Had these survivors never been genetically tested they would not have known why they had suffered a near-fatal arrest or that their families were also at risk, senior author, and director of RPAs Genetic Heart Disease Clinic Professor Chris Semsarian said.

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"These are young people some in their teens and 20s who on a particular day had a cardiac arrest and for so long we didnt know what the reason was," he said.

"You can do every test under the sun and you dont find anything wrong with their heart. It looks normal, but it carries genetic mistakes."

Without genetic testing, any medical team may come to the conclusion a cardiac arrest was likely to be a freak event, requiring no ongoing treatment, leaving patients vulnerable to a possibly fatal second attack.

The genetic testing was detecting "concealed cardiomyopathies", and could be used as an early-warning system that the hearts electrical system was short-circuiting.

The study authors suspect patients such as James Medway will develop outward signs of cardiomyopathy years later, such as an enlarged or thickening of the heart.

"This is precision medicine in action using genetics to pinpoint the precise cause of the disease, screen their families and find targeted treatments that may be as simple as lifestyle changes or medication or it may be an implanted defibrillator, but its all geared towards preventing sudden death in the future," Dr Semsarian said.

Mr Medway was in peak physical condition "fantastically fit", the Olympic rowing hopeful says as he trained at the Australian Institute of Sports training centre in northern Italy in 2019.

Then he blacked out.

When he was roused from an induced coma two days later, his teammates had to fill him in.

The 27-year-old had finished his warm-down after intense training. Just as he unstrapped his feet from a rowing machine he collapsed, hitting his head on the ergometer.

"It was such a pivotal, live-or-die moment," Mr Medway said.

James Medway, now 28 years old, in Hyde Park, Sydney. Credit:Louise Kennerley

The rower knew he was lucky. Had he been training on the lake, had one of his teammates a doctor not immediately suspected his heart and not his head injury was the critical concern, and the medical team not been nearby with a defibrillator, he could have died.

But after exhaustive testing, his heart looked normal for an elite athlete slightly on the larger side, and healthy. Within a week, he was fitted with an implantable cardioverter-defibrillator and less than two months later he was back training.

"There was a chance this was just a freak accident and I definitely still had the dream of getting to the Olympics and winning a medal," he said.

But Professor Semsarian, one of Mr Medways doctors, knew there was a small chance the episode had a genetic cause. Testing confirmed it.

Mr Medway had arrhythmogenic cardiomyopathy linked to a genetic mutation, which is estimated to affect one in 5000 people, most of whom never have a cardiac episode.

But elite-level sport was out of the question.

"It was a lot to take in," Mr Medway said. "I prided myself on being a really fit and healthy person, and the fact that I never got sick or injured. Now something I excelled at was no longer good for me.

Roughly 33,000 Australians have a sudden cardiac arrest every year; 9 per cent survive. The vast majority of cases are people over 35 with coronary heart disease and well-established risk factors like smoking and high blood pressure.

Younger patients with these unexplained cardiomyopathies are uncommon, and most can avoid having one of these near-death experiences. But for those who do, the ramifications can be tragic.

"Looking back, it would be very easy to slip into the mentality where everything I was working towards has been taken away but Im so lucky to have had that experience," said Mr Medway, who has launched into a career he loves as an investment banking analyst.

The scientists have so far genetically tested 11 first-degree relatives of survivors. Five had the same inherited genetic mutations and six were given the relieving news that they did not, the International Journal of Cardiology reported.

Dr Isbister said genetic testing could provide the answers when all other causes have been excluded, but guidelines written about a decade ago dont support this approach.

"What we have seen here is that there is perhaps more utility for patients with no clinically identified causes to have genetic testing now that our knowledge of cardiac arrest and genetics has advanced and we can cast a wide [genetic testing] net to get answers for patients," she said.

The authors suggested all sudden cardiac arrest survivors under the age of 50 could have an assessment to determine whether genetic testing was warranted.

Professor Jason Kovacic, executive director of the Victor Chang Cardiac Research Institute, said the research showed the benefits of comprehensive genetic testing.

"It does indicate that we can potentially save lives here," he said.

But the single-centre study of 36 patients was not likely to be enough to alter diagnostic practice, he said.

"Its a challenge to recruit thousands of patients because while always a tragedy this is relatively uncommon in our community," Professor Kovacic said.

"Whether this will tip the scale to make [cardiology] committees change their guidelines is yet to be determined."

Kate Aubusson is Health Editor of The Sydney Morning Herald.

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Hidden cause of cardiac arrests uncovered in perfectly healthy hearts - Sydney Morning Herald

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