When Mark Hughes, the former Manchester United and Wales manager, lost his 38-year-old son, an inquest found the cause was sudden adult death syndrome, or Sads.

He had appeared healthy. There was no obvious disease. For a family, that combination is almost impossible to make sense of. How can someone die from a heart condition when even a post-mortem might find nothing wrong with their heart?

The answer starts with something easy to forget: the heart isn’t just a pump; it’s also an electrical organ. Every heartbeat begins with a signal that ripples through the heart in a precise sequence, telling each chamber when to contract. In Sads, the fault often lies in that electrical system, not in the heart’s physical structure.

If the signal becomes unstable, an abnormal rhythm – an arrhythmia – can develop. Most are harmless. But some that start in the heart’s lower chambers, the ventricles, can be fatal. During ventricular fibrillation, electrical activity turns chaotic and the heart quivers instead of pumping. Blood stops reaching the brain, causing the person to lose consciousness.

Without CPR and a shock from a defibrillator, they can die within minutes. This is a cardiac arrest.

It’s worth being clear that a cardiac arrest isn’t the same as a heart attack, even though the terms get used interchangeably. A heart attack happens when a blocked artery starves part of the heart muscle of blood. A cardiac arrest happens when the heart suddenly stops pumping blood around the body.

A heart attack can cause a cardiac........

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