Two companies are trying to win your heart — literally.  Both Thoratec (NASDAQ:THOR) and Heartware International (NASDAQ:HTWR) make implantable devices to treat heart failure.  This chronic disease occurs when the heart muscle degenerates, reducing the pumping power of the heart. This, in turn, causes the heart to become too weak to pump blood at a level sufficient to meet the body’s demands.

Heart failure is a huge problem.  According to estimates by the American Heart Association 5.8 million people suffer from the disease in the U.S. and approximately 610,000 new cases are diagnosed each year. Heart failure kills about 300,000 people every year.  Health professionals have tried to manage the disease with drugs, but the only true cure for those in the late stages is a heart transplant. And there just aren’t enough hearts to go around.

The United Network for Organ Sharing says that in any 12-month period there are only about 2,300 hearts available for transplant in the U.S.  At any given time, approximately 2,600 patients are on the U.S. national transplant waiting list and a comparable number people are waiting in Europe. The median wait time for a donor heart is approximately nine months, and many patients have to wait as long as two years.

So Thoratec and Heartware have stepped into the breach with their respective devices, which keep people alive until they can receive a transplant. Pleasanton, Calif.-based Thoratec is the clear leader so far, primarily because its products were first to market. The company says that more than 18,000 of its devices have been implanted in heart failure patients around the world.

Thoratec’s success is reflected in its share price. Trading at just under $14 in June 2006, the company enjoyed a fairly steady climb through June 2010, when the stock hit an all-time high of $46.30. That’s when its shares began to retreat, trading as low as $23 in January of this year before jumping back to its current $35.

Certainly the impending marketing of Heartware’s competitive product has had something to do with the blips in Thoratec shares.  Operating out of Framingham, Mass., Heartware has a device that’s smaller and easier to transplant.  One study showed that the survival rate for Heartware’s product was 90% vs. Thoratec’s 78%. Evidently, even Thoratec recognized the superiority of its competitor’s device, making a run at buying Heartware in February 2009. The potential acquisition, however, flat-lined when U.S. antitrust enforcers said they would challenge it.

With the Heartware product already being used in Europe and U.S. Food and Drug Administration approval expected in the fourth quarter of 2011, expectations for the stock ran high. Investors rode it up some 260% from May 2009 to January of this year, when it nearly touched $100. But those same investors evidently had some palpitations of their own earlier this year when data released by Heartware showed that patients using its device had a greater chance of developing blood clots. As investors headed for the hills, the company’s shares dropped to $25.

Physicians who thought HeartWare was the future now aren’t so sure. Others pooh-pooh the test results, stating that there are going to be issues with any device.

So, the faint of heart might want to stick with the known quantity, Thoratec. But HeartWare still seems to be a long way from being on life support — there may be plenty of upside for investors willing to absorb some risk.