Mechanisms of injury
By convention, injury is classified into several categories —these are listed below.
Types of injury
• Nonpenetrating blunt
• Blast overpressure
• Other, including crush and barotrauma
In this chapter discussion centres on blunt, penetrating and blast injury only.
Impact between the body and an external object may result in tissue compression, stretching, tearing and other deformation ranging in severity from trivial to tissue injury beyond recoverable limits. The severity of damage is related to many factors, the most important of which are the amount of energy transferred and the nature and extent of the tissues over which it is applied.
In penetrating injury of low velocity and low available energy, tissue damage is focused over a small area, for example injuries caused by low-energy handguns, knives, sharp instruments, spikes of glass, wood or metal. Injury severity and outcome is related to the tissues involved. While low-velocity and low-energy injury to the soft tissue in the forearm will be slight, a similar injury involving the mediastinum might be lethal. In this case debates concerning velocity and energy transfer become academic. In high-velocity injury, associated
with the potential for high-energy transfer, damage to structures may extend over a wide area remote from the wound track (see later).
In blunt injuries mechanisms may be multiple and tissue damage of complex aetiology. Mechanisms are listed below.
Mechanisms of blunt injuries
• Stretch and shear
Victims of motor vehicle accidents may be injured either by rapid deceleration or by deformation with intrusion of vehicle components into the interior of the vehicle. In a crash, as deceleration occurs, the occupant’s body is thrown against the interior of the vehicle, often referred to as the ‘second collision’. There is also a ‘third collision’ between soft tissues and skeletal structures. All may contribute to injury, the extent of which will depend on the body region involved, degree of restraint and severity of the impact. Deformation and intrusion may result in blunt, penetrating or crush injury. Prolonged entrapment may exacerbate matters. Ejection of an occupant may occur, in which case rapid deceleration of the body occurs when it strikes the ground or another vehicle. Ejection is associated with increased likelihood of serious injury and death. When a pedestrian is struck by a moving vehicle there is often an acceleration injury in addition to the direct trauma at the sites of impact. In an adult, injury is commonly due to bumper (fender) impact to the limbs; in children, such an impact is over a wider area often involving the chest and abdomen, and is associated with multiple injuries and high mortality.
Patterns of injuries in road traffic accidents
Although the variety of injuries that may occur in a road traffic accident is vast, consistent patterns of injury are observed. The following are typical combinations:
• head, face and cervical spine injuries;
• cervical whiplash and sternal injuries;
• sternal fracture and dorsal spine injury;
• lower rib fractures with injury to kidneys, liver or spleen;
• intra-abdominal and diaphragmatic injuries;
• pelvic fracture with lower urinary tract injury;
• lower limb fracture with hip dislocation or spinal fracture.
While the wearing of automobile seat belts has undoubtedly saved many lives and has also reduced the incidence and severity of injuries to passengers, there are instances in which significant injury may be produced by their use. These lesions are secondary to restraint caused by the seat belt, whereby the occupant is forced by inertia against the straps as the vehicle rapidly decelerates. Injury may also be attributed to incorrect seat belt usage resulting in ‘submarining’ below the lap section of the belt on impact. While injuries to the head, face, lung, heart, aorta and liver/spleen are rarer since the introduction of seat belts, an increased incidence of injuries to stomach, duodenum, pancreas, small bowel and mesentery has been noted. Major intra-abdominal vessels may also be traumatised and fractures of the lumbar spine can occur.
There is considerable interest in the use of airbags for drivers and front seat passengers. Evidence of their effectiveness and safety is gradually emerging with increased use. While they have saved lives, deaths, particularly in front-seat child passengers, have been reported. These were related to inadvertent inflation in slow-speed impacts with inflation velocity sufficient in some cases to decapitate. More evidence is required.
Death following injury
Dr Donald Trunkey has pointed out that deaths following injury fall broadly into three groups giving a distinct trimodal pattern.
Immediate deaths (50 per cent) — those occurring immediately or within the first few minutes of injury and usually due to widespread damage to the brain or upper spinal cord, the heart or major vessels, or multiple injuries. This first peak is due to injuries which are generally lethal so that little can be done in their management that is likely to affect outcome. Reduction of this peak can only be achieved by preventive measures such as wearing of appropriate seat belts in automobiles, head protection on bicycles and motorcycles, road safety legislation and education for pedestrians.
• Early deaths (30 per cent) — those occurring within the first few hours after injury [called by some the ‘golden hour(s)’ of trauma]. These deaths are deemed preventable and are due to facial injuries with developing airwayobstruction, lethal disruption of the breathing mechanism, massive blood loss into body cavities or from multiple long bone fractures leading to collapse of the circulation, and dysfunction of the central nervous system due to space-occupying collections of blood within the skull. This became the basis of the ABCDE approach to initial assessment of the severely injured (see later).
• Late deaths (20 per cent) — those occurring days or weeks after injury, generally due to sepsis and multiple organ failure. Organ failure may involve the heart, kidney, liver, lung, brain and haemopoietic systems.
it is among those cases represented by the second and third peaks that potentially preventable deaths occur.
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