As we learn about trauma medicine, the topic of blast injuries usually gets very little coverage. The thinking on this is that blast injuries are so severe that there isn’t much hope for that patient anyways, and that blast/explosion events don’t occur that often.
The trend lately has been a blend of hazards during active shooter events and some events are simply bombings themselves. The Boston Marathon bombing shows us that simple devices can cause a very high casualty count. Reaching back farther in time, the Oklahoma City bombing is another event that caused a very high casualty count. Blended hazard events are becoming the norm lately, with explosive hazards at the Aurora theater shooter’s house, pipe bombs being used and/or found in the aftermath of the San Bernardino attack and the Orlando club shooting. Columbine, which was the tipping point in mass shooting events, was originally supposed to be a bombing event.
How Does an Explosion Happen?
There are two types of explosions, detonation and deflagration. The difference between the two is the speed at which the combustion occurs. In detonation, the combustion event occurs at supersonic speeds. This pushes a pressure wave in front of the combustion. During deflagration, the speed is subsonic, and the damage occurs more from the burning of the material itself. If a material is typically known for deflagration, that’s not to say it cannot detonate. If deflagration occurs in a confined space and pressure builds up, when the pressure releases it can detonate.
What Are the Types of Blast Injuries?
Reaching back to EMT classes, there are four classes of blast injuries: primary, secondary, tertiary and quaternary.
Primary blast injuries are caused by the pressure wave from detonation. This pressure wave is extremely fast, but the intensity of the wave causes significant damage to the air filled organs inside the body. Why? Waves — think sound waves — travel faster through water than they do through air. As the pressure wave reaches the body (water) it will speed up. Traveling through the body it will encounter air filled organs where the pressure wave will slow down, and sometimes change direction — ever so slightly. The pressure wave reaches the other side of the organ and meets the tissue again where it will speed up. The changes in speeds through the body and the slight change in direction can tear organs apart. The organs that are most sensitive to this are the lungs and GI tract. Luckily, over distance the energy from the pressure wave dissipates very quickly.
Secondary blast injuries occur as the shrapnel and fragmentation from the blast strike the body. The fragmentation may have been part of the device or from objects around the explosion. In some cases, bone fragments from casualties near the explosion have caused secondary injuries to patients farther away. These injuries will present themselves just like other penetrating injuries from a GSW or shotgun. The thing to remember is that while the pressure wave will dissipate over distance and the energy will decrease, the energy in shrapnel is going to remain fairly constant. Energy was spent sending shrapnel flying, and there’s not much to slow it down until it reaches a casualty. The take home message is that you can find secondary injuries much farther away from a blast than primary injuries. Both can be significant.
Tertiary injuries are a result of the patient flying and striking other objects around them. This could be a wall or building behind them. Cars nearby or even the ground itself might also cause tertiary injuries. The presentation of these injuries is typically going to be blunt trauma related, but if a patient strikes something sharp it could cause impalement/penetrating injuries. Coup-contra coup, “paper bag effect” and other major blunt trauma style injuries should be considered. These blunt trauma injuries are a little harder to assess than penetrating injuries, as they may not be as visible.
Quaternary injuries are any long-term effects from the blast. If there are any CBRNE (chemical, biological, radiological and nuclear) components to the explosive, this is where these problems will present themselves. Typically, CBRNE exposure is not immediately life threatening when compared to other hazards like the blast wave or fragmentation. Hasty decon should be considered to ensure that there are not any potential hazards that the patient will have prolonged exposure to, or these contaminants make their way to the hospital. Simply removing clothing can decon up to 80 percent of contaminants. Even if there were no CBRNE hazards associated with the explosive device itself, those secondary injuries caused by bone fragments of other patients/casualties may have infectious disease risks. Psychological impact of the event is another quaternary injury that is often overlooked. These patients may endure a long road to recovery and returning to a normal life from before the event.
Since blast events can potentially cause a large number of casualties, we have to decide on some quick and easy methods to assess large groups of people quickly. The first tool will start with tympanic membrane rupture.
What Did You Say?
So there are a variety of studies that focus on whether or not TM (tympanic membrane) rupture is a good/poor indicator of other primary blast injuries. Most of the studies seem to conflict themselves on the presence of TM rupture as an indicator of blast lung injury. The force required to cause blast lung injury is around four times the force required to cause TM rupture. The prevailing idea in the emergency department is that just because a patient has TM rupture doesn’t necessarily mean they have pulmonary injury. In the field, we lack the diagnostic tools to quickly determine if a patient only has TM rupture (x-ray).
In this study, 193 patients experienced primary blast injuries. Isolated TM rupture was experienced by 142, and 51 had other forms of injury. Of these other forms, 18 had isolated pulmonary blast injuries. It was determined that isolated TM ruptures were safe to be discharged from the hospital after assessment for pulmonary injuries and a brief observational period. What does this mean in the field though? Of 193 patients that were confirmed with primary blast injuries, only 18 had isolated pulmonary injuries. If we utilize TM rupture and/or the presence of dyspnea, this should catch all primary blast injury patients. If a patient has TM rupture, they need to be evaluated at the hospital for potential complications regardless of their respiratory status. According to Wolf, et al., 71 percent of patients who were hospitalized from blast injuries had pulmonary injuries. It is critical that responders are on the lookout for patients with pulmonary injuries. Where will we find the critical patients though?
Location, Location, Location
The next tool is focused on the patient’s location relative to the blast. To start, the farther a patient is from the blast, the less likely they are to have primary blast injuries. To get super specific, according to Hopkinson’s Rule, the pressure from the wave decreases inversely proportional to the cube of the distance from the blast. So a patient at 10 feet will receive eight times the pressure of someone standing at 20 feet. So does that mean the patient closest to the explosion is the sickest or most hurt? Not necessarily. Other factors come into play such as the shape of the location of the blast and the relative position of the patient. Was the patient standing in the corner of a room where a device exploded in the center? The patient at the corner potentially took a hit from the pressure wave three times as it was reflected off one wall, against the second wall and back into the room. If there was anything that could have potentially shielded the patient from the blast, such as vehicles or other large objects, the patient may have avoided some of the pressure from the wave.
These are just a few small tools for trying to rapidly triage a large group of people after a blast event. There are always additional considerations to these scenes, and any mass casualty event will quickly overwhelm the resources of initial responders that arrive to the scene. Keeping sharp with blast injury education will help responders identify potential injuries, and utilizing some simple tools can help triage patients who need further detailed evaluation vs. patients who were lucky to escape unscathed.