The mechanism by which the trauma occurs is an important factor in its severity. In cases of penetrating injuries the type of the weapon used and the history assumes a great importance in the occurrence of intrathoracic injuries. For example, major differences in injury will be observed with low- or high-energy bullets, and with the direction and distance of the gunfire. Similarly, stab wound injuries differ according to the type of knife used. The pathophysiology for blunt trauma is more complex and involves various forces. Chest trauma is frequently observed in motor vehicle accidents and falls. Speed, direction of impact, and the type of seatbelts and airbags present influence injuries by direct impact. Ejected and rollover victims are particularly at risk of deceleration injuries. In falls, the height, the site of primary impact, and the type of surface on which the patient lands modify the pattern of injury and the outcome. In high-speed impact or explosions, blast injuries of the lung may also be observed.

Previous significant medical history may effect the outcome. A history of smoking with chronic obstructive pulmonary disease or cardiac problems may contribute to respiratory failure (LpCicero and. Mattox...

From a pathophysiological point of view severe chest trauma leads to two major consequences, respiratory distress and shock. Respiratory distress

Several factors may contribute to acute respiratory distress and failure.

Pneumothoraces can occur secondary to penetrating injury, rib fractures, intra-alveolar overpressure (barotrauma), or bronchial disruption. A tension pneumothorax produces the risk of immediate respiratory insufficiency, with increasing intrapleural pressure shifting the mediastinum towards the non-injured side. This can occur with a mechanically ventilated patient, where a small well-tolerated pneumothorax in a spontaneously breathing patient may suddenly be converted to a large tension pneumothorax after positive airway pressure is instituted. If it is not relieved quickly, this condition may be fatal.

Multiple rib fractures and flail chest (at least three contiguous ribs, each fractured in two places) are commonly associated with visceral injury. Flail chest can lead to paradoxical ventilation, where the flail segment moves inwards during expiration and outwards during inspiration. This paradoxical movement modifies intrapleural pressures and generates inefficient ventilation by the pendelluft phenomenon (Moloney §L§L I960) where, with each inspiration, respiratory gases pass from the flail side (which appears to be collapsing) into the controlateral normal lung. During expiration the process reverses, with gas passing from the normal lung into the flail side (which appears to expand). The consequence of pendelluft is progressive respiratory distress with hypoxemia and hypercapnia. Treatment for this form of respiratory failure is based on systematic early mechanical ventilation (pneumatic internal stabilization) or, rarely, surgical stabilization of the chest wall.

However, the pendelluft phenomenon may not be responsible for respiratory failure in flail chest. Rather, it may be due to chest wall instability which increases the work of breathing and pain, compromising cough and ventilation and leading to atelectasis and pneumonitis. Thus not all patients require intubation but rather alleviation of pain, so that effective tidal volumes and vital capacities can be generated ( Bp||iger..and.yan E§d..e.n 1990; Cicala etal 1990).

Pulmonary contusion is common with blunt chest injury and is an important factor leading to respiratory failure. Pulmonary contusion occurs as a result of disruption of the alveolocapillary integrity and may also be seen with high-velocity bullet impact or blast injury. Initially focal, the pulmonary contusion may progress adversely, affecting gas exchange with a reduction of compliance and increased pulmonary shunting. Pulmonary contusion, and consequently hypoxia, can be made worse by shock and volume loading and may progress to acute respiratory distress syndrome (Fulton..and P.§teLl9.Z.3). The treatment of pulmonary contusion is supportive in that hypoxemia justifies mechanical ventilation with positive end-expiratory pressure.

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