Blast Physics
Modern close-combat drones in the Ukraine war are modified to carry a multitude of different explosive charges. This can range from a simple VOG or RGD-5 grenades (34g of A-IX-1 explosive, or 110g TNT, respectively), to “petal” and PMN-2 antipersonnel mines (37g VS-6D explosive, or 100g TG-40, respectively), to anti-tank mines (typically with several kg of high explosive).
High-Order explosives create supersonic blast waves (C-4, semtex, dynamite, ammonium nitrate, TNT, etc). These are commonly found in conventional military weapons.
Low-Order explosives create subsonic blast waves and less shear velocity (molotov cocktails, gunpowder, pipe bombs, etc). Damage may be severe, due to involvement of fragments, hot gases, and infectious agents. These are typically used in improvised, non-military-grade weapons.
Blasts produce two basic types of pressure waves:
Stress waves are supersonic, longitudinal pressure waves with high potential for injury, especially of gas-filled organs
Shear waves are lower velocity, longer duration transverse waves, which cause tissue in the body to move back and forth.
Understanding the Mechanisms of Injury in Blasts
Blast injury patterns are complex and unpredictable. Important variables include type of explosive device, patient proximity to the blast, soil conditions, and whether the event occurred in a closed space.
As a broad rule, casualties with traumatic amputations from conventional explosives were usually within ~1 meter of the device. These account for a large proportion of the immediate fatalities. Assume any patient with a limb amputation also has multi-system injuries.
Explosions causing shattering glass have a high incidence of penetrating eye injuries, which may initially be occult. Most eye injuries are preventable with simple eye protection.
In the case of ground explosions from mine and incoming artillery/missiles, soil conditions have a huge affect on the amount of blast energy that is delivered to the victim. During detonation, an expanding sphere of hot gas drives a shock wave ahead of it. Dry, loose sand allows this sphere and shockwave to dissipate downwards into the ground, as well as upwards. If the ground is frozen, hard, packed earth or saturated clay, energy is reflected upwards, amplifying the damage done to the victim.
Those with primary (blast wave) injuries are most often survivors of closed-space explosions. Primary blast injuries typically result from three different physical mechanisms: spallation, implosion, and shearing injuries. Spallation occurs when the pressure blast wave passes from a dense medium to a less dense medium (e.g., bowel wall into the gas-filled bowel). Implosion results from compression of tissue that is otherwise not typically compressible (e.g., solid organs). Shearing results from acceleration/deceleration, which displaces the tissue, causing tearing injuries.
Lung injuries occur at pressure increases over 40 psi. Pressure increases over 200psi in open air are almost universally fatal. Lung injuries are the most common cause of death due to primary blast effect. Signs and symptoms may be immediate, or delayed for up to 48 hours.
Ear injuries can occur from as little as 5-15psi of overpressure. Therefore these injuries may occur in open-air settings, or in patients that were not immediately adjacent to the device. Absence of tympanic rupture (symptoms are noticeable loss of hearing + some blood in ear canals) can help rule out other higher-pressure injuries. Hearing loss may be temporary or permanent.
The category of “Quinary Blast Injury” (clinical consequences of post-detonation environmental contaminants, including chemical (e.g., sarin), biological (e.g., anthrax), and radiological (e.g., dirty bombs) substances) has recently come into common use
Briefs on Common Blast Injuries
Lung Injury
“Blast lung” is a direct consequence of the HE over-pressurization wave. It is the most common
fatal primary blast injury among initial survivors. Signs of blast lung are usually present at the
time of initial evaluation, but they have been reported as late as 48 hours after the explosion.
Blast lung is characterized by the clinical triad of apnea, bradycardia, and hypotension.
Pulmonary injuries vary from scattered petechae to confluent hemorrhages. Blast lung should
be suspected for anyone with dyspnea, cough, hemoptysis, or chest pain following blast
exposure. Blast lung produces a characteristic “butterfly” pattern on chest X-ray.
Ear Injury
Primary blast injuries of the auditory system cause significant morbidity, but are easily
overlooked. Injury is dependent on the orientation of the ear to the blast. TM perforation
is the most common injury to the middle ear. Signs of ear injury are usually present at
time of initial evaluation and should be suspected for anyone presenting with hearing loss,
tinnitus, otalgia, vertigo, bleeding from the external canal, TM rupture, or mucopurulent
otorhea. All patients exposed to blast should have an otologic assessment and audiometry.
Abdominal Injury
Gas-containing sections of the GI tract are most vulnerable to primary blast effect. This
can cause immediate bowel perforation, hemorrhage (ranging from small petechiae to
large hematomas), mesenteric shear injuries, solid organ lacerations, and testicular
rupture. Blast abdominal injury should be suspected in anyone exposed to an explosion
with abdominal pain, nausea, vomiting, hematemesis, rectal pain, tenesmus, testicular
pain, unexplained hypovolemia, or any findings suggestive of an acute abdomen. Clinical
findings may be absent until the onset of complications (peritonitis, sepsis).
Brain Injury
Primary blast waves can cause concussions or mild traumatic brain injury (MTBI)
without a direct blow to the head. Consider the proximity of the victim to the blast
particularly when given complaints of headache, fatigue, poor concentration, lethargy,
depression, anxiety, insomnia, or other constitutional symptoms. The symptoms of
concussion and post traumatic stress disorder can be similar
US Marine Corps guide to Blast-Relared Injuries
American Journal of Emergency Medicine: Blast Injuries
*** Tables and selected text excepts above and below from: CDC Blast Injuries Fact Sheet
