Martin Büsen

Implementation of intrinsic lumped parameter modeling into computational fluid dynamics studies of cardiopulmonary bypass

Stroke and cerebral hypoxia are among the main complications during cardiopulmonary bypass (CPB). The two main reasons for these complications are the cannula jet, due to altered flow conditions and the sandblast effect, and impaired cerebral autoregulation which often occurs in the elderly. The effect of autoregulation has so far mainly been modeled using lumped parameter modeling, while Computational Fluid Dynamics (CFD) has been applied to analyze flow conditions during CPB. In this study, we combine both modeling techniques to analyze the effect of lumped parameter modeling on blood flow during CPB. Additionally, cerebral autoregulation is implemented using the Baroreflex, which adapts the cerebrovascular resistance and compliance based on the cerebral perfusion pressure. The results show that while a combination of CFD and lumped parameter modeling without autoregulation delivers feasible results for physiological flow conditions, it overestimates the loss of cerebral blood flow during CPB. This is counteracted by the Baroreflex, which restores the cerebral blood flow to native levels. However, the cerebral blood flow during CPB is typically reduced by 10-20% in the clinic. This indicates that either the Baroreflex is not fully functional during CPB, or that the target value for the Baroreflex is not a full native cerebral blood flow, but the plateau phase of cerebral autoregulation, which starts at approximately 80% of native flow.

Optimizing endovascular stroke treatment: removing the microcatheter before clot retrieval with stent-retrievers increases aspiration flow.

Background Flow control during endovascular stroke treatment with stent-retrievers is crucial for successful revascularization. The standard technique recommended by stent-retriever manufacturers implies obstruction of the respective access catheter by the microcatheter, through which the stent-retriever is delivered. This, in turn, results in reduced aspiration during thrombectomy. In order to maximize aspiration, we fully retract the microcatheter out of the access catheter before thrombectomy—an approach we term the ‘bare wire thrombectomy’ (BWT) technique. We verified the improved throughput with systematic in vitro studies and assessed the clinical effectiveness and safety of this method. Methods We compared aspiration flow of water through various access catheters (5–8 F) with a Rebar microcatheter (0.18 inch and 0.27 inch) and a Trevo stent-retriever using the standard technique and the BWT technique in vitro. We also retrospectively analyzed 302 retrieval maneuvers in 117 patients who received endovascular treatment with a stent-retriever between February 2010 and April 2015. Results In the in vitro experiment, removal of the microcatheter in all tested settings resulted in significantly increased aspiration flow through the access catheter (p<0.001). This effect was particularly pronounced in access catheters with a diameter of ≤7 F. In the clinical study, the revascularization rate (Thrombolysis In Cerebral Infarction ≥2b) was 91%. There were no complications associated with the BWT technique in 302 retrieval maneuvers. Conclusions The BWT technique results in improved aspiration flow rates compared with the standard deployment technique. Our clinical data show that the BWT technique is effective and safe.

In vitro flow investigations in the aortic arch during cardiopulmonary bypass with stereo-PIV

The cardiopulmonary bypass is related to complications like stroke or hypoxia. The cannula jet is suspected to be one reason for these complications, due to the sandblast effect on the vessel wall. Several in silico and in vitro studies investigated the underlying mechanisms, but the applied experimental flow measurement techniques were not able to address the highly three-dimensional flow character with a satisfying resolution. In this work in vitro flow measurements in a cannulated and a non-cannulated aortic silicone model are presented. Stereo particle image velocimetry measurements in multiple planes were carried out. By assembling the data of the different measurement planes, quasi 3D velocity fields with a resolution of~1.5×1.5×2.5 mm(3) were obtained. The resulting velocity fields have been compared regarding magnitude, streamlines and vorticity. The presented method shows to be a suitable in vitro technique to measure and address the three-dimensional aortic CPB cannula flow with a high temporal and spatial resolution.

Numerical washout study of a pulsatile total artificial heart

Purpose For blood pumps with long term indication, blood stagnation can result in excessive thromboembolic risks for patients. This study numerically investigates the washout performance of the left pump chamber of a pulsatile total artificial heart (TAH) as well as the sensitivity of the rotational orientation of the inlet bileaflet mechanical heart valve (MHV) on blood stagnation. Methods To quantitatively evaluate the washout efficiency, a fluid-structure interaction (FSI) simulation of the artificial heart pumping process was combined with a blood washout model. Four geometries with different orientations (0°, 45°, 90° and 135°) of the inlet valve were compared with respect to washout performance. Results The calculated flow field showed a high level of agreement with particle image velocimetry (PIV) measurements. Almost complete washout was achievable after three ejection phases. Remains of old blood in relation to the chamber volume was below 0.6% for all configurations and were mainly detected opposite to the inlet and outlet port at the square edge where the membrane and the pump chamber are connected. Only a small variation in the washout efficiency and the general flow field was observed. An orientation of 0° showed minor advantages with respect to blood stagnation and recirculation. Conclusions Bileaflet MHVs were demonstrated to be only slightly sensitive to rotation regarding the washout performance of the TAH. The proposed numerical washout model proved to be an adequate tool to quantitatively compare different configurations and designs of the artificial organ regarding the potential for blood stagnation where experimental measurements are limited.

In Vitro Evaluation of Intra-Aneurysmal, Flow-Diverter-Induced Thrombus Formation: A Feasibility Study

BACKGROUND AND PURPOSE: Intracranial aneurysm treatment by flow diverters aims at triggering intra-aneurysmal thrombosis. By combining in vitro blood experiments with particle imaging velocimetry measurements, we investigated the time-resolved thrombus formation triggered by flow diverters. MATERIALS AND METHODS: Two test setups were built, 1 for particle imaging velocimetry and 1 for blood experiments, both generating the same pulsatile flow and including a silicone aneurysm model. Tests without flow diverters and with 2 different flow-diverter sizes (diameter: 4.5 and 4.0 mm) were performed. In the blood experiments, the intra-aneurysmal flow was monitored by using Doppler sonography. The experiments were stopped at 3 different changes of the spatial extent of the signal. RESULTS: No thrombus was detected in the aneurysm model without the flow diverter. Otherwise, thrombi were observed in all aneurysm models with flow diverters. The thrombi grew from the proximal side of the aneurysm neck with fibrin threads connected to the flow diverter and extending across the aneurysm. The thrombus resulting from the 4.0-mm flow diverter grew along the aneurysm wall as a solid and organized thrombus, which correlates with the slower velocities near the wall detected by particle imaging velocimetry. The thrombus that evolved by using the 4.5-mm flow diverter showed no identifiable growing direction. The entire thrombus presumably resulted from stagnation of blood and correlates with the central vortex detected by particle imaging velocimetry. CONCLUSIONS: We showed the feasibility of in vitro investigation of time-resolved thrombus formation in the presence of flow diverters.

Effect of Inflow Cannula Tip Design on Potential Parameters of Blood Compatibility and Thrombosis

During ventricular assist device support, a cannula acts as a bridge between the native cardiovascular system and a foreign mechanical device. Cannula tip design strongly affects the function of the cannula and its potential for blood trauma. In this study, the flow fields of five different tip geometries within the ventricle were evaluated using stereo particle image velocimetry. Inflow cannulae with conventional tip geometries (blunt, blunt with four side ports, beveled with three side ports, and cage) and a custom-designed crown tip were interposed between a mixed-flow rotary blood pump and a compressible, translucent silicone left ventricle. The contractile function of the failing ventricle and hemodynamics were reproduced in a mock circulation loop. The rotary blood pump was interfaced with the ventricle and aorta and used to fully support the failing ventricle. Among these five tip geometries, high-shear volume ( γ ˙ ≥ 2778 / s , potential parameter of platelet activation) was found to be the greatest in the blunt tip. The cage tip was observed to have the highest low-shear volume and recirculation volume ( γ ˙ ≤ 100 / s and Vz  > 0, respectively; potential parameters of thrombus formation). The crown tip, together with conventional tip geometries with side ports (blunt with four side ports and beveled with three side ports) showed no significant difference in either high-shear volume or low-shear volume. However, recirculation volume was reduced significantly in the crown tip. Despite limited generalizability to clinical situations, these transient-state measurements supported the potential mitigation of complications by changing the design of conventional cannula tip geometries.

Experimental Approach to Visualize Flow in a Stacked Hollow Fiber Bundle of an Artificial Lung With Particle Image Velocimetry: FLOW VISUALIZATION IN AN ARTIFICIAL LUNG

Flow distribution is key in artificial lungs, as it directly influences gas exchange performance as well as clot forming and blood damaging potential. The current state of computational fluid dynamics (CFD) in artificial lungs can only give insight on a macroscopic level due to model simplification applied to the fiber bundle. Based on our recent work on wound fiber bundles, we applied particle image velocimetry (PIV) to the model of an artificial lung prototype intended for neonatal use to visualize flow distribution in a stacked fiber bundle configuration to (i) evaluate the feasibility of PIV for artificial lungs, (ii) validate CFD in the fiber bundle of artificial lungs, and (iii) give a suggestion how to incorporate microscopic aspects into mainly macroscopic CFD studies. To this end, we built a fully transparent model of an artificial lung prototype. To increase spatial resolution, we scaled up the model by a factor of 5.8 compared with the original size. Similitude theory was applied to ensure comparability of the flow distribution between the device of original size and the scaled-up model. We focused our flow investigation on an area (20 × 70 × 43 mm) in a corner of the model with a Stereo-PIV setup. PIV data was compared to CFD data of the original sized artificial lung. From experimental PIV data, we were able to show local flow acceleration and declaration in the fiber bundle and meandering flow around individual fibers, which is not possible using state-of-the-art macroscopic CFD simulations. Our findings are applicable to clinically used artificial lungs with a similar stacked fiber arrangement (e.g., Novalung iLa and Maquet QUADROX-I). With respect to some limitations, we found PIV to be a feasible experimental flow visualization technique to investigate blood-sided flow in the stacked fiber arrangement of artificial lungs.

Necessary Catheter Diameters for Mechanical Thrombectomy with ADAPT

BACKGROUND AND PURPOSE: Large-bore catheters allow mechanical thrombectomy in ischemic stroke by engaging and retrieving clots without additional devices (direct aspiration first-pass technique [ADAPT]). The purpose of this study was to establish a model for minimal catheter diameters needed for ADAPT. MATERIALS AND METHODS: We established a theoretic model for the calculation of minimal catheter diameters needed for ADAPT. We then verified its validity in 28 ADAPT maneuvers in a porcine in vivo model. To account for different mechanical thrombectomy techniques, we factored in ADAPT with/without a hypothetic 0.021-inch microcatheter or 0.014-inch microwire inside the lumen of the aspiration catheter and aspiration with a 60-mL syringe versus an aspiration pump. RESULTS: According to our calculations, catheters with an inner diameter of >0.040 inch and >0.064 inch, respectively, are needed to be effective in the middle cerebral artery (2.5-mm diameter) or in the internal carotid artery (4 mm) in an average patient. There was a significant correlation between predicted and actual thrombectomy results (P = .010). Our theoretic model had a positive and negative predictive value of 78% and 79%, respectively. Sensitivity and specificity were 88% and 64%, respectively. CONCLUSIONS: Our theoretic model allows estimating the minimal catheter diameters needed for successful mechanical thrombectomy with ADAPT, as demonstrated by the good agreement with our animal experiments. Our model will be helpful to interventionalists in avoiding selecting catheters that are likely too small to be effective.

Development of a Numerical Pump Testing Framework

It has been shown that left ventricular assist devices (LVADs) increase the survival rate in end-stage heart failure patients. However, there is an ongoing demand for an increased quality of life, fewer adverse events, and more physiological devices. These challenges necessitate new approaches during the design process. In this study, computational fluid dynamics (CFD), lumped parameter (LP) modeling, mock circulatory loops (MCLs), and particle image velocimetry (PIV) are combined to develop a numerical Pump Testing Framework (nPTF) capable of analyzing local flow patterns and the systemic response of LVADs. The nPTF was created by connecting a CFD model of the aortic arch, including an LVAD outflow graft to an LP model of the circulatory system. Based on the same geometry, a three-dimensional silicone model was crafted using rapid prototyping and connected to an MCL. PIV studies of this setup were performed to validate the local flow fields (PIV) and the systemic response (MCL) of the nPTF. After validation, different outflow graft positions were compared using the nPTF. Both the numerical and the experimental setup were able to generate physiological responses by adjusting resistances and systemic compliance, with mean aortic pressures of 72.2-132.6 mm Hg for rotational speeds of 2200-3050 rpm. During LVAD support, an average flow to the distal branches (cerebral and subclavian) of 24% was found in the experiments and the nPTF. The flow fields from PIV and CFD were in good agreement. Numerical and experimental tools were combined to develop and validate the nPTF, which can be used to analyze local flow fields and the systemic response of LVADs during the design process. This allows analysis of physiological control parameters at early development stages and may, therefore, help to improve patient outcomes.

Combined Computational and Experimental Approach to Improve the Assessment of Mitral Regurgitation by Echocardiography

Mitral regurgitation (MR) is one of the most frequent valvular heart diseases. To assess MR severity, color Doppler imaging (CDI) is the clinical standard. However, inadequate reliability, poor reproducibility and heavy user-dependence are known limitations. A novel approach combining computational and experimental methods is currently under development aiming to improve the quantification. A flow chamber for a circulatory flow loop was developed. Three different orifices were used to mimic variations of MR. The flow field was recorded simultaneously by a 2D Doppler ultrasound transducer and Particle Image Velocimetry (PIV). Computational Fluid Dynamics (CFD) simulations were conducted using the same geometry and boundary conditions. The resulting computed velocity field was used to simulate synthetic Doppler signals. Comparison between PIV and CFD shows a high level of agreement. The simulated CDI exhibits the same characteristics as the recorded color Doppler images. The feasibility of the proposed combination of experimental and computational methods for the investigation of MR is shown and the numerical methods are successfully validated against the experiments. Furthermore, it is discussed how the approach can be used in the long run as a platform to improve the assessment of MR quantification.