What Are Left Ventricular Assist Devices (LVAD)?
Left ventricular assist devices (LVADs) are battery-operated mechanical pump-type devices that are surgically implanted on the upper part of the abdomen. They take blood from the left ventricle and pump it through the aorta. LVADs are becoming more common and are often used by heart failure patients who have to wait for heart transplants.
How Do LVADs Function?
Most ventricular assist devices (VADs) operate on similar principles, including the Thoratec Heartmate II. A cannula, a tube that can be inserted into the body with the purpose of delivering or removing fluid, is inserted into the apex of the appropriate ventricle. Blood passes through the ventricle to a pump and then through a tube on the device to the aorta in the case of an LVAD, or to the pulmonary artery in the case of a right ventricular assist device (RVAD). The pump is powered through a lead that connects it to a controller and power supply. In some cases, there is also a tube to vent the pump to outside air.
The Jarvik 2000 operates slightly differently – the pump is actually located inside the left ventricle and its outflow passes through the apex of the ventricle to a tube that leads to the aorta.
Major distinguishing features between the different VADs are the pump (which can vary substantially in method of operation, size, and placement), the controller, the materials used both for the pump and the associated tubes and cannulas, and the lead between the pump and the controller/power supply.
Pulsatile Pumps vs. Continuous Flow Pumps
Pumps used in VADs can be divided into two main categories – pulsatile pumps, which mimic the natural pulsing action of the heart, and continuous flow pumps.
Pulsatile VADs use positive displacement pumps. In some of these pumps, the volume occupied by blood varies during the pumping cycle, and if the pump is contained inside the body then a vent tube to the outside air is required. The only three LVADS approved by the FDA for use in the U.S. are all pulsatile pumps. All were approved between 1994 and 1998, indicating the relative maturity of this technology.
Continuous flow VADs normally use either centrifugal pumps or axial flow pumps. Both types have a central rotor containing permanent magnets. Controlled electric currents running through coils contained in the pump housing apply forces to the magnets, which cause the rotors to rotate. In the centrifugal pumps, the rotors are shaped to accelerate the blood circumferentially and thus cause it to move towards the outer rim of the pump. Whereas in the axial flow pumps, the rotors are more or less cylindrical with blades that are mostly helical, causing the blood to be accelerated in the direction of the rotor’s axis.
An important issue with continuous flow pumps is the method used to suspend the rotor. Early versions used solid bearings; however, newer pumps, some of which are approved for use in the European Union, use either electromagnetic or hydrodynamic suspension. These pumps contain only one moving part. Manufacturers claim that these methods of suspension not only virtually eliminate wear, but also reduce damage to blood cells.