ETIOLOGY AND MECHANISMS OF INJURY The most common cause of blunt cardiac trauma is a high-speed motor vehicle accident. However, myocardial injury has been documented in accidents involving vehicles going less than 20 mph. Other causes include direct blows to the chest, industrial crush injuries, falls from heights, blast injuries, and athletic trauma.
The heart is suspended relatively freely within the chest cavity from the great vessels, and this mobility plus its location between the sternum and the thoracic vertebrae make it susceptible to injury as a result of several mechanisms: (1) sudden horizontal acceleration and/or deceleration, causing the heart to impact against the sternum and vertebrae; (2) a compression between the sternum and vertebrae following a direct forceful blow to the chest; (3) a sudden increase in intrathoracic and intracardiac pressures, causing disruption of the myocardium or cardiac valves; (4) a "hydraulic ram effect," with compression of the abdomen forcibly displacing abdominal viscera against the heart with sudden great force; and (5) strenuous or prolonged cardiac massage, particularly if done through the intact chest wall.
TYPES OF INJURIES Blunt trauma to the heart can cause a wide spectrum of injuries, including (1) rupture of an outer chamber wall, with resulting death from tamponade or bleeding; (2) septal rupture; (3) valvular injuries, of which injury to the aortic valve is the most common; (4) direct myocardial injury (contusion); (5) laceration or thrombosis of coronary arteries; and (6) pericardial injury.
Some authors have stressed the differences between myocardial concussion and myocardial contusion. With myocardial concussion, there is no anatomic cellular injury, but there is some dysfunction, as demonstrated by abnormal wall motion studies. With contusion, there is an anatomic injury, as demonstrated either by elevated CPK-MB isoenzymes or by direct visualization at surgery of autopsy. Because these are difficult to distinguish, the term blunt myocardial injury (BMI) is often used (see below).
DIAGNOSTIC PROBLEMS Blunt cardiac trauma can be very difficult to detect at times. The victim may have experienced severe multiple-system trauma, and the presence of a cardiac injury may be overshadowed by other, more obvious injuries. In addition, the forces that produce blunt cardiac trauma may cause little or no external evidence of injury. Therefore, a history of moderate to severe chest or upper abdominal injury, even without abnormalities on physical examination, should make one suspect cardiac injury (Tabje. ., 2.5.1.-4).
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