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(CNN) -- Pioneering heart transplant surgery announced Friday in Australia may lead to a new option for patients awaiting transplants by boosting the number of donor hearts available.

Doctors at St. Vincent's Hospital in Sydney said they performed three successful transplants of hearts that had naturally stopped beating in the donor, rather than using the typical method of removing donor hearts from patients who are brain-dead but still have cardiovascular function.

The donor hearts had stopped beating for as long as 20 minutes before surgeons were able to remove them from the patients.

The three surgeries were done in the past few months. All patients are doing well; one of them told reporters she feels like a "different person" who can perform more physical activity than before the surgery.

The procedure involves injecting the hearts with a preservation solution developed by the institute and hospital, then placing them in a machine that perfuses them with warm oxygenated blood. The machine keeps the heart replenished with oxygen, nutrients and hormones during transport, according to TransMedics, the maker of the machine.

Typically, donated organs are transported on ice, which carries the risk of damage.

The Victor Chang Cardiac Research Institute in Sydney, which worked with St. Vincent's on the procedure, said it "will result in a major increase in the pool of hearts available for transplantation."

The transplanting of hearts that have naturally stopped beating in donors -- which are called DCD hearts, for "donors after circulatory death" -- is already the subject of research internationally, said Dr. Joseph Woo, a cardiothoracic surgeon at Stanford Health Care and chairman of cardiothoracic surgery at Stanford Medicine in Palo Alto, California.

Stanford researchers have been studying the process in human DCD hearts, and researchers elsewhere are studying it in pigs and other animals, he said.

Transplants involving other DCD organs such as kidneys, livers and lungs are already widely accepted, he said.

In 2008, an article in the New England Journal of Medicine reported the successful transplants of DCD hearts into infants at Denver Children's Hospital. Three babies younger than 18 months received hearts from donors who had died from cardiocirculatory causes; the recipients' six-month survival rate was 100%, according to the article.

The authors, all members of the hospital's Pediatric Heart Transplant Team, said the results were promising for others awaiting critical transplants.

"Donors who died from cardiocirculatory causes offer an opportunity to reduce waiting time and waiting-list mortality among children whose survival depends on a heart transplant," wrote the authors.

"For pediatric heart donation and transplantation involving patients who die from cardiocirculatory causes to become a more frequent option for end-of-life care and to affect significantly the nationwide risk of dying while waiting, the concept of distant sharing of donated organs from these donors should be considered."

Woo said the number of heart transplants in the United States has remained at around 2,000 for years, limited by the number of viable donors. New methods like the one performed successfully in Australia may help save more lives.

"The thinking is if you change the way that families are able to permit donation, that you might have more donors," he said.

Two of the patients who received the new hearts in Sydney beamed as they spoke to reporters Friday.

Michelle Gribilas said she feels years younger. Before the surgery, she couldn't walk more than 100 meters (about 110 yards) at a time.

"I'm a different person altogether. Like I walk 3 kilometers (almost 2 miles) a day, I go up the stairs, about 120 to 100 stairs a day," she said.

"It's a wild thing to get your head around, that your heart's (come from) a stranger, someone you don't know -- part of them is now inside you," said another recipient, Jan Damen. "It's a privilege. It's an amazing thing."