Physiological effects of Behind Armour Blunt Trauma

This report is addressing Behind Armour Blunt Trauma (BABT), defined as the non-penetrating injury resulting from a ballistic impact on personal body armour. The protective vest may impede the projectile, but some of the kinetic energy is transferred to the body, causing effects such as pulmonary contusion, apnea, hypotension and occasionally death. Our aims of these studies have been to investigate physiological responses after high-velocity BABT, including EEG (study I). Furthermore, the safety criterion of "44 mm" for protective vests (study II), regulation and effects of apnea (study III), fluid resuscitation (study IV), effects of Trauma Attenuating Backings (study V) has been evaluated. Other findings include some previously unknown physiological responses after BABT, which has given us better understanding of body reactions after severe injuries. Anaesthetized pigs, wearing body armour on the right side of thorax, were shot with a standard 7.62 mm assault rifle (velocity approx. 800 m/s). We used body armours that allowed 19-42 mm impression in clay placed behind the vest. Different armour panels were used in the studies to address specific concerns. Several physiological parameters were thereafter monitored during two hours after the shot. Experimental protocol was similar in all studies, except from study III (in which one group received bilateral cervical vagotomy) and study IV, in which 2 groups received Ringer´s acetate (RA) or hypertonic saline with dextrane (HSD). In all studies we observed an immediate drop of blood pressure, desaturation of blood, increased pressure in the lung circulation, suppressed EEG-pattern and pulmonary contusion. Our observed EEG-changes indicate that high-velocity BABT induces brain dysfunction, for at least a couple of minutes, but in some cases during hours or until the animal died. Based on our results, the safety criteria of 44 mm should be considered insufficient when a vest is exposed to high-velocity bullets. Our results show that apnea after BABT is a vagally mediated reflex, that can be inhibited by vagotomy. Fluid resuscitation has limited effects on physiological parameters in our model, although HSD induces less oedema formation and a tendency to improved saturation compared to RA. In study II and IV, severe hyperkalemia was seen early after the trauma and several animals had serious arrhythmias. Haemoglobin was also elevated in seriously injured animals, a finding that was probably a defence mechanism in a lifethreatening situation. Animals that suffered from prolonged desaturation preceding death often showed dense cardiac tissue, a finding that would be interesting to evaluate in future studies.