Blast Injuries

Blast Physics

Type of Explosives

• Explosives can be ranked as either high-order (HE) or low-order (LE).¹
• HE explosives are more powerful than LE explosives and detonate to produce supersonic, overpressurized shock (blast) waves. The sources include trinitrotoluene (TNT), dynamite, nitroglycerin, ammonium nitrate fuel oil, Semtex, and many military-grade explosives.
• LE explosives, such as gunpowder, homemade bombs, and petroleum-based incendiary devices, deflagrate to produce subsonic explosions that do not generate overpressured shock waves, as observed in HE detonations.
• “Dirty bombs” are explosives mixed with nuclear and chemical agents. Biological materials are not commonly found in dirty bombs, as the heat from the explosion would denature the living organisms.²

Physics of Shock Waves

• HE explosions generate a supersonic wave known as the “dynamic overpressure” or “blast wave” (Figure 1). This wave travels at up to 8000m/s and causes shearing at air tissue interfaces.
• This is rapidly followed by a ‘blast wind’ of high-pressure compressed air travelling at up to 500m/s, which will push objects (environmental debris, bomb fragments, as well as people) away from the detonation.
• This wind may also be accompanied by thermal energy, depending on the cause of the blast. The expanding outward energy leaves a vacuum behind it, which then causes an under-pressure ‘release wind’ or negative impulse, which may suck objects back towards the epicenter of the explosion1 (Figure 2).
• The rapid expansion and movement of compressed air cause direct tissue injury and damage to structures and objects.² The collapse of structures may lead to crush trauma.
• Blast waves have properties predicted by the physics of waves. Therefore, blasts near hard surfaces or within confined spaces (such as railway carriages or in underground tunnels) become amplified two to nine times due to shock wave reflection. Individuals between the blast and a hard object may suffer two to three times the degree of injury compared to those in open spaces.

Blast Injury Classification

• The injury pattern sustained by the casualty depends on the type and size of the blast and the patient’s distance from the blast. In general, the energy of the blast is inversely proportional to distance from the blast, and casualties closer to the epicenter suffer more extensive injury. The surrounding environment (e.g., the presence of buildings or vehicles, or whether the detonation occurred in an enclosed space), and whether the patient was wearing protective equipment, are also factors.
• Blast injuries have traditionally been grouped into four types: primary, secondary, tertiary, and quaternary³ (Figure 3).

Primary Blast Injury

• Primary injuries occur when the blast waves act on the body, and organs with an air-tissue interface are particularly vulnerable.³ 
• The most common injury in this class is tympanic membrane (TM) rupture (reported in up to 47% of survivors), because the tympanic membrane has the lowest pressure threshold for injury.⁴
• The second most commonly injured organ is the lung. Primary blast lung injury (PBLI) is present in 0.6% of survivors.4 Incidence of 6-11% has been reported in military conflicts but increases to 90% in enclosed-space explosions.
• The disruption of the alveolar-capillary interface can lead to subcutaneous emphysema, pulmonary contusion, pneumothorax, pulmonary edema, hemothorax, pneumomediastinum, and acute respiratory distress syndrome (ARDS).³
• Gastrointestinal (GI) tract injury is relatively rare, with an incidence of 0.3% – 0.6% among survivors. Primary abdominal blast injury (PABI) includes rupture of the colon, mesenteric ischemia, or infarction. Two risk factors of GI injury are closed-space explosions and underwater blasts.⁴
• Other possible primary injuries include rupture, infarction, ischemia, and hemorrhage of the solid organs, rupture of the eye globes, ossicular fracture/dislocation, facial fractures, and traumatic brain injury.

Secondary Blast Injury

• These are caused by environmental debris (e.g., wood splinters) or material from the explosive device (e.g., ball bearings) being propelled at high velocity, causing penetrating injuries to any body region.⁴
• Amputation injuries may occur.
• Shrapnel may be thrown thousands of meters from the epicenter of the blast, causing a large blast injury radius.
• Although rare, Hepatitis B, Hepatitis C, and HIV transmissions caused by tissue fragments from suicide bombers have been reported.5 Similarly, wounds can be grossly contaminated and infected by environmental dirt being driven into tissue planes.
• Secondary blast injury is the commonest cause of blast injury; many improvised explosive devices (e.g., nail bombs) are specifically designed to inflict widespread secondary injury.

Tertiary Injury

• Tertiary injuries are caused by the blast wind, which throws people against objects, resulting in blunt or penetrating trauma.
• Structural collapse may also lead to crush and entrapment injury.²
• Any body part may be affected, e.g., compartment syndrome, fractures/amputations, or open or closed brain injury.
• Children are more likely to be injured due to their lower body mass.

Quaternary Injury

• Quaternary injuries are explosion-related conditions that do not fall under primary, secondary, or tertiary categories.
• Examples are chemical/thermal burns, asphyxiation by carbon monoxide or cyanide, and inhalation of toxic materials.²

Blast Injury Management

Triage Considerations

• In the event of a civilian mass casualty, consider the battlefield approach to triage. This highlights “the greatest good for the greatest number”. Providers should triage according to urgent, immediate, delayed, minimal, or expectant care.²
• Once the hospital emergency department receives alerts of blast injury, a triage area at the hospital entrance should be established with the capacity to perform decontamination procedures, if necessary.
• Decontamination of the patient, if required, is vital prior to hospital admission to prevent healthcare workers from becoming casualties.  Until patients are adequately decontaminated, healthcare workers should use appropriate personal protective equipment.
• The sickest patients are usually those closest to the blast.  However, patients from the outer radius of the blast may often present to a hospital in reverse triage order, with the ‘walking wounded’ self-presenting before emergency services can extract and transfer those most severely injured.
• Chemical, Biological, Radiological, Nuclear, and Enhanced explosive (CBRNE) incidents need specific management strategies and antidotes.

General Management Considerations

• Blast injuries can present with multi-organ polytrauma, and therefore, all blast injury victims need to be managed as per the principles of Advanced Trauma Life Support (ATLS), as the mechanisms that will kill the patient quickest remain the same.
• A primary survey should look for and treat immediately life-threatening injuries based on XABCDE (eXsanguinating hemorrhage, Airway with Cervical Spine management, Breathing, Circulation, Disability, Extremities and Environment)
• Adjuncts to the primary survey include early intravenous (IV) access and laboratory tests (including type and screen samples and ABG with lactate), radiography (particularly chest, pelvis, and extremities if injured), and extended Focused Assessment with Sonography in Trauma (eFAST).
• Once stabilized, a secondary survey should be completed. This involves a thorough head-to-toe physical exam to avoid missing injuries.¹
• Specific “trauma” computed tomography (CT) scans with IV contrast will help diagnose further injuries.  Magnetic resonance imaging in the acute setting is generally not recommended due to the risk of embedded foreign material/shrapnel.
• Special attention should be paid to the chest, abdomen, and brain. PBLI, abdominal injury, and traumatic brain injury often worsen within 1-2 days after the initial insult.6
• A tertiary survey, where the body is examined from head to toe, is usually performed within the first 48hours with the aim of ruling out occult injury.
• Patients should be transferred to appropriate facilities early if the healthcare facility lacks the capacity to manage the injuries.
• Suicide bomb victims may need to have empiric blood-borne virus prophylaxis and testing for human immunodeficiency virus and hepatitis B/C on admission and in the future.  Accelerated hepatitis B vaccination +/- hepatitis B Immunoglobulin may be needed.
• Penetrating injury and soft tissue injury from environmental debris often requires early broad-spectrum antimicrobials and antifungals, tetanus immunization, and extensive debridement.
• Whilst rare events, blasts can easily produce multiple casualties and overwhelm the medical system.  Simulation-based training scenarios help prepare the medical team for mass-casualty events.
• Fragments/clothing/tissues should be kept for future forensic investigations.

Organ-Specific Management

Eyes

• Routine ophthalmologic evaluation is mandatory.
• There should be a low threshold for ophthalmology consultation especially if there is decreased visual acuity.

Ears

• Routine otoscopy is mandatory.
• Although an otoscope exam is needed to evaluate TM injuries, rupture of the TMs is not an accurate marker for severe primary blast injury.⁴
• Treatment for most TM ruptures includes sterile irrigation, removal of debris, and prevention of further damage.¹ 
• ENT consultation is encouraged for management and future follow up.

PBLI

• Patients with significant PBLI are most likely symptomatic by the time they arrive in the hospital. Symptoms include dyspnea, hemoptysis, cough, chest pain, tachycardia, and tachypnea.⁷ 
• PBLI rarely occurs without overlapping with lung contusions and/or penetrating injuries.
• PBLI typically shows a butterfly pattern on chest X-ray.
• CT is more sensitive at detecting alveolar hemorrhage, lacerations, pneumatoceles, and pneumothoraxes.⁷  
• Patients who maintain spontaneous ventilation for 2 hours after PBLI are unlikely to need mechanical ventilation.⁷
• The ventilatory strategy for PBLI patients should follow lung-protective ventilation as used in ARDS management. High FiO₂ might be helpful to dissolve gas emboli (rare complication) If patients are able to tolerate permissive hypercapnia, high frequency oscillatory ventilation and airway pressure release ventilation may be considered.⁷
• Pneumothorax and bronchopleural fistulae are common.  Bilateral chest decompression with indwelling chest drains should be considered prior to positive pressure ventilation or air transport.
• Extracorporeal membrane oxygenation (ECMO) can be used for refractory hypoxia, but it carries the potential risks of ECMO, particularly pulmonary hemorrhage in the setting of systemic anticoagulation.¹

PABI

• There should be a high index of suspicion, as this is easily missed.
• Gas-filled structures (in particular colon) are at increased risk of injury.
• Bowel shearing, contusion, and perforation can occur and are often delayed in presentation.
• Mesenteric shear injury needs repeated clinical exam/eFAST and may be missed on the initial trauma CT scan.
• PABI is managed as per other abdominal trauma.¹

Primary Brain Blast Injury

• It may present with hematoma, contusion, and diffuse axonal injury.
• They should be managed as per other traumatic brain injuries.

Cardiovascular System

• Consider inotropic support and fluids if blast-related cardiac dysfunction is suspected – transthoracic or transesophageal echo may help to titrate therapy.
• If there is clinical concern for air emboli, the patient should be placed in the left lateral or Trendelenburg position. High FiO₂ and hyperbaric oxygen therapy may be beneficial.
• If the patient requires positive-pressure ventilation, peak airway pressures should be minimized to prevent further embolization.

Pregnancy and Fetus¹

• During pregnancy, direct blast injury to the fetus is uncommon as the fetus is protected by the amniotic fluid.
• The placenta and its attachment to the uterine wall are, however, at risk of injury. Therefore, women who are in the second and third trimester exposed to blast injuries should be admitted to Labor and Delivery for further evaluation (see OA summary on obstetric trauma for more details Link)