The pigs had been dead for an hour. The cause: cardiac arrest. But six hours after researchers at Yale University connected their bodies to a machine pumping a nutrient-rich fluid, their organs began to show signs of life again.

Though the organs didn’t suddenly start working normally, some of the cellular damage brought on by loss of blood flow after death appeared to be reversed. The pigs’ hearts emitted electrical activity. Cells in their kidneys, livers, and lungs were functioning again and showed signs of repairing themselves. The discovery, published Wednesday in the journal Nature, suggests that cell death could be delayed longer than currently possible. If those processes could be slowed down, it could mean saving more organs for transplantation.

“This new system showed that not only can we slow down cellular damage, but that we can actually activate processes at the genetic level for cellular repair,” says Brendan Parent, an assistant professor of bioethics at New York University, who was not involved in the study but authored a commentary in Nature alongside it. “This might force us to reconsider what we decide is ‘dead.’”

In 2019, the Yale team challenged the idea that brain death is final when they reported that they had partially revived the brains of pigs for hours after the animals had been slaughtered. For the current experiment, the researchers wanted to see if the same method, in which a blood substitute is carried into the animal’s circulatory system, could also be used to revive other organs.

“We restored some functions of cells across multiple vital organs that should have been dead without our interventions,” author Nenad Sestan, a Yale neuroscientist, told reporters on a call Tuesday. “These cells are functioning hours after they should not be, and what this tells us is that the demise of cells can be halted and their functionality restored in multiple vital organs, even one hour after death.”

Deepali Kumar, president of the American Society of Transplantation and professor of medicine at the University of Toronto, says that with further refinement, the system could one day be used to expand the pool of human organs available for donation. “There is a significant shortage of organs for transplantation, and we certainly need new technologies that can help improve the organ supply,” she says.

In the US, around 106,000 people are on the national transplant waiting list, and every day 17 people die waiting for an organ, according to the federal Health Resources and Services Administration. Despite the huge need, around 20 percent of organs are discarded every year due to poor quality. That could mean they’re too old or damaged, which can happen when organs are cut off from an oxygen-rich blood supply for too long.

The standard practice for preserving organs for transplant is static cold storage. Cooling organs quickly after removal reduces their oxygen demand and can prevent cell death, but doesn’t save every organ. There’s also growing interest in using a technique called extracorporeal membrane oxygenation, or ECMO, for patients who can’t be resuscitated, in order to preserve their organs for transplantation. Typically used as life support for patients whose heart or lungs are badly damaged, an ECMO machine pumps blood outside the body to remove its carbon dioxide and add oxygen, and then returns it back to the body.