There is hope for transplant patients, from technology which may keep a heart alive for 24 hours.
At the moment, a donated heart can usually only last around four hours as it is rushed to the person who needs it.
But by pumping fluid through its blood vessels, and using pulses of oxygen which mimic a heartbeat, scientists have been able to keep pigs' hearts for 24 hours.
They say the technology could be available for human hearts in a year.
The breakthrough, using a device which fits into a small suitcase, could cut waiting lists for transplants.
The latest figures show there are 328 people in the UK on the waiting list for a heart, including 39 children.
Many are forced to wait until it is too late, and figures suggest around three-quarters of donated hearts cannot be used in the UK.
Dr Rafael Veraza, from the University of Texas Health at San Antonio, presented the findings on pig hearts, which still appeared to be oxygenated with viable cells after 24 hours, at the annual meeting of the American Association for the Advancement of Science in Seattle.
He said: 'The first heart was transported more than 50 years ago by putting it on ice, and decades later it is done much the same way.
'But being able to keep a heart viable for 24 hours means you could transport it almost anywhere in the world, and this could save many lives.'
While the main reason that hearts are not taken from UK donors is because they are unsuitable, experts say the time between removing an organ and transplanting it is 'crucial'.
The average person waits almost three years - 1,085 days - for a heart transplant, with NHS Blood and Transplant reporting in 2018 that one man, 45-year-old Gareth Evans from Stockport, had been on the waiting list for nine years.
The key to preserving hearts for a full day is to mimic the conditions in the human body as much as possible.
That means flushing a preservation solution through arteries, delivering 60 pulses of oxygen per minute like a heartbeat, and suspending the heart in a solution, similarly to how it sits in the body, to keep it from collapsing under its own pressure.
The US researchers tested their system, called ULiSSES, in five pig hearts, finding they appeared viable after 24 hours, with little of the swelling which indicates cell death.
The next step is to transplant the hearts back into pigs, to ensure that they function properly, before moving on to human hearts.
The researchers plan to test donated hearts which are not suitable for transplant within three months, and are already discussing this with hospitals.
If the necessary official approvals are granted, the scientists say hearts kept for 24 hours could be given to people within a year.
They also hope to use the technology to preserve and transport limbs lost in war, road crashes or industrial accidents, so these could be reattached, although this has only been done using pigs' limbs so far.
The researchers say they have a grant from the US Government to research this.
The hearts are currently being stored at 4C (39F), but are hoped in future to be kept at room temperature, doing away with the need for organs to be carried on ice.
Donor hearts, stored in this way, have previously appeared to work normally for a short time in dogs.
Dr Veraza, a post-doctoral fellow at UT Health, said: 'The important thing about our research is how portable this device is.
'We hope it will make more organs available, which can be kept at a good quality, so that there are more matches and fewer rejections.'
The waiting list for a heart transplant has more than doubled from 126 people in March 2010 to 295 in March of last year.
John Forsythe, medical director for organ donation and transplantation at NHS Blood and Transplant, said: 'For a heart transplant to be successful, the time between retrieval and transplantation is particularly crucial.
'The use of machines to stabilise and preserve organs is one important area of development, and the use of oxygen 'perfusion' techniques has proven extremely successful in other organs, including liver and pancreas.
'UK teams are exploring the use of similar techniques for heart transplantation, through our world-leading DCD (death after circulatory death) heart programme - and also looking at how the machines and techniques being pioneered as part of this programme, might also be applied for hearts transplanted after brain stem death.
'We welcome any research that builds on this.'