Taking a baseball to the chest hurts. Sometimes, it leaves a bruise. Sometimes, it knocks the wind out of you. If it hits at the wrong millisecond, it can kill.

illustration by Roxanne Palmer

Every once in a while, a baseball or a hockey puck becomes a sinister projectile.  When an object traveling at precisely the right speed and having precisely the right mass strikes a person, particularly a child, in precisely the right place, at precisely the right time, his heart stops.

Because so many factors must be present at once, the phenomenon of sudden death after a blunt blow to the chest, known as commotio cordis, is exceedingly rare.  There have only been 190 commotio cordis deaths recorded during the past 100 years, hardly making it a public health crisis.  But it kills children and it kills athletes, shattering idealistic societal images of innocence and vitality.  The average age of the victims is 14, and one-third of them are under age 10.  Ninety-five percent are boys.  Approximately 60% of the deaths occur during organized sports, primarily baseball, hockey, and lacrosse, where small, dense projectiles dominate the action.  Another 25% happen during backyard or playground sports.  (The remaining cases are attributable to chest blows suffered in other situations like car accidents, fights, or domestic violence.)  Even more disturbing, no practical strategies for prevention exist.  Equipment marketed to protect children from commotio cordis is ineffective at best, and life-saving defibrillators (along with people appropriately trained to use them) are unavailable at most youth sporting venues.

Commotio cordis, derived from the Latin for disturbance of the heart, appeared in medical textbooks as early as 1857, but it has entered the public consciousness only recently.  The first report of a sports-related commotio cordis death hit the press in 1898, when a British newspaper reported on a 13-year-old cricket player who died instantly after he was hit by a pitch in the chest.  Scientists began studying the phenomenon in the laboratory, and by the 1930s, they understood that a blow to the chest could trigger an abnormal heart rhythm called ventricular fibrillation.  They did not, however, determine why.  Then, commotio cordis disappeared from the medical literature for 60 years.  In 1995, a cardiologist in Minnesota published a case report on 25 young athletes who died over an 18-year span during competitive or recreational sports after suffering blows to the chest.  The reframing of commotio cordis as a problem that affects innocent children engaging in wholesome athletic activities sparked renewed scientific and public interest.

Over the past 13 years, researchers have made significant progress in solving the electrical mystery of commotio cordis.  In order to trigger a disturbance in the heart’s rhythm, an object must strike the chest squarely over the heart during the so-called vulnerable period.  The vulnerable period is a 10-millisecond window—about two flaps of a honeybee’s wings—during the time between heart beats when the heart muscle is recharging (a process that takes about 200 milliseconds in total).    A blow to the chest at that literal split second causes the pressure inside the left ventricle, the heart’s strongest chamber, to rise very quickly, which upsets the electrical balance in the heart.  Instead of fully recharging, the ventricle squeezes again.  The improperly timed beat sets off a wave of abnormal electrical activity that causes the heart to quiver rather than pump.  This rhythm, called ventricular fibrillation, kills patients without exception, unless they are quickly shocked with defibrillation paddles—and even then, only about 15% of victims survive.

No one is exactly sure why commotio cordis most frequently strikes children.  One major reason is probably because children’s chests are relatively soft.  Ribs, which contain a large percentage of pliable cartilage at birth, do not turn completely to hard bone until age 15; they do not fuse to the sternum, or breastbone, until around age 25.  So, it takes less force to distort children’s chest walls—and an external impact is more easily transmitted to the structures inside, like the heart.  Additionally, more children than adults participate in competitive sports, putting them at more risk.

Balls and pucks also travel at slower speeds in youth sports, and, somewhat surprisingly, only blows by objects traveling 30-50 miles per hour induce commotio cordis.  Logically, slower projectiles do not carry enough force to trigger the necessary increased pressure in the heart, but it is less intuitive why faster ones do not cause electrical problems.  For one thing, an extremely forceful blow may crack a rib or the breastbone, in which case the bone, rather than the heart, absorbs the brunt of the impact.  Even if the heart does take the direct hit by an object traveling faster than 50 miles per hour, the muscle will be stunned, according to Dr. Mark Link, as cardiologist at Tufts-New England Medical Center and a national expert on commotio cordis.  Link said a stunned heart cannot conduct electricity at all.  So, rather than creating an electrical disturbance, the affected portion of the muscle “sits out” while the normal electrical cycle continues in the rest of the heart.

The tendency for commotio cordis to afflict more boys than girls is even more difficult to explain.  At least historically, more boys play sports like baseball, hockey, and lacrosse.  There are probably other factors, though, according to Dr. Link.  He speculated that some physiological difference probably makes boys more susceptible to arrhythmias, but said no one knows what that difference is.

Because of the intangibility of electricity and the lack of physical evidence associated with the condition, major research advances in commotio cordis are hard to come by.  Other than perhaps an external bruise on the chest, there are no autopsy findings whatsoever. The heart suffers no structural damage—not even bruising or bleeding.  The affected children are otherwise completely healthy with normal hearts.  Unlike other heart conditions that can be dangerous for athletes, it is impossible to screen for commotio cordis risk.

With no way to predict commotio cordis, many parents and coaches encourage children to wear chest protectors to prevent it.  Unfortunately, none of the commercially available chest protectors have proven to be effective.  In fact, of the 85 athletes known to have experienced commotio cordis between 1985 and 2006, 32 of them (38%) were wearing protective equipment.

“Sometimes youth baseball leagues call me and say ‘we’re going to mandate chest protectors—what do you think?’” said David Halstead, a PhD in biomechanics and the technical advisor to the National Operating Committee on Standards for Athletic Equipment (NOCSAE).  “I say, you’re going to kill more kids from heat exhaustion.”

Until someone invents an effective chest protector, the only hope for victims of commotio cordis is immediate CPR and defibrillation (shock).  Unfortunately, these basic life-saving measures are not always readily available at youth sporting events.  While Little League Baseball does require each individual league to submit a generic safety plan to the national office, they do not mandate the contents of the plan.  Many leagues choose not to require coaches and umpires to be formally trained in basic life support and CPR.  It is rare for automated external defibrillators (AEDs) to be present at youth fields and arenas.  According to the American Heart Association, AEDs cost $1500-2000, prohibitive for many youth sports organizations, especially given the extremely low likelihood of needing to use them.

While researchers continue to unravel the mysteries that surround commotio cordis and youth sports leagues revise their safety polices, there is probably no way to completely eliminate this rare but fatal condition.  Although balls and pucks can pack a lethal punch, the infinitesimal risk should not preclude participation.