SciTech

How Things Work: Mechanical hearts

According to the Current Science and Technology Center, 16,000 Americans are in need of a heart transplant. Only a couple thousand of these patients, though, receive treatment.

Shortages in treatment for heart-failure patients has led researchers to develop mechanical hearts that are capable of doubling people’s life expectancies.

But what all goes into a mechanical heart? There is more than just nuts and bolts to a device that keeps people living.
William Devries developed the first replacement heart in 1982, called the Jarvik-7.

This aluminum and plastic device is driven by air that travels from an external compressor and into the patient’s chest through external wiring.

The Jarvik-7 replaces the left and right ventricles of the patient’s heart, which are the two main parts of a heart, and pumps blood to the body using rubber diaphragms.

The more recent version of the Jarvik-7 is SynCardia’s CardioWest Temporary Total Artificial Heart. This air-driven mechanical heart performs the work of the ventricles and four heart valves.

The CardioWest completely replaces the original heart. Like the Jarvik-7, it connects to an air hose that originates from a machine containing pressurized air.

The CardioWest operates by dividing each ventricle into two regions. One region of the ventricle fills with air, placing pressure on the other region, which pushes blood out of the heart and into the body.

The CardioWest is meant to be temporary. Approved by the FDA in 2004, the CardioWest is designed to support the body’s circulatory system only until a heart donor is found.

Similarly, ventricular assist devices (VADs) are artificial hearts that assist the actual heart, usually by pumping blood from the left ventricle to the aorta. VADs are used to support hearts that have been weakened by heart attack or surgery.

The Jarvik-7 and CardioWest both require external wiring, called drivelines, for operation. However, Massachusetts health-care company Abiomed developed a completely self-contained artificial heart in 2001. It is called the AbioCor Replacement Heart.

The AbioCor heart pumps more than 10 liters of blood per minute, thereby supporting the body’s entire circulatory system. It replaces only the ventricles of the heart, however, leaving the left and right atria, aorta, and pulmonary artery.

In an actual human heart, the atria contract simultaneously and send blood into the ventricles. The ventricles then contract and send blood into the body. AbioCor, though, only pumps blood out of one ventricle at a time.

The two-pound AbioCor heart consists of a hydraulic pump, porting valve, and wireless energy transfer system.

The hydraulic pump performs most of the work. The pump pushes fluid to the left and right of the heart through the porting valve, causing blood to circulate to the lungs and throughout the body.

This pumping action can be managed by the controller, which is located within the abdomen.

The wireless energy system is the means by which energy is transferred from the external battery to the internal battery of the artificial heart. The energy system transfers energy by sending it across the skin from the system’s external coil to its internal coil.

Through this process, the larger battery recharges the smaller battery.

Prior to surgery, doctors perform X-ray and CAT scans to ensure that the heart will fit in the patient’s chest. During the seven-hour surgery, the left and right ventricles of the original heart are completely removed and replaced by the AbioCor heart.

The AbioCor heart is only meant for heart-failure patients who have a life expectancy of less than 30 days. It is intended to double the life expectancies of these patients in order to give them more time to find a transplant.

Abiomed is currently developing the AbioCor II heart, which will be 30 percent smaller than the AbioCor I heart and will have a five-year reliability.