A blow to the chest can cause sudden, cardiac death. It’s called commotio cordis. Essentially, the trauma causes a brief electrical spike. If it happens during a particular point of the cardiac cycle, it can stop the heart immediately. It is much much more common in young children due to the increased compliance of the chest wall. Most cases that I’ve ever heard of occur with a strike to the sternum, usually a punch or a ball – like in baseball or lacrosse.
It’s extremely rare. Only about 10 to 20 cases a year are reported. I don’t think I’ve ever heard of a case in anyone over the age of 20 years of age. Every case I’ve heard of, or attended to, involved baseball. I would think shoulder pads would prevent the anterior chest from this type of injury. Modern pads cover the upper/mid sternum and anterior chest. Resuscitation was historically dismal, but that has improved with better recognition. There is some speculation that certain individuals are more predisposed given a possible underlying dysrhythmia. The variables that are necessary for this to occur include the shape of the projectile, the velocity and timing of the incident. Without going into the specifics of the cardiac electrical cycle and repolarization, the most common underlying dysrhythmias include long QT syndrome, and a condition called Brugada syndrome.
A blow to the chest does transmit electrical activity through the chest wall into the heart. If this occurs, it creates an electrical spike which (if timed to occur exactly) can cause a "R on T phenomena". The waves of the EKG are labelled PQRST (U). This causes ventricular fibrillation which must be immediately electrically corrected (defibrillation) or else the heart will stop. Whenever I’m at the bedside and a patient’s heart stops (rare, but it happens enough) the quickest way I have to get it restarted is a thump, essentially a punch into the chest of the patient. It is thought that a normal punch to the chest confers approximately 2-5 J of energy. A punch, hit, ball, etc is more like 20-40J. Enough to stop the heart if it happens at the precise wrong time. To be clear, this is an enormously rare event. Just an awful situation. Defibrillation will almost surely save him if that's the issue. I'm sure the med staff on the field know this stuff cold and the defibrillators are all right there.
Edit: Docsplaining, clarity