Understanding Cardiac Assist Devices

Abstract

What is a cardiac assist device? And what do I, as a biomedical technician, need to know about them?These devices are sometimes called heart assist devices. They are used throughout the world for short-term support of the left ventricle, right ventricle, or both ventricles of the heart. At the hospital where I work, I've seen these devices used most often for what is called postcardiotomy support—used on patients who have had heart surgery and then developed heart failure. Postcardiotomy is not the only reason this device would be used. Other reasons include: acute heart attack, disease of the heart muscle, acute viral myocarditis, donor heart failure after transplant, and trauma to the heart.Let's look at how the cardiac assist device works. The pump is connected to the patient by a specially designed hose. The blood flows from the heart to the inflow tube, through the tube to the ventricular assist device, to the outflow tube, and then blood is delivered to the major vessels of the body. The pump is connected to the computer with a small cable that leaves the body through a small opening. The patient needs to have anticoagulant medications to prevent clotting. The patient is more likely to get an infection than to have the pump malfunction. This is the number one cause of death when using the assist device.Most heart assist devices are controlled by microprocessors and driven by pneumatic pumps. Most of them can adjust the heart rate based on the flow in and out of the pump. Given the nature of their use, it might be easy to assume that these devices are very complicated, but that is not necessarily true. The pneumatic block diagram for these is fairly simple. There is a compressor, several solenoid valves, a couple of check valves, and a couple of pressure and vacuum transducers. On the other side of the coin, the electrical block diagram contains the PCA controller and power management. Those of us in the electronics discipline would find the layout fairly simple. The simplicity of the circuitry belies the vital function this device provides. Through the PCA schematic, there are watchdog circuits and plenty of redundancy to prevent malfunction or a catastrophic event.Usually there is an automatic self-test upon start-up to ensure the safe operation of the system. Our system has dual redundancy to ensure operation at all times. Should the microprocessor fail, there is a hand pump to use to ensure the system functions. The battery backup on most units will last close to an hour, depending on how many functions are being used. The blood pumps can be located externally or implanted in the patient. For an external pump, a canula that is grafted into the patient's ventricle, but the pump itself is external to the patient. For an internal pump, the canula is implanted into a patient's abdomen along with the pump. The computer is left outside the body. Several companies make these devices, both internal and external. With the invention of smaller electrical components, these devices are themselves getting smaller and smaller. This allows for more mobility of the patient and a better quality of life.