Trauma epidemiology

The most common cause of death from birth to the fourth decade, and the fourth most common cause overall, is trauma. For individuals between 15 and 24 years, trauma leads to three times as many deaths as any other cause. On average, for individuals of working age, heart disease and cancer result in the loss of 10 years of potential life, but road traffic accidents (RTAs) alone cause the loss of 30–35 years. The economic cost is staggering. Patients with trauma occupy 10% of hospital beds and, globally, trauma accounts for 1–2% of gross national product.

However, experience of trauma cannot be extrapolated from other countries to predict events or outcomes. For example, in the UK, RTAs, falls and interpersonal violence account for the majority of major trauma (Fig. 7.1). Fewer than 1 in 10 patients with major trauma have penetrating injury, and this is usually caused by knives. In the USA, approximately 20% of the population owns a gun, and RTA deaths are matched by firearm injuries. Staggeringly, having a gun in the home increases the risk of homicide three-fold, and that of suicide five-fold; and for 15–24-year-olds, these figures increase by a factor of 10. A recent UK study found that the total number of homicides over a 2-year period for a population of 0.8 million was similar to that seen in a single day among the same-sized population of many American cities. In so-called developed countries, annually, 1 person in 50 will be involved in an RTA. Of these, 1% will die, 10% will need hospital treatment and 25% will be temporarily disabled.

Fig. 7.1 Relative proportions of causes of trauma deaths.

(Data kindly provided by the Scottish Trauma Audit Group 1992–1997.)

It is often quoted that trauma deaths have a trimodal distribution. The first ‘peak’, representing deaths occurring immediately after or within a few seconds of injury, contributes up to 50% of the total. The second ‘peak’, up to 4 hours after injury, accounts for 30% of deaths, and the final 20% take place (usually in an intensive care unit) days or weeks after the event. Much significance has been placed upon this alleged temporal relationship, particularly the second peak. On the basis that interventions for the second group of patients offered great potential for preventing unnecessary deaths, the provision and nature of pre-hospital and hospital trauma services in the USA and the UK were changed profoundly. Unfortunately, the ‘second peak’ is a myth, at least in the UK, where the vast majority of deaths occur immediately after or within a very few minutes of injury. Furthermore, the subsequent deaths do not cluster into peaks. So, although attempts to improve care for those who initially survive must continue, the overwhelming message is that trauma prevention is far more important than any other aspect.

Trauma is a common cause of death and morbidity in children. After the first year of life, it is the most common cause of death in the paediatric population. The most common causes of serious injury seen in children are RTAs and falls. Non-accidental injury accounts for a significant number of the remainder.

The pattern of injury seen in children differs from that in adults. The small mass of the child is less able to disperse the kinetic energy of impact; as a consequence, multi-system injury is more common. The large paediatric head (in proportion to the rest of the body) means that head injuries are common. The larger body surface area to body mass found in infants and children results in increased heat loss after injury. There are often significant psychological sequelae to major trauma in children. It has been estimated that as many as 60% of such children are left with behavioural or learning difficulties after a serious accident.

Injury biomechanics and accident prevention

To anticipate the injuries from any given trauma event, the clinician must understand the biomechanics involved. An accurate history can identify or predict the great majority of an individual patient’s injuries.

The magnitude of injury is related to the energy transferred to the victim during the event, the volume/area of tissue involved, and the time taken for the interaction. Tissue characteristics, such as elasticity, plasticity and fluid content, are also important. These factors are summarized in the formula:

where E = energy transfer, T = time, V = volume of tissue, and C = tissue factors (a constant).

Kinetic energy, the energy of motion, is proportional to the mass of the object but to the square of its velocity. This can have unexpected effects. For example, a pedestrian struck by a car of mass 700 kg travelling at 100 km/h receives over three times more destructive energy than if hit by a heavy lorry of mass 5000 kg travelling at 40 km/h. If the car travels at 160 km/h, over 10 times the energy is involved. The longer the time frame during which the kinetic energy is transferred to the body, the less the acceleration/deceleration force sustained and the less the trauma that results.

These physical principles underpin strategies of accident prevention and protection. Obviously, reducing the chance of direct contact helps; separating pedestrians and traffic is the single most important factor in reducing pedestrian injury rates. This is illustrated by the fact that, in the US, less than 2% of traffic fatalities are pedestrians, whereas in the UK they account for 36% of the total. In a similar fashion, the central reservation barriers on motorways dramatically reduce the chances of high-speed head-on collisions (Fig. 7.2).

Fig. 7.2 Car involved in a high-speed, head-on collision, showing the magnitude of structural damage.

If impact does occur, then limitation of the velocities involved is the most important determinant in reducing injury. One in 10 drivers involved in RTAs has been travelling inappropriately fast. Even a 1 mph (1.6 km/h) reduction in average road speed reduces fatal accidents by 8%. The 20 mph (32 km/h) zones in residential areas, together with traffic calming measures, significantly reduce deaths and serious injuries, in particular to children and the elderly.

Contact factors can be minimized by vehicle design: crumple zones, energy-absorbing materials, preventing the ejection of passengers from the vehicle and reducing intrusion into the passenger compartment. For the occupants, seatbelts, airbags, collapsible steering columns and soft fascia compartments enable contact deceleration to take place over a longer time period, reducing the potential for injury (Fig. 7.3). Properly used, seatbelts reduce the risk of death/serious injury by 45%. Airbags further reduce the risk of death by 10% for belted drivers, and by 20% for unbelted front-seat passengers, but may not provide protection from side-impact events, or if the vehicle rolls over.

Fig. 7.3 Impact sites in an unrestrained head-on collision.

These devices can also modify the patterns of injury, particularly if they are incorrectly positioned. Seatbelts and airbags do reduce deaths overall but certain injuries, e.g. sternal fractures and soft-tissue neck injuries, may be associated with their use. If lap belts alone are used, pancreatic, renal, splenic and liver injuries are relatively more common and hyperflexion of the trunk over the belt can produce anterior compression fractures of the vertebrae. Finally, seatbelts are only protective when used. A recent study showed that 90% of all injured rear-seat passengers were unrestrained. These passengers increase the severity of their own injuries, as well as causing injury to restrained individuals in the front seats.

Alcohol and drugs

The message is often unwelcome, but few episodes of trauma are without direct human failing or causation. There is, for instance, a four-fold increase in the risk of being involved in an RTA while using a mobile telephone, even if hands free, a level similar to that seen when driving with a blood alcohol level at the UK legal limit (blood: 80 mg alcohol/ 100 ml of blood; breath: 3g mcg/100 ml breath).

The combination of youthful over-confidence, incompletely developed motor skills, and ready availability of high performance vehicles accounts for an extraordinarily high rate of accidents. These rates decline with age and experience, but at the other end of the spectrum, the elderly have a disproportionately high incidence of trauma because of co-existing medical conditions and visual/motor impairments that affect judgement.

At all ages, alcohol is the major causal factor for all types of trauma; 60% of individuals sustaining trauma in assaults have consumed alcohol. For burns, homicides and drowning, alcohol is implicated in 30–50% of events. Its combination with young males and road vehicles is particularly lethal; in this group, one-third of all fatalities, and 10% of all injuries, involve alcohol consumption. Drink-driving laws do reduce the proportion of fatal crashes involving intoxicated drivers, but high-risk behaviour remains common. Although death rates from alcohol-related events have fallen, the risk of being involved in an accident with a blood alcohol at the current UK driving limit is twice that for an individual with no alcohol in their blood. At higher levels, the risk dramatically increases even further. About 20% of RTA deaths are thought to be related to drug or substance misuse, but the difficulties of testing and the involvement of prescribed medications such as sedatives may mean that this is a considerable underestimate.

Wounds

Gunshot wounds

Gunshot wound data (Fig. 7.4) highlight the gulf between UK and US practice. In the US, deaths from gunshot wounds are the fourth leading cause of years of potential life lost before the age of 65. Guns are used in over 60% of suicides and 70% of all homicides. Non-fatal gunshot wounds outnumber fatal ones two- to three-fold.

Fig. 7.4 Chest X-ray showing a bullet to the right of the upper mediastinum.

(Courtesy of Miss Kate Wilson.)

As with other injury, the exchange of energy is crucial. Low-velocity missiles cause local injury, involving tissue tearing and compression. When velocities exceed 500–600 m/s, cavitation injury – a temporary space torn in tissues at right angles to the direction of travel – is also produced. This process develops in microseconds and, depending upon the body tissues involved and their elasticity, can involve a volume many times the diameter of the bullet itself. The wounding potential can be further magnified by features specifically designed to increase the area of injury and the release of energy; examples include bullets that tumble in tissues and others designed to deform or fragment on impact (dum-dum or semi-jacketed bullets).

Shotgun events are relatively more common in the UK than handgun or rifle injuries (http://www.crimeandjustice. org.uk/). The muzzle velocity of these weapons is relatively high, but dissipation of the shot and air resistance on the pellets quickly decrease their velocity and limit the wounding potential (Fig. 7.5). These weapons are lethal at close range but, unless ‘choked’, are relatively less wounding at greater distances, where they tend to cause superficial injury to skin and subcutaneous tissues.

Fig. 7.5 X-ray of the lower chest and upper abdomen, showing pellets from a shotgun injury.

Note the wide dissipation of the pellets.

Falls

The major determinant of injury and the chance of death is directly proportional to the height fallen, as the accelerating force of gravity is constant. For practical purposes, the velocity at impact (v) is given by the equation:

where g = gravitational constant (9.8 m/s) and h = height fallen in meters

Thus a body falling two storeys (10 m) has an impact velocity of around 50 kph. At impact, the deceleration forces are determined by the individual’s mass, the nature of the landing surface and the body’s orientation on landing. Surfaces such as mud, snow, soft earth and, to a lesser extent, water can permit an increased duration of impact, reducing deceleration forces and hence injury. For an ‘average’ man, a 5 m fall on to a concrete surface produces a deceleration force of approximately 700 g; if the landing is a soft, yielding surface, the stopping distance may be several centimetres, decreasing the force 10–20-fold.

The body’s position during landing affects the contact area and the propagation of energy since, if the same force is dissipated over a larger area, there is less force per unit area and hence less damage (Figs 7.6 and 7.7). Feet-first falls involve a relatively small area of contact, but deceleration forces can be reduced by flexing the knees and hips. Regardless of the position on landing, however, for falls > 5 m there is a high incidence of deceleration injuries to intrathoracic and intra-abdominal structures, particularly where these are relatively immobile or tethered: for example, the aortic root and the mesenteric arteries. Overall, falls on to an unyielding surface from 15–20 m are associated with a greater than 50% mortality. Nevertheless, bizarre descriptions of survival from high falls do exist. There are well documented accounts of individuals surviving after falling from aircraft onto trees and deep snow. Clothing may also play a role in slowing the rate of fall: in 1885, Miss Sarah Ann Henley survived after a fall from the Clifton suspension bridge (∼︀75 m) as her hooped crinolines acted as a parachute and the tide was out so that the landing surface was thick mud.

Fig. 7.6 Lateral view of ankle and grossly comminuted fracture of calcaneum, involving the subtalar joint.

The mechanism of injury was a fall on to the feet from 15 metres.

Fig. 7.7 Compression (wedge) fracture of the L2 vertebral body (arrow), caused by a fall from height.

Injury severity assessment

Audit of trauma patients, both individually and as a group, is essential. To allow objective comparison between systems or hospitals, injury classifications have become standardized.

Two types of classification are used. The first measures the severity of anatomical injury. The most commonly used system is the Abbreviated Injury Scale (AIS). Once the patient’s injuries have all been identified (this may only be possible at discharge or autopsy), each separate injury is assessed from a scoring ‘dictionary’ and awarded a numerical score. The Injury Severity Score (ISS) is then derived from the three highest AIS scores within six body areas (head and neck, abdomen and pelvic contents, bony pelvis and limbs, face, chest and body surface). ISS provides an internationally recognized objective evaluation of anatomical injury.

The second type of classification is physiological. The best-known physiological scoring system is the Glasgow Coma Scale (Table 7.1), which is used to assess the neurological state of injured patients objectively, and which also has prognostic value. The Glasgow Coma score (GCS), in conjunction with two other physiological recordings, systolic blood pressure and respiratory rate, can be used to produce the ‘Revised Trauma Score’. Although widely used, physiological scoring systems have intrinsic problems. Some patients with severe injury may not be identified initially, usually because the assessment has been performed before detectable physiological compromise has had time to occur. The system may also overestimate injury severity if physiological changes occur (due, for example, to alcohol) that are not reflected in the measured parameters, or which modify these factors. In addition, no allowance is made for factors such as comorbid features e.g. underlying cardiac or pulmonary disease or medications.

Table 7.1 Glasgow Coma Scale