Image Number 3 for United States Patent #6264601.
An implantable ventricular assist device (VAD) has a small size to provide full-implantable capabilities. The VAD has two variable-volume chambers and an actuator for expelling blood from the chambers in sequence and to a common outlet, thus operating as a positive-displacement pump. The variable-volume chambers may be flexible sacs and the pump is operable at a substantially continuous flow, such as during the systolic phase of the assisted ventricle. A method of operation includes operating the pump at a first frequency during systole and a second lower frequency during diastole. The VAD pump includes a pair of coils housed within a frame and disposed in a spaced relationship to generate a coil flux through a pair of poles. A plate including an armature and a magnet is disposed within the frame such that the armature is between the poles and the magnet is between the coils. Gaps are defined between the armature and each of the poles and the coil flux displaces the armature across the gap. The coil flux follows a donut-shaped path including the frame, one of the poles, one of the gaps, the armature, the other the gap, and the other the pole, and no coil flux passes through the magnet. The magnet is not subject to depolarization and may be made substantially smaller than those of conventional devices. The magnet generates a bias flux that offsets a portion of the coil flux around the frame. A spring may be provided to counteract the unstable action of the bias magnet. The volume of the chambers is a fraction of the ventricular volume, and may be about 20 ml. To pump a typical 80-ml volume of a left ventricle, the controller may activate the coils four times during systole, or once every 40 msec.