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Last week, surgeons successfully transplanted a genetically modified pig's heart into a man with life-threatening heart failure. It's a medical first that is raising hopes of a new, viable alternative for people at risk of dying before limited human cadaver hearts become available and for those too sick or ineligible for human heart transplantation.

But heart failure specialists say any optimism should be buffered with caution, as questions about short- and long-term safety and survival remain unanswered.

"There is promise embedded within this story, but also peril," said Dr. Clyde Yancy, chief of cardiology at Northwestern University Feinberg School of Medicine in Chicago and a past president of the 91ÊÓƵ. "I don't think we celebrate just yet."

The Food and Drug Administration has not approved the use of porcine, or pig, heart transplants. But it made a one-time "compassionate use" exception for the Jan. 7 surgery in a 57-year-old man.

Heart transplants become necessary when there is severe heart failure, which occurs if one or both heart ventricles stop functioning properly. Human hearts for transplant come from organ donations made available when people die.

However, the need far exceeds availability and either die before they reach the top of the list or become too ill for a successful transplant. According to the federal , there are 3,484 people awaiting hearts as of Jan. 14.

Researchers have long been searching for alternatives to human heart transplants. The most successful efforts have come through advances in technology, such as the left ventricular assist device – LVAD – which helps the heart pump blood and has for people awaiting transplant over the past several decades.

Animal organ transplants, called xenotransplants, hold promise but come with a host of unknowns, Yancy said. While pig hearts are similar to human hearts and therefore good candidates for transplant, there are unforeseen risks, such as whether they could transfer porcine viruses into their human hosts.

"There are lots of reasons to be celebratory, but we need to pause and consider all of the questions that go along with this discovery," he said, "starting with any evidence of harm and a careful review of safety."

Another question is how long – and how well – a pig's heart can work in a human host. Human heart transplants are highly successful with a . On average, heart transplant recipients live another 12 to 13 years.

"Will these genetically modified pig hearts meet the now standard thresholds for transplantation?" Yancy asked. "Will they last as long as a human cadaver heart can last? Will they be as functional? Will the patient feel better?"

Whether they do or not, the fact that the recipient's body did not immediately reject the new heart is encouraging, said Dr. Sandra Chaparro, director of the advanced heart failure program at Miami Cardiac & Vascular Institute at Baptist Health South Florida.

"It's already a success because the patient did not develop acute rejection in the first 24 hours," she said. "Obviously it's going to take time to understand the long-term implications of the human body adapting to a pig organ transplant. But this is a historical step in the direction of getting therapies for end-stage heart failure patients."

Some also have raised ethical concerns over raising an animal for the sole purpose of creating an organ genetically tailored to save a human life, she said.

Other questions to be answered are how much it costs to raise and genetically modify a pig sufficiently for its heart to be transplanted into a human and how to make such transplants available in an equitable fashion, Chaparro said. "We obviously want to make sure access is equal for all patients who need this, not only the ones with the resources to pay for it."

While researchers continue to explore xenotransplantation and other solutions to end-stage heart failure, Yancy said he'd like to see more emphasis on the lifestyle behaviors that can help people maintain good heart health. These include not smoking, staying physically active, eating a heart-healthy diet, maintaining an appropriate weight and keeping blood glucose, cholesterol and blood pressure levels under control.

"As encouraging as it is to see this potential new resource for heart transplantation and the ability to extend definitive interventions for patients," Yancy said, "it is equally important to give consideration for how we can prevent the condition from the outset. The best treatment for heart failure is never having it."

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