They say home is where the heart is, but until recently patients who had suffered biventricular failure could survive only with the help of an artificial heart tethered to large, immobile driver system to maintain blood circulation while they awaited a heart transplant.

This could be changing; artificial heart–maker SynCardia Systems, Inc., in Tucson, Ariz., last month announced that three patients surgically implanted with the company's technology have been able to walk out of their respective hospitals and wait for donated replacement hearts in the comfort of their own homes.

These departures were made possible by SynCardia's six-kilogram Freedom portable driver, which blows packets of air through two clear tubes (each about one centimeter in diameter) connected to the company's temporary Total Artificial Heart. These air packets force the blood in the implanted artificial heart's ventricles to be pumped to the rest of the body. The 75-centimeter-long tubes run from the artificial ventricles, exiting the body just below the rib cage. They connect to the Freedom driver, which can be worn in a backpack or shoulder bag.

SynCardia's temporary Total Artificial Heart—previously known as the CardioWest temporary Total Artificial Heart—is designed to replace the two ventricles (lower chambers) of a person's original heart while still relying on the natural left and right atria (upper chambers) and aorta (main artery) to supply the pumped blood to the rest of the body, says SynCardia CEO Rodger Ford. The pulmonary artery also remains intact. The company's artificial heart, which costs about $125,000, has been approved by the U.S. Food and Drug Administration since October 2004 as a bridge for transplant patients dying from end-stage failure of their hearts' right and left ventricles, which collect blood from the atria and pump it to the lungs and body, respectively.

The SynCardia's artificial heart's volume is 70 cubic centimeters, which Ford says is small enough to fit into about 75 percent of males and 25 percent of females. The company is developing a 50-cubic-centimeter heart expected to be small enough to fit all adults as well as some teenagers.

SynCardia has received conditional approval from the FDA to conduct an Investigational Device Exemption (IDE) clinical study of the Freedom portable driver in the U.S. An IDE allows a device to be used in a clinical study in order to collect safety and effectiveness data required to support a Premarket Approval (PMA) application, which SynCardia obtained from the FDA. As part of that trial, 30 stable Total Artificial Heart patients (as opposed to those still recovering from surgery) must be discharged from the hospital using the Freedom driver for 90 days (or shorter if they can find a donor heart in that time frame).

The study's purpose is to determine whether the Freedom driver is a suitable pneumatic pump for stable Total Artificial Heart patients, and if it can be safely used at home, Ford says. There are several portable Freedom drivers in use in Europe and one in the U.S. The company expects more to follow in the latter because 22 hospitals here are seeking permission from their institutional review boards to offer Freedoms to artificial heart patients.

Originally, patients were tethered via a 1.5-meter-long tube to a 180-kilogram driver system—nicknamed "Big Blue"—until they could find a donor heart to replace SynCardia's technology. The company saw the opportunity to create the Freedom while it was developing a 23-kilogram alternative in-hospital driver to the Big Blue system. The 23-kilogram Companion driver has been available in Europe since October 2009; SynCardia plans to submit it for FDA approval in September.

A challenge to shrinking down the driver technology to three percent of its original weight was to retain enough energy output to operate the artificial heart. The Freedom driver has a primary motor as well as a backup in case there is a problem with the main system; the motors operate a piston, which compresses air and then pushes it through the tubes to power the artificial heart.

SynCardia has plans to further shrink its driver technology, although Ford says current technology would prevent the device from weighing less than four kilograms. The downsizing could be done over the next two years and would mostly come from using a smaller motor controller, smaller lithium ion batteries (the Freedom uses two) and replacing analogue components with digital technology.

SynCardia's technology is one of a few available to people suffering from heart failure. The only other FDA-approved artificial heart is AbioMed's AbioCor, which is a self-contained replacement heart with an internal battery charged by a transcutaneous energy transmission (TET) system, meaning that no wires or tubes penetrate the skin and therefore there is less risk of infection. The AbioCor is intended for end-stage heart failure patients who have a life expectancy of less than 30 days and are not eligible for a natural heart implant.

Another technology, left ventricular assist devices (LVAD), have been on the market since 1994 for heart patients awaiting a permanent transplant, but they work only if a recipient's heart can still function to some degree on its own. An LVAD generally consists of a tube that pulls blood from the damaged heart's left ventricle into a pump, which then sends blood into the aorta, assisting the weakened ventricle. The pump, placed in the upper part of the abdomen, has a second tube that extends outside the body through the abdominal wall, where it is attached to the pump's battery and control system.