Patricia Cooper's father was sliding toward death more quickly than anyone had anticipated. His only hope of surviving liver disease was a transplant, which his doctors agreed was all but a fantasy. In a nation with a severe shortage of donor livers, Wayne Cooper was too far down the list to have any chance of receiving one.

At his home in McLean, Va., the 70-year-old was losing weight quickly. He slept all day. Patricia, an immigration attorney in Miami, called frequently. "He was too tired to hold the phone," she said.

On Dec. 5, Patricia donated more than half her liver to her father, an obstetrician who 34 years earlier had personally brought her into this world.

"It wasn't even a choice. It's my dad. Of course I'm going to do this," she said. "I knew the mental anguish of him dying and me thinking I could have done something to save his life was going to be worse than any physical pain."

Though living-donor liver transplants have been performed in the United States for nearly three decades, they are still rare. In 2016, just 345 of the 7,841 livers used in transplants came from living donors, according to the United Network for Organ Sharing, which coordinates transplant lists. U.S. hospitals more commonly use living donors for kidney transplants. Elsewhere, including Turkey and South Korea, live donors are the standard for liver transplants.

In 2016, more than 14,000 people were on waiting lists for livers across this country. About 1,000 die each year because organs aren't found in time.

The critical difference in live-liver donation is that, unlike other organs, livers regenerate. If the donor and recipient match in a variety of ways, surgeons can take a portion of a donor's liver and transplant it, skipping what can be a years-long wait for a cadaver liver that too often ends in a patient's death.

Yet living-donor transplants expose two people — one of them perfectly healthy — to the rigors and risks of major surgery.

"This is one of the few transplants we do where you could have two people die instead of one person die," said Rolf N. Barth, director of liver transplantation at the University of Maryland School of Medicine in Baltimore, who participated in the surgeries on the Coopers.

They had been turned down by at least one other program, which considered their case too risky. Wayne's age put him near the limit that many physicians will consider for a transplant. He also has diabetes and, most important, heart problems that make liver surgery difficult.

Not long before the operation, doctors had to insert a stent to keep a coronary artery open, something that normally would have put him on blood thinners for six months and precluded surgery. Instead, they used a new kind of device that requires only six weeks of blood thinners.

"We've been a little more expansive in tackling difficult cases," Barth said.

The family's discussion started last summer after Patricia Cooper, the oldest of three children, remembered writing stories for her high school newspaper about a classmate who received a kidney transplant. As she began to bring herself up to date on transplantation, she came across information on living donation.

Her father rejected the idea, until she pointed out that he would have done anything to help his own father, who died at 58 of heart disease.

"She's a lawyer," Wayne Cooper said. "She's good with words. So she turned the tables on me."

Once it was clear that their blood types matched, the family discussed the possibility with one of the transplant centers where Wayne was being evaluated. The main concern was the disparity in their body sizes: Patricia's liver had to be large enough to provide an organ for her father and still leave her with adequate liver function during recuperation. The doctors there said she was too small. The wait for a cadaver liver resumed.

Then Cooper and his wife, Bonnie, brought up the idea of living donation at the end of a conversation with Barth and John C. LaMattina, director of the medical school's living-donor liver transplantation program. The surgeons were intrigued and began checking into the possibilities.

By October, Wayne was growing sicker and could barely eat. "I knew he was not going to get a cadaver [liver]," Patricia said. "He was going to die of malnutrition in a month or two. I just wanted to give him the chance to live."

The doctors modeled the two livers before surgery. But nothing is ever certain until they see inside the patients. Patricia's liver required an unusual amount of microsurgery to connect to her father's bile ducts and arteries.

On the day of the operations at the University of Maryland Medical Center, the two were placed in adjacent rooms. LaMattina and another surgeon began removing the lobe of Patricia's liver. About 90 minutes later, Barth and a fourth surgeon began preparing Wayne to receive the transplant. Timing was critical to minimizing how long the organ was outside both bodies. In between, it was stored in a cold solution.

Patricia gave her father the entire right lobe of her liver, about 59 percent of its total volume, LaMattina said. Typically, the lobe is between 60 and 70 percent of a donor's liver volume. He said studies suggest the organ grows to most of its original volume — in both donor and receipient — within two months.

In Maryland's program, which does about a dozen live-liver transplants each year, recipients fare as well or slightly better than people who receive cadaver livers, Barth and LaMattina said. The same is true nationally, according to UNOS data. Research has not shown a large impact on living donors, though a small number have suffered complications.

Today, Wayne Cooper is recovering well at home. He is on immunosuppressants so that his body doesn't reject his daughter's organ, and he takes about 20 pills a day, including diabetes and heart medications. The twin surgeries cost hundreds of thousands of dollars, most of which will be covered by Medicare.

Cooper and the doctors expect that he will die one day of something other than liver disease.

"We tell all our patients that's our goal — that they would die of old age in their sleep," LaMattina said.

Patricia recently returned to work part-time. A competitive master's swimmer, she is suffering most from fatigue and the incision made in the thick muscles of her abdomen.

"When these [surgeries] go well, they are wonderful, wonderful things," LaMattina said. "This is a story about the donor. The donor is going through something they don't need; that's a big deal, that's a risk, to do something for someone they care about."