LOS ANGELES — Rob Evans, a 61-year-old social worker from Apache Junction, Ariz., got the good news on Father’s Day: After 3½ years, doctors had found him a heart and were preparing to bring it to the University of California- Los Angeles, where he was being treated for a slow, steady decay of his cardiac muscle. Evans had been hospitalized at UCLA for six weeks.

Excited, hopeful and anxious all at once, Evans dared imagine a different life: out in the garage remodeling his ‘69 Nova, riding his horse, wrestling with his grandson and helping his wife, Gail, take care of their barn instead of sitting, exhausted, in his chair all day.

But the heart destined for Evans’ chest was not like most that transplant recipients get. Instead of coming to him in icy slumber inside a cooler, it was warm and beating. Though it had been excised from its donor, blood fl owed through it until shortly before it was stitched into Evans’ body on June 19.

Evans is taking part in a nationwide clinical trial to test whether such “beating hearts” are as good as, or better than, hearts packed in ice and slowed down with potassium and other chemicals. Doctors at UCLA, one of the study’s leading sites, have enrolled multiple patients, and other transplant centers across the country will soon start recruiting too.

“The human heart was never meant to be on ice,” said Dr. Abbas Ardehali, the surgical director of UCLA’s heart and lung transplantation program and the trial’s principal investigator. Chilling donor hearts before a transplant slows the rate of cell death and buys time to get the organ to a recipient, but only a little bit, he said. “That’s why having a human heart in a warm, beating state has always been exciting.”

From the outside, it looks like an ordinary grayand- white storage unit on wheels. At 40 inches tall, 30 inches wide and 20 inches deep, it weighs less than 100 pounds and fi ts comfortably in the back of an SUV.

But inside, it houses a living human heart.

The beating organ rests in a clear, sterile chamber equipped with a pacemaker in case it needs a jumpstart. Warm blood fl ows through tubing connected to the aorta because the veins that normally bring blood to the heart are sewn shut. The blood travels through the heart, feeding it oxygen and nutrients so it can function while removing waste products, which are fi ltered out by a machine. Fresh oxygen and nutrients are added to the donor’s blood before it is pumped back in. All the while, doctors can keep tabs on the heart’s rhythm, blood pressure and other vital signs to make sure it is healthy for surgery.

“The heart believes it’s still in the body,” said Dr. Waleed Hassanein, who founded TransMedics, the Massachusetts-based medical device company that developed the apparatus and is sponsoring the trial.

The technology was tested in Europe in a 2006 clinical trial of 20 heart transplant patients. Nineteen of these patients were out of intensive care within 24 hours, a much shorter stay than the two to fi ve days patients usually spend in the ICU, said Dr. Bruce Rosengard, the surgical director of cardiac transplantation at Massachusetts General Hospital in Boston and one of the fi rst surgeons to perform warm transplants in Europe.

“That’s a testament that those hearts function quite well,” he said. The device, known as the Organ Care System, has been commercially available in Europe since 2009.

Worldwide, more than 100 warm transplants have now been performed, all of them with positive results, Hassanein said. In the U.S., 24 patients have received warm transplants, including Evans.

To be sure, transplants of hearts packed in ice are lifesavers: More than 2,400 such hearts were transplanted in the U.S. last year, according to data from the United Network for Organ Sharing, the nonprofi t organization in Richmond, Va., that oversees the country’s transplant system. Almost 90 percent of recipients survive the fi rst year after surgery, and close to 50 percent live for at least 10 years with their new hearts, according to the National Heart, Lung and Blood Institute.

But there are drawbacks. Once a heart is disconnected from its blood supply, surgeons cool it to put the brakes on damage that occurs because cells are deprived of oxygen and nutrients. Damage still occurs in spite of the cooling, though — so much so that after about six hours outside a body, the hearts must be thrown away. As a result, a heart in Los Angeles, for example, can’t be used to save a matched patient in Miami because the heart would be too debilitated by the time it arrived at the hospital for transplant.

In addition, surgeons only have time to use factors such as blood type, weight, height and gender to pair up donors and recipients. They don’t have the luxury of running more sophisticated tests, like they do for kidney transplants, because the heart won’t tolerate the delay. “If we could test for proteins that mount an immune response, it could improve outcomes for transplant patients,” Ardehali said