Can the heart itself be damaged by an external force, such as being struck forcefully in the chest? What would be the result of this type of injury?

Because the heart is a muscle, if an external force acts upon the heart, it can be damaged. One of the most common injuries of this type is myocardial contusion .

This usually occurs because of blunt force to the anterior thoracic wall , such as hitting a steering wheel of a car when not wearing a seatbelt. Often, it is accompanied by rib fractures and punctured lung (pneumothorax).

Patients with a myocardial contusion will present with chest pain, a fast irregular pulse, low blood pressure, and difficulty breathing.

If the contusion is severe enough, it can result in blood building up around the heart ( cardiac tamponade ). This can be life-threatening, because the heart is not able to fill completely and circulate oxygenated blood to the vital organs.

The strongest evidence that cardiac tamponade is present is Beck's Triad : distended neck veins, muffled heart sounds, and low blood pressure. Narrowed pulse pressure and pulsus paradoxus (a drop of blood pressure upon inspiration) are also often present.

One of the other life-threatening complications of sudden force to the heart is a dysrhythmia, an irregular heartbeat.

The force of the impact of a small object such as a baseball, hockey puck, or bat hitting a person's chest with a great deal of velocity can be enough to cause a number of cardiac dysrhythmias, including ventricular fibrillation which would necessitate CPR, defibrillation, and advanced measures.

The most profound injury that can result is cardiac rupture . Blunt force trauma can be strong enough to rupture a heart chamber - this is rarely a survivable injury.

A more detailed explanation can be found in this article from The Journal of Trauma.