People who had heart transplants as children have an increasing risk of kidney failure over time, researchers found when they connected databases that separately tracked the two conditions in thousands of people.

The study findings showed kidney failure was more extensive than previously thought, said Swati Choudhry, M.D., lead author of the study and a pediatric cardiology fellow at Washington University School of Medicine in St. Louis, Missouri.

In addition, the risk of developing kidney failure increased over time. People surviving 10 years after a heart transplant had a 3 percent risk, compared to 16 percent for those surviving 20 years, according to the researchers.

“A heart transplant is a miraculous, life-saving surgery for patients who suffer from irreversible heart conditions,” said Choudhry. “Unfortunately, some heart transplant recipients may end up with devastating kidney failure over time, a setback which compromises the quality and the longevity of life for these patients.”

In 2016, there were about 444 heart transplants in children ages birth to age 17, according to the Organ Procurement and Transplantation Network. Since 1988, there have been more than 8,751 heart transplants in that age group.

Researchers tracked nearly 25 years of records from more than 6,900 heart transplant patients across the Scientific Registry of Transplant Recipients and the United States Renal Data System while navigating privacy laws, said senior author Charles E. Canter, M.D., medical director of the cardiac transplant program at St. Louis Children’s Hospital.

When the databases were joined, it highlighted that 214 heart transplant recipients had end-stage renal failure. That was 25 percent more people with kidney failure after heart transplants than was tracked by the transplant registry alone.

In 1982, the introduction of cyclosporine, a new type of drug that kept the immune system from attacking transplanted organs, dramatically improved three-year survival in heart transplant patients — from about 40 percent to 70 percent. Tacrolimus is another commonly used immune-suppressing drug. But both medications are toxic to the kidneys.

“The advent of cyclosporine and tacrolimus has been a godsend for transplantation,” said Stuart Goldstein, M.D., director, Center for Acute Care Nephrology and co-medical director of the Heart Institute Research Core at Cincinnati Children’s Hospital in Ohio.

However, the residual damage of the medications has led to kidney transplants along with a second heart transplant, he said. Goldstein said researchers are “absolutely” looking for transplant medications less toxic to the kidneys. But the drugs aren’t available now, said Canter.

Overall, the study results were a “combination of good news, bad news,” Canter said. The good news, he said, is that heart transplant patients who received a heart before adulthood are surviving much longer. But these heart transplant survivors are at risk of developing kidney failure over time.

Other risks may also lead to kidney failure, the study showed. The likelihood of kidney failure inched up by more than 2 percent in heart transplant recipients with high blood pressure or diabetes. Treating these conditions could help patients avoid or delay kidney failure, Canter said.

Kidney transplants appeared to improve survival more than dialysis, according to preliminary study results. “This project provides reassurance that proceeding to kidney transplant gives patients the best chance for overall survival,” Canter said.

The study also highlights the loss of data for heart transplant patients as they become adults, Canter said. Moving from specialized pediatric care to adult medical care is a challenge faced by many patients affected by chronic diseases since childhood.

“These patients in transition really struggle,” Goldstein said.

The study, presented in November at the American Heart Association’s Scientific Sessions, was likely a one-time undertaking due to the intense effort required, researchers said.