The human heart consists of four chambers that contract and expand in sequence to pump blood throughout the vascular system. In order to do this, the heart has a carefully synchronized electrical network that must fire signals in a rhythmic manner.
The muscular heart chambers have thick walls that allow for the maximum volume of blood to be ejected with each beat. Aerobic training reduces the amount of effort necessary for the heart to function.
A recent study looked at the incidence and outcomes of cardiac arrest associated with marathon and half-marathon events from January 2000 to May 2010. The number of participants steadily grew from 1 million in 2000 to 2 million in 2010.
Of the 11 million people studied over the 10-year span, only 59 suffered cardiac arrest during a race. Cardiac arrest was most common in males who participated in full marathons as opposed to half-marathons. Men were also most likely to die from the arrest and are clearly more susceptible to exertional cardiac arrest.
Surprisingly, the most common cause of arrest was not due to blockage of coronary arteries but an oversized left ventricle. The increased size (hypertrophy) eventually blocks the outflow of blood from the heart.
Cardiac hypertrophy is most common in younger athletes and associated with poor outcome.
The rate of cardiac arrest should in no way discourage participation in long-distance athletic events. Instead, it should alert athletes to the need for proper pre-participation physical examinations.