杏吧原创

BASEBALL doesn鈥檛 feature high on most people鈥檚 list of dangerous sports, but even it can get ugly. Players are sitting ducks on the rare occasions when balls are accidentally bashed straight at them. The consequences are especially dire for younger players: between 1973 and 1995 in the US alone, 88 youngsters died from head injuries, most of them pitchers or infielders brained by batted balls (see 鈥淭he bat鈥檚 too good鈥�).

But baseball players also face other, less obvious hazards: mysterious accidents in which young players are inexplicably sent into fatal cardiac arrest by ball strikes to the chest that are sometimes so mild they leave no trace. Between 1975 and 2001, over 60 baseball and softball players died this way. The idea that a strapping youth can drop dead from a tap on the chest seems to make little sense. Yet researchers are finally unravelling the mystery of how a light blow to the chest can kill, fuelling hopes that these accidents can finally be prevented.

These strange deaths are believed to be caused by the condition known as commotio cordis or shaking heart, so named for the way the heart quivers before it finally judders to a halt. The condition has been recognised for 150 years: a report from 1876 describes a porter dying when the yoke he was pulling snapped and he fell to the ground chest-first. But the true extent of commotio cordis has emerged only recently. Barry Maron of the Minneapolis Heart Institute in Minnesota has compiled a register of 128 cases, using medical records and crime investigations, which reveals that baseball is the single most risky activity 鈥� although others, even everyday activities, can be fatal too.

Sixty-seven of the 128 cases involved baseballs or softballs; others featured hockey pucks, or lacrosse, cricket or soccer balls. Five children were struck by parents disciplining them, six by play-boxing friends. One victim was hit by a snowball, another by a plastic bat. Among the most shocking cases were a child struck by a friend trying to cure his hiccups, and another who was thumped in the chest by the family dog.

鈥淐ommotio cordis drives the imagination,鈥� concludes Peter Kohl, head of the cardiac mechano-electric feedback lab at the University of Oxford. 鈥淚t has the air of a hard-to-understand, black magic thing 鈥� but it isn鈥檛.鈥�

Each beat of the heart is triggered by a sudden flow of ions through channels that cross the membranes of heart tissue cells, altering the potential difference across the membranes. Before the next contraction can occur, pumps embedded in the membranes must restore the distribution of ions to their previous levels. This 鈥渞echarging鈥� doesn鈥檛 occur instantaneously across the whole heart. The left ventricle, which pumps oxygen-rich blood from the lungs to the rest of the body, recharges gradually, the process starting at one side of the ventricle and moving to the other. And if a contraction is triggered before the entire ventricle has recharged, the muscle鈥檚 single, concerted contraction can degenerate into chaos.

So could a gentle blow to the heart trigger premature contraction? Mark Link of the Tufts-New England Medical Center in Boston has been trying to answer that question by firing balls at the chests of pigs, each shot timed to strike at specific moments in a pig鈥檚 heartbeat cycle. He has discovered that commotio cordis results from incredibly bad luck. The blow must come at the right moment and the right location at the right speed. In pigs, it is triggered when collisions occur in a 15-millisecond window during a heartbeat. Impact must also occur directly over the heart, and balls fired at around 65 kilometres an hour seem most likely to cause it (see Graphic).

Link also found a drug that prevents commotio cordis in pigs. It blocks a potassium ion channel in heart muscle cell membranes that plays a role in contraction. He believes that a blow can cause a blood pressure spike in the left ventricle that stretches the heart muscle. This seems to open the potassium ion channels, starting an early contraction and triggering commotio cordis.

The bad news is that there appears to be no predisposing factor. Anyone struck on the chest in exactly the wrong way can have an attack. Commotio cordis deaths are more common in young people, but only because they have springier ribcages.

The good news is that victims can be saved by quick action with a defibrillator. Unfortunately, delays in getting help to them often occur, and only 16 per cent of the victims studied survived. 鈥淎 kid gets hit and people don鈥檛 even start CPR for three minutes, because no one thinks that getting hit by a ball will kill,鈥� says Link.

Although soft safety baseballs reduce commotio rates in pigs, the chest protectors sometimes worn in sports like hockey, lacrosse and baseball do nothing at all. The pig model may help researchers design chest protectors that do prevent commotio.

But improved chest protectors worn during sports will not be a panacea, since the agent of doom can be a dog鈥檚 head, a snowball or a fall from a swing. In these cases, commotio cordis 鈥� just like stray meteorites or frozen chickens falling from the sky 鈥� may simply be part of the risk of living.