MECHANISMS OF INJURY AND INJURY PATTERNS IN EXPLOSIONS

Explosions produce specific injury patterns and the potential to cause life-threatening multisystem or multidimensional injuries. These patterns are the result of the composition and type of bomb, the delivery method, the distance between the victim and the blast, whether the blast occurred in a closed or open space² and surrounding environmental barriers or hazards. Blast injury is a general term that refers to the biophysical and pathophysiological events and the clinical syndromes that occur when a living body is exposed to blast of any origin. Explosions are caused by a rapid chemical conversion of a solid or liquid into a gas with resultant energy release. Explosives are either high or low order. High-order explosives are designed to detonate quickly, generate heat and loud noise, fill the space with high-pressure gases in 1/1000th second, and produce a supersonic overpressurization shock (the increased pressure above normal atmospheric pressure from a blast) that expands from the point of detonation outward in a pressure pulse. The level of overpressure depends on the following: (1) the energy of the explosion, (2) the distance from the point of detonation, (3) the elapsed time since the explosion, and (4) the measurement technique. Blast strength is described as the ratio of overpressure to ambient pressure. The blast wave (positive wave) moves in all directions away form the explosion, exerting pressures of up to 700 tons (Figure 1). Shock waves possess the quality of brisance (shattering effect). The displaced air then compresses and forms a vacuum returning to the point of detonation (negative wave). The negative phase is not considered to result in blast injury. High-order explosives include TNT, C-4, Semtex, nitroglycerin, dynamite, and ammonium nitrate fuel oil. Low-order explosives produce a subsonic explosion without overpressurization wave. Energy is released slowly and burns by a process of deflagration. Low-order explosives include pipe bombs, gun powder, Molotov cocktails, and pure petroleum-based bombs. Explosives have several effects: the blast pressure wave as described previously, the fragmentation effect, the blast wind, the incendiary thermal effect, secondary blast pressure effects, and ground and water shocks for explosions that occur below ground or water.⁹

Figure 1 Propagation of blast wave over time.

(Data from Jensen JH, Bonding P: Experimental pressure induced rupture of the tympanic membrane in man. Acta Otolaryngol 113:62–67, 1993.)

Fragmentation effect occurs from projectiles included in the container, projectiles produced from the destruction of the container, and from objects surrounding the detonator and target. These projectiles can travel up to 2700 feet per second. The blast wind is created by the motion of air molecules responding to pressure differentials generated by the blast. These winds may be as high as those seen in hurricanes but are not sustained. The incendiary thermal effect is different for high- and low-order explosives. High-order explosives produce higher temperatures for shorter periods of time, usually resulting in a fireball at the time of the detonation. Low-order explosives have a longer thermal effect and cause secondary fires. Secondary blast pressure effects are caused by the blast wave’s reflection off surfaces prolonging and magnifying the effect, particularly in enclosed spaces. Greater transfer of energy to the body occurs. Underground and underwater explosions propagate the shock waves farther and with more force than air.¹⁰

Bombs are weapons and defined as any container filled with explosive material whose explosion is triggered by a clock or other timing device. Terrorist bombs, IEDs, are usually custom-made, may use a number of designs or explosives, and are of two types: conventional (filled with chemical explosives containing the compounds of hydrogen, oxygen, nitrogen, and carbon) or dispersives (filled withchemical or other projectiles such as nails, steel pellets, screws, and nuts) designed to disperse. Nuclear devices rely on nuclear fission or fusion. Explosions can produce unique patterns of injury. They have the potential to inflict multisystem life-threatening injuries on many persons simultaneously. The injury patterns following such events are a product of the composition and amount of the materials involved, the surrounding environment, and the delivery method, the distance between the victim the blast, and any intervening protective barriers or environmental hazards. Because explosions are relatively infrequent, blast-related injuries can present unique triage, diagnostic, and management challenges to providers or emergency care personnel.¹¹

BLAST INJURY: CLINICAL ASPECTS

Blast-related injuries are now very common and have become a threat for populations all over the world. Powerful explosions that result in different tissue and air interactions have the potential to inflict complex and unique injuries. Severely injured patients from explosions often sustain combined blunt, penetrating, and burn injuries.¹² Therefore, knowledge of the mechanisms of blast effect and early recognition of the potential injuries are of paramount importance in the management of blast-injured patients. The injury patterns vary depending on setting (open or closed space), amount of explosive, and chemical properties of the explosive.¹³ Explosions in confined spaces are significantly more deadly and associated with high incidence of primary blast injuries than those in open air. Improvised explosive devices are accompanied by heavy shrapnel that result in various injury patterns involving more body regions and increase severity. This makes management of blast injuries more challenging.⁴ According to their underlying mechanisms, blast injury is classified into four categories: primary, secondary, tertiary, and quaternary (Table 1).¹⁴ Quinary blast injury also has recently been described.

Table 1 Categories of Blast Injury