D’Zhana Simmons was only 14 years old when she and her parents learned earlier this year that she needed a heart transplant. D’Zhana and her parents later flew from their home in Clinton, S. C. to Holtz children’s Hospital in Miami for a transplant operation.

Unfortunately, the newly transplanted heart didn’t work properly, so doctors were left with few options. They decided to implant a dual ventricular assist device (VAD) into D’Zhana’s chest to keep her alive until a new donor became available.

It worked. The blood pumping system took the place of her natural heart for nearly four months, at which time she underwent a second heart transplant procedure.

D’Zhana’s story is unusual because she is so young to have gone through such an experience. However, VADs have been implanted into thousands of patients to keep them alive for extended periods. Many of these life-saving devices are powered by pneumatics. This should come as no surprise, considering the cushioning effects that are easily achieved with compressed air and the high reliability of the technology.

The marvel of pneumatics
Our natural hearts consist of four chambers — left and right atria, and left and right ventricles. The right ventricle pumps blood to the lungs, and the left ventricle pumps it to the body. The left ventricle has to work the hardest, so when it can no longer pump blood to the body effectively, a heart transplant may be an option.

Thoratec’sVADs have proven to sustain life for weeks or months — either to help a patient’s weakened heart recover from surgery or for bridge-to-transplant periods. But what’s important to the patients is the quality of life these portable devices allow. That is, patients are able to recover at home and carry on most of their normal activities.

Pneumatics to the rescue
For more than 30 years, Thoratec Laboratories Corp., Pleasanton, Calif., has been developing and manufacturing advanced medical technologies to improve patient survival and quality of life. Its Implantable VAD is the only implantable biventricular support option for advanced heart failure patients requiring months or years of support, and Thoratec’s VADs are the first and only biventricular support systems approved for home discharge.

Thoratec’s Implantable VAD consists of three main components:

• blood pump, which takes the place of one or both of a patient’s ventricles,
• TLC-II pneumatic driver, which consists of an air compressor, controls, valves and other components, and
• cannulae, a network of tubing that routes air between the blood pump and pneumatic driver.

A modified version of Humphrey’s modelThe main component of the pneumatic driver is, of course, the compressor. According to Eric Lee., Ph. D., business unit manager, intermediate circulatory support at Thoratec, the pneumatic driver’s reciprocating compressor uses three pistons to pressure and vacuum to actuate the blood pump. Lee explained that because the air does not come in direct contact with any blood or body tissue, quality of the compressed air (moisture and contamination) posed no particular challenge, so normal industry standards were followed.

Lee said that from the compressor, air or vacuum is routed to accumulators (left pump, right pump, vacuum) that each store a quantity of compressed air or vacuum. This stored energy serves as a reserve to allow disconnecting different drivers to suit the particular situation. The accumulators also stabilize pressure to eliminate the pulsations inherent to the compressor.

Valves provide long life
Not surprisingly, pressure requirements are low; the left ventricle typically requires maximum pressure of only 200 to 250 mm-Hg (3.87 to 4.83 psig), the right ventricle requires 120 to 170 mm-Hg (2.32 to 3.29 psig), and vacuum is 0 to –50 mm-Hg.

From the accumulators, air is routed to a series of normally closed poppet valves provided by Humphrey Products. According to Humphrey’s Dale Dratt, Thoratec uses a custom engineered version of Humphrey’s model 320 valves, which are direct-acting, solenoid actuated poppet valves.

Dratt said that Thoratec had been using different valves, but wanted to increase their rated life to 5 million cycles or more. He explained that the model 320 valves are normally rated to 20 million cycles, but Thoratec’s testing showed them to exceed 100 million.