Systematic Design of a Magnetically Levitated Brushless DC Motor for a Reversible Rotary Intra-Aortic Blood Pump.

The IntraVAD is a miniature intra-aortic ventricular assist device (VAD) designed to work in series with the compromised left ventricle. A reverse-rotation control (RRc) mode has been developed to increase myocardial perfusion and reduce ventricular volume. The RRc mode includes forward rotation in systole and reverse rotation in diastole, which requires the IntraVAD to periodically reverse its rotational direction in synchrony with the cardiac cycle. This periodic reversal leads to changes in pressure force over the impeller, which makes the entire system less stable. To eliminate the mechanical wear of a contact bearing and provide active control over the axial position of the rotor, a miniature magnetically levitated bearing (i.e., the PM-Coil module) composed of two concentric permanent magnetic (PM) rings and a pair of coils-one on each side-was proposed to provide passive radial and active axial rotor stabilization. In the early design stage, the numerical finite element method (FEM) was used to optimize the geometry of the brushless DC (BLDC) motor and the maglev module, but constructing a new model each time certain design parameters were adjusted required substantial computation time. Because the design criteria for the module had to be modified to account for the magnetic force produced by the motor and for the hemodynamic changes associated with pump operation, a simplified analytic expression was derived for the expected magnetic forces. Suitable bearings could then be designed capable of overcoming these forces without repeating the complicated FEM simulation for the motor. Using this method at the initial design stage can inform the design of the miniature maglev BLDC motor for the proposed pulsatile axial-flow VAD.

In Vitro Evaluation of the Dual-Diffuser Design for a Reversible Rotary Intra-Aortic Ventricular Assist Device.

The intra-aortic ventricular assist device (IntraVAD) is a miniature intra-aortic axial-flow ventricular assist device (VAD) that works in series with the left ventricle (LV) to assist the compromised heart. Previous in vitro results have shown that the IntraVAD can successfully increase coronary perfusion and offload ventricular volume by operating in reverse-rotation control (RRc) mode. The RRc mode includes forward rotation in systole and reverse rotation (RR) in diastole. It is necessary to derive a new diffuser design that can be used for the bi-directional rotation of the IntraVAD. In this work, a dual-diffuser set (DDS) was proposed to replace the conventional inducer and diffuser upstream and downstream of the pump. The DDS comprised two diffusers, located on both sides of the impeller, omitting the conventional inducer and diffuser. Different configurations of the DDS were designed and manufactured with various combinations of curved and straight blades. All configurations were initially tested in continuous flow, then in a pulsatile mock circulatory loop. A weighted normalized scalar (WNS) was proposed to comprehensively evaluate the hemodynamic effect of the DDS with different configurations. The results show that the maximum of WNS occurred when the upstream diffuser had equal numbers of curved and straight blades and the downstream diffuser had only curved blades. This indicates such a dual-diffuser design for the IntraVAD can give an optimal cardiac assistance potentially improving ventricular contractility, thereby restoring heart function.

Replication of pressure-volume loop with controllable ESPVR and EDPVR curves on a personalized mock circulatory loop based on elastance function.

In the development of a left ventricular assist device (LVAD), it is important to evaluate the LVAD's hemodynamic effect on the compromised left ventricle (LV) before surgical implantation. The mock circulatory loop (MCL) is widely accepted as an in vitro test platform to evaluate LVADs across a wide range of operational conditions as a way to examine how the device and the cardiovascular system interact. Unfortunately, most MCLs represent an oversimplified model of cardiac function, with disease states simulated through generalized changes in heart rate and stroke volume. Because heart failure (HF) severity varies substantially among patients, an MCL is needed that can mimic the pressure-volume loop of an individual patient. In this work, two numerical elastance models, derived from a specific pressure volume loop template, were used to control the LV simulator of the MCL to simulate different degrees of HF. The numerical elastance model was then scaled to change the slopes of the end-systolic (ESPVR) and end-diastolic (EDPVR) pressure volume relationship curves to simulate systolic and diastolic dysfunction. The resulting experimental pressure volume loops are consistent with theoretical loops, demonstrating the feasibility of creating an MCL that can be customized for the patient.

Hemodynamic effects of synchronizing an intra-aortic VAD in reverse-rotation control with left ventricle: a mock loop study.

The IntraVAD is an intra-aortic left ventricular assist device (LVAD) to be located in the ascending aorta. In order to enhance unloading and promote coronary flow for the left ventricle (LV), an operating mechanism, reverse-rotation control (RRc) mode, has been developed for the IntraVAD and tested in vitro in a mock circulation loop (MCL). The RRc mode consists of forward rotation (FR) and reverse rotation (RR). The synchronization between the IntraVAD and the LV was studied to offload the ventricle more effectively and to improve myocardial perfusion. The percentage time length of the FR period in the cardiac cycle (Tlf) and time offset between the central-lines of the FR period and the LV systole (Toc) are two parameters of the RRc mode that were varied to adjust the synchronization between the IntraVAD and the LV. The ejection fraction (EF), coronary perfusion pressure (CPP), and arterial pulsatility index (API) were measured at different Tlf and Toc values. These hemodynamic results closely correlated to the LV unloading, coronary perfusion, and peripheral arterial pulsatility. The EF, CPP and API were fed into a weighted normalized scalar (WNS) which was implemented to comprehensively evaluate the hemodynamic influence. The WNS result shows that the overall hemodynamic response is more sensitive to the changes in Toc value than Tlf value. The result shows a significant reduction in LV afterload by starting the FR before LV contraction, then switching to RR at the onset of ventricular dilation. The optimal phase shift of -π/5 was found to precede LV contraction, indicating that changes in LV afterload are more sensitive to the phase shift at the start of the ventricular systole than at the end. Thus, a phase advance between intra-aortic pumps and the LV is critical to unload the ventricle and promote myocardial recovery.

In vitro study of an intra-aortic VAD: Effect of reverse-rotating mode on ventricular recovery.

Cardiac recovery has been observed in end-stage heart failure patients with mechanical circulatory support. An intra-aortic ventricular assist device (IntraVAD) is a novel rotary blood pump designed to operate in the ascending aorta behind the aortic valve, working in series with the compromised left ventricle (LV). Such a device requires optimal motion control in order to enhance the myocardial perfusion and thus promote cardiac recovery. Therefore, a reverse-rotating control (RRc) mode has been proposed to increase the mean arterial pressure (MAP) in diastole where the most coronary flow occurs. The RRc mode consists of two motions - forward rotating speed (FS) and reversely rotating speed (RS). The capability of cardiac recovery of three control modes, including `continuous', `on/off ' and `RRc' modes, was evaluated in vitro. Stroke work (SW), ventricular volume, coronary perfusion pressure (CPP), and arterial pulsatility index (API) were used to evaluate LV unloading, myocardial perfusion and arterial pulsatility. The results show that, all three modes increased the LV stroke work (0.98W vs 1.00W vs 1.01W for continuous, on/off and RRc, respectively; baseline 0.9W) and decreased both end-diastolic volume (EDV) and end-systolic volume (ESV). The "RRc" mode improved CPP significantly (78.4 mm Hg compared to 66.4 mmHg and 70.9 mm Hg for continuous and on/off modes; baseline 71 mm Hg). The arterial pulsatility was higher in `RRc' mode (0.84 compared to 0.43 and 0.59; baseline 0.48). In summary, the IntraVAD operating in the RRc mode can successfully unload the LV, enhance the myocardial perfusion, and restore the arterial pulsatility; therefore, it could be a promising therapeutic option to bridge heart failure patients to recovery.

An extended computational model of the circulatory system for designing ventricular assist devices.

An extended computational model of the circulatory system has been developed to predict blood flow in the presence of ventricular assist devices (VADs). A novel VAD, placed in the descending aorta, intended to offload the left ventricle (LV) and augment renal perfusion is being studied. For this application, a better understanding of the global hemodynamic response of the VAD, in essence an electrically driven pump, and the cardiovascular system is necessary. To meet this need, a model has been established as a nonlinear, lumped-parameter electrical analog, and simulated results under different states [healthy, congestive heart failure (CHF), and postinsertion of VAD] are presented. The systemic circulation is separated into five compartments and the descending aorta is composed of three components to accurately yield the system response of each section before and after the insertion of the VAD. Delays in valve closing time and blood inertia in the aorta were introduced to deliver a more realistic model. Pump governing equations and optimization are based on fundamental theories of turbomachines and can serve as a practical initial design point for rotary blood pumps. The model's results closely mimic established parameters for the circulatory system and confirm the feasibility of the intra-aortic VAD concept. This computational model can be linked with models of the pump motor to provide a valuable tool for innovative VAD design.