Left Ventricular Assist Devices: Engineering Design Considerations

Abstract

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 body's 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