A new study published in the New England Journal of Medicine looks at cardiac arrest in young athletes. The good news is that the risk of cardiac arrest among athletes younger than 45 is rare—less than one case per 100,000 competitive athletes in Ontario, Canada.

The bad news is that these fatal cardiac arrest episodes are not likely predictable, which makes prevention difficult. Researchers discovered that more than 80 percent of the cases will not be caught during what is called pre-participation screening. This type of screening is based on the idea that doctors can detect underlying heart abnormalities, and then keep young people out of sports.

In some countries, such as Europe, electrocardiograms (ECGs) are also used to detect abnormalities early. But in general, these screenings rely on physical exams and questions about alarming symptoms and family history.

Cardiac arrest is fatal within minutes without medical intervention. It is characterized by the heart stopping, often due to a sudden rhythm disturbance.

The study looked at cardiac arrests that were treated by paramedics in a large region of Ontario, while focusing on young athletes (aged 12 to 45) who suffered a cardiac arrest while playing sports. Of the 16 who had cardiac arrest, only three had an underlying heart condition that could have been detected with a screening.

While cardiac arrest typically cannot be predicted, coaches and other players can be prepared to jump into action by calling 911 and beginning chest compressions immediately.