Eduardo Divo

Detection of ventricular suction in an implantable rotary blood pump using support vector machines

A new suction detection algorithm for rotary Left Ventricular Assist Devices (LVAD) is presented. The algorithm is based on a Lagrangian Support Vector Machine (LSVM) model. Six suction indices are derived from the LVAD pump flow signal and form the inputs to the LSVM classifier. The LSVM classifier is trained and tested to classify pump flow patterns into three states: No Suction, Approaching Suction, and Suction. The proposed algorithm has been tested using existing in vivo data. When compared to three existing methods, the proposed algorithm produced superior performance in terms of classification accuracy, stability, and learning speed. The ability of the algorithm to detect suction provides a reliable platform in the development of a pump speed controller that has the capability of avoiding suction.

Computational Fluid Dynamics in Congenital Heart Disease

Computational fluid dynamics has been applied to the design, refinement, and assessment of surgical procedures and medical devices. This tool calculates flow patterns and pressure changes within a virtual model of the cardiovascular system. In the field of paediatric cardiac surgery, computational fluid dynamics is being used to elucidate the optimal approach to staged reconstruction of specific defects and study the haemodynamics of the resulting anatomical configurations after reconstructive or palliative surgery. In this paper, we review the techniques and principal findings of computational fluid dynamics studies as applied to a few representative forms of congenital heart disease.

A new current-based control model of the combined cardiovascular and Rotary Left Ventricular Assist Device

Rotary Left Ventricular Assist Devices (LVAD) are mechanical pumps implanted in patients with congestive heart failure to assist their heart in pumping the required amount of blood in the circulatory system. Until recently, the combined mathematical model of the LVAD coupled with the left ventricle has assumed the availability of the rotational speed of the pump as the independent control variable. In reality, however, the device is controlled by the pump motor current which, in turn, produces the desired rotational speed of the pump motor. Therefore, the actual implementation of any desired speed controller for the device requires the solution of an inverse problem in order to determine the corresponding motor current that yields the desired pump speed. Recently, it has been observed from in-vivo experiments that an LVAD that is controlled by a motor current with a given profile (constant or ramp-like) has yielded a corresponding pump speed that exhibits a superposition of an oscillatory component which is synchronized with the pulsatility of the heart hemodynamic variables. Because of this, it has become evident that the solution of this inverse problem is extremely difficulty to accomplish. In this paper, we reformulate the existing combined LVAD and left ventricle model in such a way so as to introduce the pump motor current instead of the pump speed as the control variable, hence avoiding the inverse problem altogether. This new model is not only a more realistic representation of the LVAD control variable but also is much more practical in that it allows for the derivation of a controller directly in terms of the pump motor current rather than indirectly in terms of its rotational speed. Validation of this model and the challenges involved in using it when designing a feedback controller for the LVAD are also discussed.

Feedback control of a rotary left ventricular assist device supporting a failing cardiovascular system

A new feedback control system for a current-based model of the combined cardiovascular system and a rotary left ventricular assist device is presented. The system consists of a suction detection subsystem and a feedback controller subsystem. The suction detection subsystem is based on a Lagrangian Support Vector Machine model. In the absence of suction in the left ventricle, as indicated by the suction detection subsystem, the proposed feedback controller automatically adjusts the pump motor current in order to meet the blood flow requirement of the patients body at different physiological states. The performance of the feedback control system has been tested to show the ability to autonomously adjust the pump current, while sustaining required cardiac output and mean arterial pressure. Simulation results show that the controller can keep cardiac output and mean arterial pressure within acceptable physiologic ranges under different conditions of the patient activities. Robustness to noise of the controller is also discussed.

Left Ventricular Assist Devices: Engineering Design Considerations

Patients with end-stage congestive heart failure awaiting heart transplantation often wait long periods of time (300 days or more on the average) before a suitable donor heart becomes available. The medical community has placed increased emphasis on the use of Left Ventricular Assist Devices or LVADs that can substitute for, or enhance, the function of the natural heart while the patient is waiting for the heart transplant (Poirier, 1997; Frazier & Myers, 1999). Essentially, a rotary LVAD is a pump that operates continuously directing blood from the left ventricle into the aorta by avoiding the aortic valve. Generally speaking, the goal of the LVAD is to assist the native heart in pumping blood through the circulatory system so as to provide the patient with as close to a normal lifestyle as possible until a donor heart becomes available or, in some cases, until the patient’s heart recovers. In many situations, this means allowing the patient to return home and/or to the workforce. The amount of blood pumped by the LVAD into the circulatory system depends on many factors, the most important of which is the rotational speed of the pump which is directly controlled by the pump motor current. The higher the speed of the pump the more blood is forced into the circulatory system. Because of this, an important engineering challenge facing the increased use of these LVADs is the development of an appropriate controller for the speed of the rotor. Such a controller, in addition to being robust and reliable, must satisfy two important criteria: 1. It must be able to adapt to the daily activities and physiological and emotional changes of the patient by regulating the pump speed in order to meet the bodys requirements for cardiac output (CO) and mean arterial pressure (MAP) (Olsen, 2000; Boston et al., 2003; Schima et al., 1992; Marieb, 1994). 2. It should ensure that the rotational speed does not exceed an upper limit beyond which the pump will be attempting to draw more blood from the ventricle than available. The occurrence of this phenomenon, known as suction, for a brief period of time may cause collapse of the ventricle resulting in damage to the heart muscle (Yuhki et al. 1999; Vollkron et al., 2006; Ferreira et al., 2006). Suction, therefore, must be detected quickly and the pump speed reduced before any damage to the heart muscle occurs. The eventual goal of an LVAD speed controller is therefore to meet the above two requirements so that an LVAD recipient patient could potentially leave the hospital and return home to a safe and normal lifestyle. Given that the pump is continuously interacting

A New Control System for Left Ventricular Assist Devices Based on Patient-specific Physiological Demand

A left ventricular assist device (LVAD) is a mechanical pump that helps patients with a heart failure (HF) condition. This pump works in parallel to the ailing heart and provides a continuous flow from the weak left ventricle to the ascending aorta. The current supplied to the pump motor controls the flow of blood. A new feedback control system is developed to automatically adjust the pump motor current to provide the blood flow required by the level of activity of the patient. The systemic vascular resistance (RS ) is the only undeterministic variable parameter in a patient-specific model and also a key value that expresses the level of activity of the patient. The rest of the parameters are constants for a patient-specific model. To determine the level of activity of the patient, an inverse problem approach is followed. The output data (pump flow) are observed and using an optimized search technique, the best model to describe such output is selected. Furthermore, the estimated RS is used in another patient-specific cardiovascular model that assumes a healthy heart, to determine the blood flow demand. Once the physiological demand is established, the current supplied to the pump motor of the LVAD can be adjusted to achieve the desired blood flow through the cardiovascular system. This process can be performed automatically in a real-time basis using information that is readily available and thus rendering a high degree of applicability. Results from simulated data show that the feedback control system is fast and very stable.

A Localized Radial-Basis-Function Meshless Method Approach to Axisymmetric Thermo-Elasticity

Current methods for solving thermoelasticity problems involve using finite element analysis, boundary element analysis, or other meshed-type methods to determine the displacements under an imposed temperature/stress field. This paper will detail a new approach using localized meshless methods based on multi-quadric radial basis function interpolation to solve these types of coupled thermoelasticity problems. Here, a point distribution is used along with a localized collocation method to solve the Navier equation for the components of the displacement vector. The specific application considered in this paper is that of axisymmetric thermo-elasticity. With rapidly increasing availability and performance of computer workstations and clusters, the major time requirement for solving a thermoelasticity model is no longer the computation time, but rather the problem setup. Defining the required mesh for a complex geometry can be extremely complicated and time consuming, and new methods are desired that can reduce this time. The proposed meshless method features the complete elimination of a mesh, be it structured or unstructured, and the associated complexities involved in its generation and control. The reduction of initial model setup time makes the meshless approach an ideal method of solving coupled thermoelasticity problems. Several examples with exact solutions are used to verify this method for various geometries and boundary condition combinations.

BEM Model of Ablation Characteristics of a Thrust Vector Control Vane

A dual reciprocity boundary element method is implemented to predict ablation in model TVC vanes. A moving front algorithm is described. Experimental data are available from tests performed on scaled vanes. Numerical results for recession of quarter-scale and half-scale vanes compare well with experimental data. Future work includes accounting for temperature variation of the thermophysical properties and full coupling of the flowfield and conduction solutions.

Computational Fluid Dynamics Simulation of United Launch Alliance Delta IV Hydrogen Plume Mitigation Strategies

During the launch sequence of the United Launch Alliance Delta IV launch vehicle, large amounts of pure hydrogen are introduced into the launch table and ignited by RadialOutward-Firing-Igniters (ROFIs). This ignition results in a significant flame, or plume, that rises upwards out of the launch table due to buoyancy. The presence of the plume causes increased and unwanted heat loads on the surface of the vehicle. A proposed solution to this problem is to add a series of fans and structures to the existing launch table configuration that are designed to inject ambient air in the immediate vicinity of the launch vehicle’s nozzles to suppress the plume rise. In addition to the air injection, secondary blockages and fan systems can be added around the launch table openings to further suppress the hydrogen plume. The proposed air injection solution is validated by computational fluid dynamics simulations that capture the combustion and compressible flow observed during the Delta IV launch sequence. A solution to the hydrogen plume problem will have direct influence on the efficiency of the launch vehicle: lower heat loads result in thinner vehicle insulation and thus allow for a larger payload mass. Current results show that air injection around the launch vehicle nozzles and air suppression around the launch table openings significantly reduces the size of the plume around the launch vehicle prior to liftoff.

The aortic valve dynamics role in the recovery treatments of patients with left ventricular assist devices

This paper intends to define an optimal range for the pump speed of Rotary Left Ventricular Assist Devices (LVADs) that are used in bridge-to-recovery treatments. If the pump is operating within that optimal range, the aortic valve will be working properly (i.e. opening and closing) in each cardiac cycle. The proper operation of the aortic valve is a very important factor in helping the heart muscle recovers. The optimal range varies depending on the severity of the Heart Failure (HF) and the level of activity of the patient. A comparison is shown between the total flow produced as a result of operating the pump within the optimal range and the physiological demand of the patient. The comparison suggests that for cases of mild to moderate HF the flow produced is close to the physiological demand, but in severe cases the flow is significantly less than what the patient requires. Furthermore, our results suggest that data from the pump flow and the left ventricle volume signals can be used to test whether or not the aortic valve is experiencing permanent closure. Also an investigation of the aortic valve opening duration is presented for two cases: first, for mild HF case with varying Heart Rate (HR) and then for fixed HR and mild to severe HF cases. These Simulation results are obtained using a 6th order mathematical model of the cardiovascular-LVAD system.