Commotio cordis is a cardiac arrest caused by a blunt, non-penetrating blow to the chest over the heart, in someone whose heart is entirely normal. There is no bruising or tearing of the heart itself. Instead, an impact landing at one precise and unlucky moment of the cardiac cycle throws the heart into ventricular fibrillation, and the athlete collapses.
It matters in sport because it strikes young, healthy people during ordinary play, most often a child or young adult hit in the chest by a cricket ball, a hockey puck or an opponent's knee or elbow. For the sport and exercise medicine (SEM) doctor and anyone providing pitchside cover, this is a first-contact emergency where seconds count. The single most important idea is that a collapse after a chest blow is a cardiac arrest until proven otherwise, and must never be dismissed as the athlete being winded. Because the heart is structurally normal, prompt recognition and early defibrillation give a genuinely good chance of survival, which places the outcome largely in the hands of whoever is there at the time.
The heart in commotio cordis is structurally normal, which sets it apart from a cardiac contusion, where a harder impact actually bruises the heart muscle. Here the damage is electrical, not mechanical. The blow delivers a mechanical shock to the heart that, if it arrives at exactly the wrong instant, disrupts the heart's electrical rhythm and triggers ventricular fibrillation (VF), a chaotic rhythm in which the heart quivers rather than pumps.
Timing is everything. The vulnerable moment is a very narrow window on the upstroke of the T wave, the part of the heartbeat when the ventricles are recovering electrically. A blow landing in this brief window can trigger ventricular fibrillation, whereas the same blow a fraction of a second earlier or later does nothing. Two other factors matter: the impact has to fall over the centre of the chest, directly over the heart, and it has to carry a certain amount of energy, since a very light contact does nothing and a very heavy one tends to cause structural injury instead. The chest wall of children and young people is thinner and more pliable, which is thought to transmit the impact to the heart more readily, and this helps explain why commotio cordis mainly affects the young, and predominantly young males.
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