Stefan Boës

Control of the fluid viscosity in a mock circulation

A mock circulation allows the in vitro investigation, development, and testing of ventricular assist devices. An aqueous-glycerol solution is commonly used to mimic the viscosity of blood. Due to evaporation and temperature changes, the viscosity of the solution drifts from its initial value and therefore, deviates substantially from the targeted viscosity of blood. Additionally, the solution needs to be exchanged to account for changing viscosities when mimicking different hematocrits. This article presents a method to control the viscosity in a mock circulation. This method makes use of the relationship between temperature and viscosity of aqueous-glycerol solutions and employs the automatic control of the viscosity of the fluid. To that end, an existing mock circulation was extended with an industrial viscometer, temperature probes, and a heating nozzle band. The results obtained with different fluid viscosities show that a viscosity controller is vital for repeatable experimental conditions on mock circulations. With a mixture ratio of 49 mass percent of aqueous-glycerol solution, the controller can mimic a viscosity range corresponding to a hematocrit between 29 and 42% in a temperature range of 30-42°C. The control response has no overshoot and the settling time is 8.4 min for a viscosity step of 0.3 cP, equivalent to a hematocrit step of 3.6%. Two rotary blood pumps that are in clinical use are tested at different viscosities. At a flow rate of 5 L/min, both show a deviation of roughly 15 and 10% in motor current for high rotor speeds. The influence of different viscosities on the measured head pressure is negligible. Viscosity control for a mock circulation thus plays an important role for assessing the required motor current of ventricular assist devices. For the investigation of the power consumption of rotary blood pumps and the development of flow estimators where the motor current is a model input, an integrated viscosity controller is a valuable contribution to an accurate testing environment.

ConVes: The Sutureless Aortic Graft Anastomotic Device

PURPOSE Less invasive left ventricular assist device implantation became feasible with the development of smaller devices. This study evaluated a sutureless aortic anastomosis device to facilitate the implant procedure. DESCRIPTION The novel anastomotic device deploys and anchors an acute-angled stent in the aortic wall to create a sutureless outflow graft anastomosis in the ascending aorta. Four aortic anastomoses were performed on the beating hearts of two pigs without cross-clamping or cardiopulmonary bypass. EVALUATION The procedure was fast and simple. The time of anastomosis averaged 8.1 minutes, with merely oral instructions to the operating surgeon. The design of the stent allowed the outflow graft to be implanted with the intended angulation of 45 degrees. CONCLUSIONS This proof-of-concept study demonstrates the feasibility and short-term success of the proposed sutureless anastomotic device. Further preclinical studies are necessary to evaluate long-term durability of the anastomosis.

Viscosity Prediction in a Physiologically Controlled Ventricular Assist Device

Objective: We present a novel machine learning model to accurately predict the blood-analog viscosity during support of a pathological circulation with a rotary ventricular assist device (VAD). The aim is the continuous monitoring of the hematocrit (HCT) of VAD patients with the benefit of a more reliable pump flow estimation and a possible early detection of adverse events, such as bleeding or pump thrombosis. Methods: A large dataset was generated with a blood pump connected to a hybrid mock circulation by varying the pump speed, the physiological requirements of the modeled circulation, and the viscosity of the blood-analog. The inlet pressure and the intrinsic signals of the pump were considered as inputs for the model. Gaussian process yielded models with the best performance, which were then combined using a variant of stacked generalization to derive the final model. The final model was evaluated with unseen testing data from the dataset created. Results: For these data, the model yielded a mean absolute deviation of 1.81% from the true HCT, while it proved to correctly predict the direction of the HCT change. It showed to be independent of the set speed and of the condition of the simulated cardiovascular circulation. Conclusion: The accuracy of the prediction model allows an improvement of the quality of flow estimators and the detection of adverse events at an early stage. The evaluation of this approach with blood is suggested for further validation. Significance: Its clinical application could provide the clinicians with reliable and important hemodynamic information of the patient and, thus, enhance patient monitoring and supervision.

Fluid dynamics in the HeartMate 3: Influence of the artificial pulse feature and residual cardiac pulsation

Ventricular assist devices (VADs), among which the HeartMate 3 (HM3) is the latest clinically approved representative, are often the therapy of choice for patients with end-stage heart failure. Despite advances in the prevention of pump thrombosis, rates of stroke and bleeding remain high. These complications are attributed to the flow field within the VAD, among other factors. One of the HM3s characteristic features is an artificial pulse that changes the rotor speed periodically by 4000 rpm, which is meant to reduce zones of recirculation and stasis. In this study, we investigated the effect of this speed modulation on the flow fields and stresses using high-resolution computational fluid dynamics. To this end, we compared Eulerian and Lagrangian features of the flow fields during constant pump operation, during operation with the artificial pulse feature, and with the effect of the residual native cardiac cycle. We observed good washout in all investigated situations, which may explain the low incidence rates of pump thrombosis. The artificial pulse had no additional benefit on scalar washout performance, but it induced rapid variations in the flow velocity and its gradients. This may be relevant for the removal of deposits in the pump. Overall, we found that viscous stresses in the HM3 were lower than in other current VADs. However, the artificial pulse substantially increased turbulence, and thereby also total stresses, which may contribute to clinically observed issues related to hemocompatibility.

In-vivo evaluation of ConVes a novel device for a sutureless aortic graft anastomosis

Abstracts from the 44th ESAO and 7th IFAO Congress, 6-9 September 2017, Vienna, Austria.Background: Computational Fluid Dynamics is a useful tool for developing Ventricular Assist Devices (VADs). However, the results are not necessarily trusted, and validation studies are essential to increase confidence. Validation studies usually require expensive, time consuming, for example Particle Image Velocimetry (PIV). Simpler validation methods, which could be incorporated more naturally into the design process, are therefore desirable. Aim: The aim of this work was to investigate the extent to which design changes in the computational domain produced measureable effects on the experimental pressure-flow characteristics, with a view to using rapid prototyping of early design iterations to increase confidence in CFD. Methods: A small pump, similar to a VAD, was designed using CAD. The geometry was meshed and CFD calculated using ANSYS CFX. Mesh studies were conducted, and several turbulence methods were investigated, to assess errors. Transient simulations were performed to estimate the steady flow pressure- flow curves for a range of speeds. Based on examining the results a series of manual design changes were made and the simulation results were updated for each design iteration. A physical prototype of the pump was created from 3D printed parts; these fitted together allowing replacement of individual components. The pump was driven with an external motor and shaft. The pump is currently being tested in a custom designed rig. Results: For the original design the operating speed to reach the design point (100 mmHg at 5 l/min) was 10,500 rpm. At this speed the design iterations resulted in changes to the pressure head of between 10 and 200 mmHg; alternatively speed changes of 600 to 5000 rpm were required to produce the design point. Conclusions: These pressure differences are greater than both CFD and transducer measurement errors, meaning the design changes should produce measurable effects. However, rapid prototyping also has inherent errors. Good agreement between CFD and experimental pressure-flow curves in early design iterations could be extrapolated to assume good agreement at the later design stages.