Saturday, February 09, 2008

Blast injuries (including neurologic complications)

Depalma RG, Burris DG, Champion HR et al. NEJM 2005; 352: 1335-1342.

1. Blast wave has a shock wave of high pressure followed by a blast wind of air motion. The physics are nonlinear and complex, and damage decreases exponentially with distance from the source. Inddors, reflections and reverberations produce additional unpredictable damage. Enhanced blast explosive devices outdoors can be more damaging. The pressure effects damages organs at air-fluid interfaces, and the wind propels fragments and people causing penetrating or blunt injuries.

2. There are 4 types of blast inuries: primary (effect of pressure), secondary (effect of projectiles), tertiary (effects of wind) and quaternary (burns, asphyxia and exposure to toxic inhalants).

Primary blast injuries-- barotrauma-- at air fluid interfaces are due to interaction of a high frequency stress wave and a lower frequency shear wave. Most important are rupture of the tympanic membranes, pulmonary damage and air emboli (including to brian and spinal cord) rupture of the hollow viscera. The eardrum can rupture with an increase of pressure as small as 5 psi above ATM (about 1/3 atm or 250 mm Hg). Deafness, tinnitus and vertigo results. Pressure gradients of 56-76 psi (3.9-5.2 atm) are needed to damage other organs. Occassionally people have other damage without eardrum damage. Lung injuries include bihilar butterfly pattern on CXR, PTX, hemothorax, pneumomediastinum, and and subcutaneous emphysema. Frothing at mouth (pulmonary edema) with whiteout CXR has a grave prognosis. Body armor protects against projectiles but not against barotrauma including pulmonary barotrauma.

Systemic gas emboli affecting the brain or spinal cord must be diferentiated from head trauma and concussion.

Colonic rupture or occassionally other organ rupture or infarction may be initially difficult to detect and are associated with proximity to blast center. Barotrauma to the eye can cause globe rupture, serous retinitis or hyphema.

Secondary blast injuries-- penetrating fragments are the leading cause of death and injury in attacks, except for major building collapse.

tertiary injuries-- includes crush injuries (buildings), leading to oliguric renal failure; compartment syndromes, characterized by pain out of proportion to the injury. In an earthquake in Japan (1995) mortality increased in patients trapped more than 24 hours due to tertiary injuries.

Quaternary-- includes everything else

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