Electric Ventricular Assist Device Research Articles

A new mock circulatory loop and its application to the study of chemical additive and aortic pressure effects on hemolysis in the Penn State electric ventricular assist device.

A new mock circulatory loop was developed for hemolysis studies associated with the Penn State electric ventricular assist device (EVAD). This flow loop has several advantages over previously designed loops. It is small enough to accommodate experiments in which only single units of blood are available, it is made out of biocompatible materials, it incorporates good geometry, and it provides normal physiological pressures and flows to both the aortic outlet and the venous inlet of the pumping device. Experiments with reduced aortic pressure but normal cardiac output showed that hemolysis in a loop with normal aortic blood pressure was significantly higher than that in a loop with lowered aortic pressure, thereby illustrating the importance of maintaining loop pressures as close as possible to those found in vivo. This data also imply that blood traveling through the left ventricle in an artificial heart may be subject to higher hemolysis rates than that traversing the right ventricle. Another set of experiments to determine the effects of 4 hemolysis or drag-reducing agents (Pluronic F-68, Dextran-40, Polyox WSR-301, and Praestol 2273TR) on blood trauma due to the EVAD and associated valves was performed. Results indicated that none of the additives significantly reduced hemolysis under the conditions found in the mock loop. Finally, a compilation of data gathered in these experiments showed that the index of hemolysis (IH) is dependent on hematocrit (HCT), which suggests that another parameter, IH/HCT, may be more suited to the quantification of hemolysis.

Aortic pressure estimation with electro-mechanical circulatory assist devices.

An adaptive technique for the estimation of the time history of aortic pressure (from applied voltage and position feedback) has been designed, implemented, and bench tested using the Penn State Electric Ventricular Assist Device (EVAD). This method, known in the field of automatic control as a dynamic observer, utilizes gains which were determined using experimental data collected while the EVAD was running on a mock circulatory system. An adaptive scheme provides the observer with a method of changing its initial conditions on a stroke-by-stroke basis which improves observer performance. In both determining the feedback gains and developing the adaptation scheme, a range of beat rates and pressure loads was taken into account to yield satisfactory observer performance over a range of operating conditions. The observer was implemented, its performance was verified in vitro and results are reported. In the six experimental operating conditions, the beat rate ranged from 56-104 beats per minute (bpm) and the span of the mean systolic aortic pressure was 10.7-18.7 kPa (80-140 mmHg). For these cases, the mean deviation between the actual and estimated aortic pressure during the latter two-thirds of systole was 0.41 kPa (3.1 mmHg).

An optimal controller for an electric ventricular-assist device: theory, implementation, and testing.

This paper addresses the development and testing of an optimal position feedback controller for the Penn State electric ventricular-assist device (EVAD). The control law is designed to minimize the expected value of the EVAD's power consumption for a targeted patient population. The closed-loop control law is implemented on an Intel 8096 microprocessor and in vitro test runs show that this controller improves the EVAD's efficiency by 15-21%, when compared with the performance of the currently used feedforward control scheme.

Sensorless position control of a brushless motor in circulatory assist devices

An innovative technique to implement position feedback and commutation of brushless D.C. motors without explicit position sensing is presented. The procedure has been successfully implemented for the Pennsylvania State University Electric Ventricular Assist Device (EVAD). Laboratory tests indicate that the method is successful for pumping operations in a mock circulatory loop for beat rates up to 60 beats min −1 and over several days of uninterrupted operation. This method of sensorless commutation is presently being investigated for inclusion as a backup system for the Penn State circulatory assist devices.

An Output Feedback Pusher Plate Controller for the Penn State Electric Ventricular Assist Device: Stability Analysis

The development of an optimal output feedback controller for the Penn State electric ventricular assist device (EVAD) is addressed. The controller is designed to minimize the electric energy consumption of the EVAD, while utilizing the measured pusher plate position as the only feedback signal. The controller incorporates statistical information of the targeted patient population and optimizes the expected value of the system performance. The present paper shows that the optimal output feedback control scheme is asymptotically stable when the EVAD operates in a steady beat rate mode, and bounded-input/bounded-output stable when the beat rate undergoes a transient. Since the EVAD operates in either steady or transient beat rate modes, this optimal control scheme is stable.

A Two‐Phase Fluid Volume Compensation Chamber for an Electric Ventricular Assist Device

A volume compensation chamber is a device used to reduce large pressure fluctuations created in electric ventricular assist devices during the emptying and filling of the blood sac. In this study, the effect of motor casing pressure variation (pressure swing) on the performance of the Penn State electric ventricular assist device (EVAD) was investigated. Design criteria were established for the maximum pressure swing tolerated by the EVAD and the optimal mean chamber pressure at which to operate. At the chosen mean chamber pressure of -15 mm Hg, it was found that pressure swing should be maintained below 45 mm Hg. A two-phase fluid volume compensation chamber was developed that reduced the pressure swing enough to ensure adequate pump performance. The device consists of a metal chamber with a high-heat-flux porous coating applied to the inside surface. The chamber uses Freon as the working fluid and is isolated from the EVAD by a metal bellows. It was found that the high-flux coating significantly reduces the pressure swing, in some cases by as much as 50% when compared with an identical chamber with no coating. In the coated chamber the pressure swing was maintained between 22 and 30 mm Hg at a beat rate of 60 beats/min, for a wide range of Freon volumes (4-38 ml). Even at 100 beats/min the pressure swings obtained with the coated chamber are well within an acceptable range.

An adaptive aortic pressure observer for the Penn State electric ventricular assist device

This paper addresses the development of an aortic pressure observer for the Penn State Electric Ventricular Assist Device (EVAD). The observer estimates the aortic blood pressure by measuring the voltage of the electric motor and the pusher plate position. The estimated pressure is fedback to the EVAD's blood flow controller, which adjusts the beat rate of the device to accommodate the varying demand of cardiac output. The gains of the observer are deterministically optimized such that the optimal values are independent of the (often unknown) system state initial conditions. To improve the performance of the pressure observer an adaptation scheme is developed. In this scheme the initial pressure estimate of the succeeding systolic cycle is adjusted when the pressure does not match its corresponding second estimated value. In vitro test runs of the developed observer show that is is robust to parameter variations, and the error of the resultant estimated pressure is less than 5%.

A Novel Output Feedback Pusher Plate Controller for the Penn State Electric Ventricular Assist Device

This paper addresses the development of an output feedback controller for the Penn State Electric Ventricular Assist Device (EVAD). The control law is designed to minimize the electric power consumption of the motor, while utilizing the measured pusher plate position as its only feedback signal. The control algorithm results in a suboptimal performance. The feedback gain function is calculated such that the expected value of the deviations between the suboptimal and full state feedback power consumption values is minimized. The system state initial conditions are treated as random variables with specified probability density functions. Numerical simulations indicate that the output feedback controller of the EVAD has a near optimum performance (the excessive electric power consumption is less than 1 percent), and a time shift manipulation of a single feedback gain function can drive the EVAD in various speeds with minimal energy losses.

Hot-film wall shear probe measurements inside a ventricular assist device.

Wall shear rates at eleven sites within the Penn State Electric Ventricular Assist Device (EVAD) were determined with the pump operating under conditions of 30 and 50 percent systolic duration and a mean flow rate of 5.8 L/min using a flush-mounted hot-film probe. Probe calibrations were performed with the hot-film in two orientations relative to the flow direction: a standard orientation and an orientation in which the hot-film was rotated by 90 deg from the standard orientation. The magnitude and direction of the wall shear stress at each site within the EVAD were estimated from ensemble averaged voltage data recorded for similar standard and rotated film orientations. The results indicate that, during diastole the wall shear stress direction around the pump's periphery for both operating conditions is predominantly perpendicular to the inflow-outflow plane (in the direction of the pusher plate motion) and reaches a peak value of approximately 350 dynes/cm2. The highest wall shear stresses were found near the prosthetic aortic valve (inside the EVAD) under the 30 percent systolic duration condition and are estimated to be as high as 2700 dynes/cm2. Peak shear stress values of 1400 dynes/cm2 were observed in the vicinity of the prosthetic mitral valve under both operating conditions. The results suggested that the valve regions are substantially more hemolytic than other wall regions of the EVAD; the magnitudes of the wall shear stresses are sensitive to operating conditions; and that wall shear in the direction of pusher plate motion can be significant.