Design and Evaluation of a Hybrid Mock Circulatory Loop for total Artificial Heart Testing

Aims

A hybrid mock circulatory loop (MCL) was developed for total artificial heart (TAH) performance evaluation. The hybrid MCL consists of hydraulic hardware components and a software computer model.

Design

The hydraulic components are divided into the systemic and pulmonary circulation, each of which includes electrically controlled compliances, resistors, and a venous volume which can be adjusted for a wide range of physiological and pathological conditions. The software model simulates the baroreflex autoregulatory response by automatically adjusting the hydraulic parameters according to changes of condition in the MCL.

Results

The experimental results demonstrated a good representation of the human cardiovascular system and the capability of real-time variation of physiological and pathological conditions. The functionality of the baroreflex autoregulatory mechanism was evaluated by simulation of a postural change.

Conclusions

The hybrid MCL that we developed allows variable and continuous in vitro evaluation of mechanical circulatory support devices in TAH configuration and particularly their control algorithms in response to various cardiovascular conditions. The system has been built in a modular configuration to allow testing of different types of devices and thus provides a valuable test platform prior to animal experiments.

The influence of device position on the flow within the Penn State 12 cc pediatric ventricular assist device

Ventricular assist devices are a commonly used heart failure therapy for adult patients as bridge-to-transplant or bridge-to-recovery tools. The application of adult ventricular assist devices in pediatric patients has led to increased thrombotic events. Therefore, we have been developing a pediatric ventricular assist device (PVAD), the Penn State 12 cc PVAD. It is designed for patients with a body weight of 5-15 kg and has a stroke volume of 12 cc. Clot formation is the major concern. It is correlated to the coagulability of blood, the blood contacting materials and the fluid dynamics within the system. The intent is for the PVAD to be a long term therapy. Therefore, the system may be oriented in different positions according to the patient's behavior. This study evaluates for the first time the impact of position on the flow patterns within the Penn State 12 cc PVAD, which may help to improve the PVAD design concerning chamber and ports geometries. The fluid dynamics are visualized by particle image velocimetry. The evaluation is based on inlet jet behavior and calculated wall shear rates. Vertical and horizontal model orientations are compared, both with a beat rate of 75, outlet pressures of 90/60 mm Hg and a flow rate of 1.3 l/min. The results show a significant change of the inlet jet behavior and the development of a rotational flow pattern. Vertically, the inlet jet is strong along the wall. It initiates a rotational flow pattern with a wandering axis of rotation. In contrast, the horizontal model orientation results show a weaker inlet jet along the wall with a nearly constant center of rotation location, which can be correlated to a higher risk of thrombotic events. In addition, high speed videography illustrates differences in the diaphragm motion during diastole. Diaphragm opening trajectories measurements determine no significant impact of the density of the blood analog fluids. Hence, the results correlate to human blood.

Three-dimensional flow characteristics in ventricular assist devices: impact of valve design and operating conditions

OBJECTIVE

The use of paracorporeal ventricular assist devices has become a well-established procedure for patients with cardiogenic shock. However, implantation of ventricular assist devices is often associated with severe complications, such as thrombosis inside the ventricular assist device and subsequent embolic events. It was the purpose of this study to use flow-sensitive 4-dimensional magnetic resonance imaging for a detailed analysis of the 3-dimensional (3D) flow dynamics inside a clinical routine ventricular assist device and to study the effect of different system adjustments and a new valve design on flow patterns.

METHODS

A routinely used clinical paracorporeal ventricular assist device was integrated into a magnetic resonance-compatible mock loop. Flow-sensitive 3D magnetic resonance imaging was performed to measure time-resolved 3-directional flow velocities (spatial resolution ∼ 1.2 mm, temporal resolution = 42.4 ms) in the entire device under ideal conditions (full fill, full empty, ejection fraction = 88%), insufficient filling (ejection fraction = 81%), and insufficient emptying (ejection fraction = 67%) of the pump chamber. In addition, a new valve design was evaluated. Flexible control and monitoring of pressures at inlet and outlet were used to generate realistic boundary conditions.

RESULTS

Flow pattern changes for different operating conditions were clearly identified and included reduced velocities during systolic outflow for impaired filling (78% reduction in pump flow compared with optimal operating conditions) and impaired clearing of the pump chamber for insufficient emptying (52% reduction). For all operating conditions, 3D visualization revealed vortex flow inside the ventricular assist device at typical locations of thrombus formation near the valve systems. Most noticeably, the new valve design provided similar global ventricular assist device function (pump flow 3.6 L/min), but vortex formation was eliminated.

CONCLUSIONS

The results of this study provide insight into the mechanisms underlying possible thrombus formation inside a ventricular assist device and the effect of different system adjustments. The presented methods may permit the optimization of future ventricular assist device systems with respect to optimal flow conditions.