. Platelet adhesion and aggregation were observed with distinctness in aggregate size, surface coverage, aggregate size, probability of an embolic event, and platelet contraction. The probability of an embolic event increased at lower flow rates. Additionally, platelet contraction was affected by the flow rate. Raising the flow rate intensified the platelet contraction. With this setup, the microembolization caused by surface contact and flow and platelet contraction can be detected in a real-time direct observation. This capability addresses both technical and clinical issues, such as thrombus and embolus formation, and may improve the research on the hemocompatibility of biomaterials.
[Show abstract][Hide abstract] ABSTRACT: Intravital microscopy (IVM) is a well-established imaging technique for real-time monitoring of microscale lung tissue dynamics. Although accepted as a gold standard in respiratory research, its characteristic image features are scarcely understood, especially when trying to determine the actual position of alveolar walls. To allow correct interpretation of these images with respect to the true geometry of the lung parenchyma, we analyzed IVM data of alveoli in a mouse model in comparison with simultaneously acquired optical coherence tomography images. Several IVM characteristics, such as double ring structures or disappearing alveoli in regions of liquid filling, could be identified and related to the position of alveoli relative to each other. Utilizing a ray tracing approach based on an idealized geometry of the mouse lung parenchyma, two major reflection processes could be attributed to the IVM image formation: partial reflection and total internal reflection between adjacent alveoli. Considering the origin of the reflexes, a model was developed to determine the true position of alveolar walls within IVM images. These results allow thorough understanding of IVM data and may serve as a basis for the correction of alveolar sizes for more accurate quantitative analysis within future studies of lung tissue dynamics.
No preview · Article · Jun 2015 · Journal of Biomedical Optics
[Show abstract][Hide abstract] ABSTRACT: Exit-site infections remain one of the main complications for percutaneous devices, such as catheters for peritoneal dialysis or drivelines for ventricular assist devices. Many efforts have been made to create a biological seal, yet without long-term success. This study investigates a new kind of percutaneous device which is coated with an extricable polymeric membrane. The bionic approach applies the naturally outwards directed growth of skin structures to technology: by pulling the protective membrane it slowly grows out of the body and a developing sulcus is exposed to dry air and an infection is avoided. In a feasibility study this kind of device was shown to reduce the rate of infection. To further investigate these devices, they were implanted in the skin of goats and observed for a period of more than 500 days. The membranes were pulled with a force of up to 2 N and the resulting movement was recorded. When being pulled, the membranes moved 0.4-0.9 mm per week, showing that the application of a continuously acting, defined force on the protective membrane causes the desired slow movement.
No preview · Article · Mar 2015 · Biomedizinische Technik/Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: Drivelines for ventricular assist devices and catheters for peritoneal dialysis are percutaneous implants which are designed for a dwell time of at least one year. The testing of such percutaneous devices in animal experiments should represent this long period, since infection episodes can occur a long time after implantation. Usually, these animal experiments are the final step in the previous development of the implant and represent a high monetary value. However, a single malfunction is already sufficient to end the experiment. As part of a research project preclinical testing of percutaneous implants was performed in goats. Although the percutaneous devices do not cause pain, the animals will bite and pull at them and endanger the experiment. Therefore, protection of the implant is required. Standard wound dressing is not sufficient and appropriate protective garments for goats are not commercially available. Therefore, custom fit protective waistcoats were designed and manufactured. To this end the size of the goats was measured and a pattern for sewing was created. A polyester mesh fabric provides the necessary resistance and good breathability. The waistcoats were reinforced with polyethylene foam to prevent biting through the garment. Side-release buckles were chosen as fasteners. A total of six waistcoats were manufactured. They fit the goats tightly, while preserving the full range of motion. The goats tolerate them well. They are durable, secure and effectively protected the implants for a period of over one year.
[Show abstract][Hide abstract] ABSTRACT: Continuous non-invasive blood pressure measurement for long-term application remains an unsolved technical challenge. Determining blood pressure by measuring pulse transit times is a promising technique; however, it needs repeated recalibration. The correlation between blood pressure and pulse transit time changes with the elastic properties of the arteries. Experimental data are required to develop a longer calibration interval with a model-based time series analysis. A polymeric vascular model of three artery sections connected by an arterial bifurcation was set up with physiological flow and pressure curves along with physiological pulse transit times. The elastic properties of the three modelled arteries can be changed separately within a physiological range during the experiments. The vascular model provides the pressure signal and pulse wave signal upstream and downstream of the bifurcation; the flow is determined in the inlet. Physiologic pulse transit time changes in the model are mainly realized through changes in the elasticity and not variations in fluid pressure.
[Show abstract][Hide abstract] ABSTRACT: With the ever-increasing clinical application of intra vascular implants like valves, stents, grafts and ventricular assist devices the problem of thrombo-embolism has received new attention. Such a thrombo-embolic adverse event starts with the adhesion of platelets. This is investigated with the stagnation flow chamber. Objective of this paper is to research the stagnation point flow with new methods in order to elucidate questions like: How does a thrombus form, what is the role of shear rate? In the new experiments reported here fresh human blood was used. It was drawn by venipuncture from a healthy donor into 1 mM citrate solution. Flow rates were 20 and 40 ml/h and the blood entered the circular flow chamber through a tube 650 mu m in diameter. Height of the flow space was 480 mu m. The platelets were dyed with Mepacrine. Before the blood entered the flow chamber a platelet-activating agent was added. This agent was mixed with the blood with the help of a micro mixer. The flow chamber was placed in an inverted microscope and platelets deposited on the bottom plate of the flow chamber. This was recorded with fluorescent video microscopy. With the help of image processing the surface coverage (coveraged area / total area) was determined and was plotted as a function of time and shear rate at the bottom plate. The shear rate was derived from computations of the blood flow using computational fluid dynamics (CFD). The platelet deposition of nearly 50 experiments was recorded, and of these a selection of 9 experiments were analyzed and described in this paper.
[Show abstract][Hide abstract] ABSTRACT: Pressure drop associated with coarctation of the aorta (CoA) can be successfully treated surgically or by stent placement. However, a decreased life expectancy associated with altered aortic hemodynamics was found in long-term studies. Image-based computational fluid dynamics (CFD) is intended to support particular diagnoses, to help in choosing between treatment options, and to improve performance of treatment procedures. This study aimed to prove the ability of CFD to improve aortic hemodynamics in CoA patients. In 13 patients (6 males, 7 females; mean age 25 ± 14 years), we compared pre- and post-treatment peak systole hemodynamics [pressure drops and wall shear stress (WSS)] vs. virtual treatment as proposed by biomedical engineers. Anatomy and flow data for CFD were based on MRI and angiography. Segmentation, geometry reconstruction and virtual treatment geometry were performed using the software ZIBAmira, whereas peak systole flow conditions were simulated with the software ANSYS(®) Fluent(®). Virtual treatment significantly reduced pressure drop compared to post-treatment values by a mean of 2.8 ± 3.15 mmHg, which significantly reduced mean WSS by 3.8 Pa. Thus, CFD has the potential to improve post-treatment hemodynamics associated with poor long-term prognosis of patients with coarctation of the aorta. MRI-based CFD has a huge potential to allow the slight reduction of post-treatment pressure drop, which causes significant improvement (reduction) of the WSS at the stenosis segment.
No preview · Article · Sep 2014 · Annals of Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: Coils and flow diverters or stents are devices successfully used to treat cerebral aneurysms. Treatment aims to reduce intra-aneurysmal flow, thereby separating the aneurysmal sac from the blood circulation. The focus and this manuscript combining literature review and our original research is an analysis of changes in aneurysmal hemodynamics caused by endovascular treatment devices. Knowledge of post-treatment hemodynamics is a path to successful long-term treatment. Summarizing findings on hemodynamic impact of treatment devices, we conclude: coiling and stenting do not affect post-treatment intra-aneurysmal pressure, but significantly alter aneurysmal hemodynamics through flow reduction and a change in flow structure. The impact of treatment devices on aneurysmal flow depends, however, on a set of parameters including device geometry, course of placement, parent vessel and aneurysm geometry.
No preview · Article · Jun 2014 · Expert Review of Medical Devices
[Show abstract][Hide abstract] ABSTRACT: Hunterian ligation affecting hemodynamics in vessels was proposed to avoid rebleeding in a case of a fenestrated basilar artery aneurysm after incomplete coil occlusion. We studied the hemodynamics in vitro to predict the hemodynamic changes near the aneurysm remnant caused by Hunterian ligation. A transparent model was fabricated based on three-dimensional rotational angiography imaging. Arteries were segmented and reconstructed. Pulsatile flow in the artery segments near the partially occluded (coiled) aneurysm was investigated by means of particle image velocimetry. The hemodynamic situation was investigated before and after Hunterian ligation of either the left or the right vertebral artery (LVA/RVA). Since post-ligation flow rate in the basilar artery was unknown, reduced and retained flow rates were simulated for both ligation options. Flow in the RVA and in the corresponding fenestra vessel is characterized by a vortex at the vertebrobasilar junction, whereas the LVA exhibits undisturbed laminar flow. Both options (RVA or LVA ligation) cause a significant flow reduction near the aneurysm remnant with a retained flow rate. The impact of RVA ligation is, however, significantly higher. This in vitro case study shows that flow reduction near the aneurysm remnant can be achieved by Hunterian ligation and that this effect depends largely on the selection of the ligated vessel. Thus the ability of the proposed in vitro pipe-line to improve hemodynamic impact of the proposed therapy was successfully proved.
No preview · Article · May 2014 · The International journal of artificial organs
[Show abstract][Hide abstract] ABSTRACT: Cardiovascular diseases are the main cause of death in Germany. A long-term blood pressure measurement is crucial to identify hypertension which can lead to cardiovascular disease. Conventional techniques use the automatic arm cuff method which is painful and provides only intermittent results. A new method for continuous measurement is developed using a DOPPLER ultrasound sensor on a superficial artery and a small balloon. A voice coil actuator is used to change the balloon pressure using a control loop. Holding the control variable - the ultrasound signal - constant and low by con-trolling the balloon pressure permits a continuous measurement. The system was tested using a blood pressure simulator with variable pressure curves and abrupt pressure changes. The controller-induced balloon pressure tracks the pressure in the model artery very closely.
[Show abstract][Hide abstract] ABSTRACT: Aortic coarctation (CoA) accounting for 3-11% of congenital heart disease can be successfully treated. Long-term results, however, have revealed decreased life expectancy associated with abnormal hemodynamics. Accordingly, an assessment of hemodynamics is the key factor in treatment decisions and successful long-term results. In this study, 3D angiography whole heart (3DWH) and 4D phase-contrast magnetic resonance imaging (MRI) data were acquired. Geometries of the thoracic aorta with CoAs were reconstructed using ZIB-Amira software. X-ray angiograms were used to evaluate the post-treatment geometry. Computational fluid dynamics models in three patients were created to simulate pre- and post-treatment situations using the FLUENT program. The aim of the study was to investigate the impact of the inlet velocity profile (plug vs. MRI-based) with a focus on the peak systole pressure gradient and wall shear stress (WSS). Results show that helical flow at the aorta inlet can significantly affect the assessment of pressure drop and WSS. Simplified plug inlet velocity profiles significantly (p < 0.05) overestimate the pressure drop in pre- and post-treatment geometries and significantly (p < 0.05) underestimate surface-averaged WSS. We conclude that the use of the physiologically correct but time-expensive 4D MRI-based in vivo velocity profile in CFD studies may be an important step towards a patient-specific analysis of CoA hemodynamics.
Full-text · Article · Aug 2013 · Annals of Biomedical Engineering
[Show abstract][Hide abstract] ABSTRACT: Background and study aims:
Image-based computational fluid dynamics (CFD) provides a means for analysis of biofluid mechanical parameters of cerebral aneurysms. This may enable patient-specific rupture risk analysis and facilitate treatment decisions. Application of different imaging methods may, however, alter the geometrical basis of these studies. The present study compares geometry and hemodynamics of an aneurysm phantom model acquired by means of magnetic resonance imaging (MRI), computed tomography (CT), and rotational angiography (3DRA).
Materials and methods:
The phantom model of a basilaris artery aneurysm was fabricated based on data generated by CT angiography. This model underwent imaging by means of CT, MRI, and 3DRA. We compared the geometrical reconstructions using the original dataset with those obtained from CT, MRI, and 3DRA. Similarly, CFD analyses were performed using the four reconstructions (3DRA, MRI, CT, and original dataset).
MRI and the 3DRA-based reconstructions yield mean reconstruction errors of 0.097 mm and 0.1 mm, which are by a factor of 2.5 better than the CT reconstruction. The maximal error for the aneurysm radius (7.11 mm) measurement was found in the 3DRA reconstruction and was 3.8% (0.28 mm). A comparison of calculated time-averaged wall shear stress (WSS) shows good correlations for the entire surface and, separately, for the surface of the aneurysmal sack. The maximal error of 8% of the mean WSS calculation of the whole surface was found for the CT reconstruction. The calculations of the aneurysmal sack mean WSS from the MRI reconstruction were estimated to have a maximal error of 7%.
All three imaging techniques (CT, MRI, 3DRA) adequately reproduce aneurysm geometry and allow meaningful CFD analyses.
No preview · Article · May 2013 · Journal of Neurological Surgery. Part A: Central European Neurosurgery
[Show abstract][Hide abstract] ABSTRACT: In the course of pulmonary research, understanding alveolar tissue dynamics plays a critical role in the treatment of patients suffering from acute lung diseases. As a gold standard technique for monitoring micro scale changes of lung tissue, real-time intra-vital microscopy (IVM) has been established to evaluate the behavior of the alveolar tissue. To allow profound qualitative and quantitative conclusions, characteristic features of the obtained images have to be thoroughly understood. These factors are strongly influenced by the imaging setup and physiological condition of the lung. To circumvent misinterpretations, a ray-tracing approach has been applied in this study using an idealized geometry of the mouse lung parenchyma deduced from optical coherence tomography (OCT) as a complementary imaging technique. Basic features of IVM images are double ring structures and disappearing of alveoli related to liquid infiltration. Ray propagation analysis reveals the formation of these features by two major reflection processes: partial reflection and total internal reflection. The results give rise to quantification errors of the alveolar area related to reflexes misinterpreted as alveolar borders and should further be used to yield a correction factor for future IVM lung tissue studies.
[Show abstract][Hide abstract] ABSTRACT: The suggested concept of rhinorespiratory homeostasis is a new theoretical model for the discussion of physiologic and physical principles of nasal breathing. This model is based on a comprehensive view of nasal functions that takes comparative animal physiology into account. Consequently, it has a universal cross-species character and emphasizes the central role of nasal secretion. In contrast to the established view, the focus is transferred from the inspired air to the nasal wall. This concept considers the parietal effect of airflow represented by wall shear stress with special regard to the epithelial lining fluid. It delivers one possible mechanism of an inherent triggering of the nasal cycle. Furthermore, the issue of biological fluid-structure interaction is introduced. This article presents a rethinking of nasal breathing that was inspired by clinical experience and results of flow field investigations through computational fluid dynamics.
Full text under: https://www.thieme-connect.com/ejournals/pdf/10.1055/s-0033-1341590.pdf
No preview · Article · Apr 2013 · Facial Plastic Surgery
[Show abstract][Hide abstract] ABSTRACT: Platelet deposition to collagen-coated surface under low shear conditions was investigated using an experimental model. The flow chamber was created by combining a stationary and a rotational glass plates spaced 50μm apart. Blood filled into this space was subjected to a simple Couette flow. Both glass plates were covered with albumin to render them anti-thrombogenic. However, one spot 1×1mm in size was covered with collagen. This spot was where the platelets deposited. The device was mounted on an inverted microscope and the platelet deposition was recorded. Platelets were dyed to render them fluorescent. The blood used was human blood from healthy volunteers. It was subjected to a range of low shear rates (below 7001/s) to find out how they act on platelet deposition. The results show a characteristic curve with elevated platelet deposition in the range of 1501/s. For the interpretation of these results a numerical model was developed. It applies the Monte Carlo method to model a random walk of platelets. This diffusive motion was superimposed on the convective motion by the Couette flow. A satisfactory match to the experimental data was achieved.
No preview · Article · Nov 2012 · Journal of Biomechanics
[Show abstract][Hide abstract] ABSTRACT: Cerebral arterial vasospasm leads to delayed cerebral ischemia and constitutes the major delayed complication following aneurysmal subarachnoid hemorrhage. Cerebral vasospasm can be reduced by increased blood clearance from the subarachnoid space. Clinical pilot studies allow the hypothesis that the clearance of subarachnoid blood is facilitated by means of head shaking. A major obstacle for meaningful clinical studies is the lack of data on appropriate parameters of head shaking. Our in vitro study aims to provide these essential parameters.
A model of the basal cerebral cistern was derived from human magnetic resonance imaging data. Subarachnoid hemorrhage was simulated by addition of dyed experimental blood to transparent experimental cerebrospinal fluid (CSF) filling the model of the basal cerebral cistern. Effects of various head positions and head motion settings (shaking angle amplitudes and shaking frequencies) on blood clearance were investigated using the quantitative dye washout method. Blood washout can be divided into two phases: Blood/CSF mixing and clearance. The major effect of shaking consists in better mixing of blood and CSF thereby increasing clearance rate. Without shaking, blood/CSF mixing and blood clearance in the basal cerebral cistern are hampered by differences in density and viscosity of blood and CSF. Blood clearance increases with decreased shaking frequency and with increased shaking angle amplitude. Head shaking facilitates clearance by varying the direction of gravitational force.
From this in vitro study can be inferred that patient or head shaking with large shaking angles at low frequency is a promising therapeutic strategy to increase blood clearance from the subarachnoid space.