Mechanisms of chest injury


Mechanisms of chest injury are usually divided into sub-sections as the pattern of injuries varies with the mechanism:

  1. 1.blunt (acceleration/decelaration)

  2. 2.crush

  3. 3.penetrating (stab, low velocity, medium velocity and high velocity)

  4. 4.blast

  5. 5.inhalation burns

  6. 6.foreign body aspiration

Many organs, such as the lung, are very elastic and will survive massive forces and function satisfactorily, whereas ribs will fracture. The lung well inflated is elastic and has a specific gravity of 0.4. High velocity missiles passing cleanly through the lung have produced surprisingly little damage in our experience.

Acceleration/deceleration injuries

Acceleration/deceleration injuries commonly occur as the result of a fall from a height, or from a road traffic accident at speed. A fall from 5 metres will result in an impact velocity of around 36 km per hour while from 20 metres the impact speed is approximately 72 km per hour. In sudden deceleration the intrathoracic organs will take on large apparent weights. In a fall from 10 metres with deceleration onto a hard surface, a cushion the apparent weight of the heart, which is normally 0.350 of a kg at rest, becomes 14 kgs. The heart can act like the striker in a bell causing widespread distruction to its attached vasculature and other intrathoracic contents. The G force of 40g associated with a fall of 10 metres can also be noted in deceleration injuries with a typically designed modern car striking a wall at 72 km per hour. If this same vehicle strikes a wall at 108 km per hour the deceleration forces produced are of the order of 90g.

While there is considerable dynamic tolerance within the human body, particularly in the chest, none-the-less, where there has been massive deformity of a motor vehicle or a history of a fall of the order of 5 metres or more major intrathoracic injuries should always be suspected. The physical nature of the chest wall means that it has considerable elastic recoil particularly in young subjects and it has a good "memory" for shape. Therefore the degree of injury within the thoracic cavity may often be judged initially by the deformity to any motor vehicle rather than the appearance of the patient. Also in cases of falls it is important that the height of free fall is accurately determined if we are to have the correct level of suspicion regarding intrathoracic injuries. Blunt injuries to the thoracic occur in three major directions:

(1) antero-posterior

(2) lateral

(3) transdiaphragmatic

Antero-posterior deformity as shown in Figure 1 results in relative backward motion of the heart. This can result in disruption of the continuity of the aorta at the level of the ligamentum just below the left subclavian artery (Figure 2), or the so called 'wishbone' fracture of the proximal bronchus (Figure 3), which is also believed to be caused by this backward and upward arc which the heart describes following AP displacement during deceleration injuries. Injuries to the heart itself are not uncommon and with deceleration injuries, with or without fractured sternum it has been estimated that there is myocardial injury in up to half of these patients. (4)

Intracardiac trauma may also result from such impact with destruction of the valvular mechanism of the heart.

Deceleration injuries with impact to the back of the thorax result in relatively few intrathoracic injuries; thus the rationale for reversing the direction of seats within our commercial aircraft. Lateral compression of the chest during deceleration injury results in fractures typically of the lower ribs and it is important to realise that here, as in all thoracic trauma, the occurence of other injuries elsewhere in the body must be considered. With fractures of 7th to 10th ribs laterally consideration should be given to splenic, renal and hepatic injuries. Where lateral compression results in so called 'flail segment' the damage to the thoracic cavity is usually relatively small and most frequently limited to contusion and laceration of lung parenchyma.

With the introduction of seat-belts there has been a dramatic reduction in the injuries sustained as a result of road traffic accidents particularly to the face and neck. There is, however, some evidence that with massive deceleration producing sudden rise in intra-abdominal pressure due to the lap belt, together with shearing and twisting of the upper trunk results in an increased incidence of rupture of the diaphragm. Massive transdiaphragmatic gradients will result typically in bursting tears of the diaphragm particularly on the left. There is propensity for failure to recognise such injuries in the early stages following trauma.

Penetrating injuries

Penetrating injuries of the lung result in parenchymal damage which has more to do with the permanent track of the missile or stabbing implement than with any relationship to the velocity of the bullet passing through the lung. The lung has a low density, great elasticity and has good properties of healing. However, more solid structures such as the major vessels and the heart will obviously suffer greater injury where high velocity missiles are the penetrating mechanism. Whether the penetrating mechanism is a stabbing implement, shrapnel or missile projectiles the most lethal complication of this mechanism of injury is haemorrhage and therefore rapid admission and surgery will give these patients their optimum chances for survival.

Crush injuries

Crush injury of the thorax can be defined as injury where the elastic limits of the chest and its contents have been exceeded. The crushing forces must be maintained for a period of time to result in injury. Usually patients who have suffered this injury have antero-posterior deformity. As with penetrating and many blunt injuries to the chest, crush injury patients are in shock when admitted and require rapid resuscitation with volume infusion. The majority of the patients will have flail chests with multiple rib fractures, pneumothorax or haemothorax. Most will have pulmonary contusion, while injuries of the heart, diaphragm and aorta are also common. As with patients suffering lateral blunt injury to the chest in deceleration trauma rupture of the liver, kidney and spleen are also relatively common. This mode of injury carries a high mortality. Another group of crush injury patients are those suffering the 'traumatic asphyxia syndrome'. In these patients constrictive forces are applied over a wide area for a period as short as 2-5 minutes. The mechanism of injury in this situation is profound venous hypertension associated with relative stasis. There is widespread capillary dilatation and rupture. Sub-conjunctival haemorrhage in this condition is virtually always present. Haemorrhage in the retina is almost always present also. Simultaneous injuries such as meningeal haemorrhage and cerebral haemorrhage must be suspected in patients who present in this way. Again this mode of injury carries a relatively high mortality rate.

Occasionally other mechanisms of lung injury are seen in trauma. Inhalation burns to the trachea are common with facial burns. The patient initially appears well and the initial chest X-ray looks normal. Oedema then leads to asphyxia or pneumonia. Respiratory burns are frequently missed with mortalities of around 50%. Further mechanisms of lung injury include the unexpected aspiration of teeth or fragments of glass, caustic inhalation and electrocution. However, these will not be discussed.