Mechanisms of Injury

"Blast" injury refers to the biophysical events and consequent pathophysiological changes occurring as a result of the effects of detonation and generated shock waves (260). Explosive forces decline rapidly from the blast source (261,262). Chemical explosions involve condensed reactants, which are of either low-order (<16 psi or lb/in.2) such as fireworks, or high-order (e.g., dynamite [263,264]). Diffuse reactant explosions involve a mixture of gas and/or particulate material with air (e.g., silo and coal mine explosions [263,265]). An explosion is sparked by either an ignition source (e.g., match), friction, static electricity, or overheating (265,266). All manners of death are possible (267,268).

"Pure" blast injury of internal organs can occur without any signs of external trauma (260). The direct effects are caused by a shock wave (primary blast injury [260,261,265,269-271]). A fraction of this shock wave is absorbed and propogated as a pressure wave in the body (260,272). Pressure greater than 300 psi destroys a human body (263). From the perspective of wave propagation, body tissues are of high viscosity, and as the wave passes through different tissues and organs, it is altered (260).

The most vulnerable parts of the body are air- and gas-containing organs (260,261). Hollow structures are more susceptible because of the bursting effect by stress waves,

Fig. 41. Blast effect. Pleural surface of right lung; pulmonary hemorrhage.

which concentrate at the interface between media having large differences in density (260,261). This is enhanced when a pressure wave breaks out of a liquid medium into a gaseous environment (e.g., alveoli, GI tract; refs. 260,273, and 274). Lung injury occurs if the chest wall is depressed at a velocity of at least 20 m/s (46 mph) in less than 0.5 ms (260,261). Blast forces also generate shear waves, which grossly distort or overcome the normal elasticity of tissue and organs by differential motion of adjacent, connected structures (261). Solid organs are susceptible to this type of acceleration-deceleration mechanism (275). An underwater blast exerts its injurious effects over greater distances than an air blast (260).

Table 4 Blast Injuries

• External injuries

° Total body disintegration indicates high-order condensed explosive at close range (260,262,263,274)

° Mangling of body near explosion with parts of extremities amputated (262,274). Craniofacial injuries, if suicide (267). Lower limb amputations typical of standing or seated individual (Fig. 37; ref. 277). Hand injuries, if explosive device held (Fig. 38; refs. 267, 271, 278, and 279) ° Projectile injury (263,265,274; Fig. 39) ° Punctate lacerations (274)

° Dust tattooing (274). Black soiling from explosive materials (263) ° Injury from fallen rubble (274)

° Burns (flash burns and singed hair seen on victims in immediate vicinity; see Fig. 40 and ref. 274)

° Propulsion of victim into other structures leading to secondary injuries (271)

• CNS hemorrhages (meninges, brain, nerve roots [260])

• Retinal hemorrhage (271)

• Eardrum rupture (a primary blast effect [260,271,272,275,276,280,281])

• Neck laceration and laryngeal fractures (265)

• Lung (primary blast effect; see Fig. 41 and ref. 281)

° Lacerations, hemorrhage (unless respiratory passages are obstructed by blood leading to asphyxia) does not necessarily cause death, survivors can progress to adult respiratory distress syndrome (see Chapter 1, Subheading 4.3. and refs. 260,261,265, 271,276, and 280) ° Pleural and subpleural hemorrhages (occur at areas of stress concentration—posterior surface of lungs, adjacent to heart, and diaphragm [261 ]) ° Acute emphysema, subpleural bullae, tension pneumothorax (261) ° Air embolism (see Chapter 5, Subheading 14.3.; Chapter 7, Subheading 12.1.;Chapter 8, Subheading 12.4.; and ref. 261).

• Cardiac dysrhythmia (282)

° Bradycardia (vagal reflex from pulmonary injury [260,283]) ° Commotio cordis (possible when blast victims have no obvious fatal injury; see Chapter 8, Subheading 6.4. and refs. 260 and 261)

• Myocardial hemorrhage (260)

• Cardiac laceration (275)

• Aortic laceration (265)

• Hepatic laceration (265,275,276)

• Esophageal, gastric or intestinal perforation (also intramural hemorrhage, transection of fixed parts [260,265,271,276,280])

• Gallbladder rupture (275)

• Kidney laceration (265,275)

• Urinary bladder rupture (271,275)

• Splenic laceration (265,275)

• Adrenal laceration (275)

Upper and lower extremity fractures, spinal fractures (260,265,271)

CNS, central nervous system.

Other mechanisms of injury include penetrating trauma by secondary missiles that can travel great distances (secondary blast injury), displacement of victim by blast wind (tertiary blast injury), burns from flash or structural fires, exposure to noxious gases, and crush (260,261,263,265-272,276). If there is a fire, the leading edge of the flame travels up to 200 m/s (450 mph) following the blast (266).

Explosions are associated with characteristic injury patterns (Table 4).

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