Severe, transient pulmonary ventilation-perfusion mismatch in the lung after porcine high velocity projectile behind armor blunt trauma

Abstract

Background: Behind armor blunt trauma (BABT) is a non-penetrating injury caused by the rapid deformation of body armor, by a projectile, which may in extreme circumstances cause death. Although there is not a high incidence of high energy BABT, the understanding of the mechanisms is still low, in relation to what is needed for safety threshold levels. BABT is also useful as a model for blunt thoracic trauma, with a compressive speed between traffic accidents and blast caused by explosives. High velocity projectile BABT causes severe hypoxia. The mechanisms are not fully known. We investigated the acute pulmonary consequences in the individual lungs, and the effects of alveolar recruitment. Methods: 12 swine (mean weight 62.5 kg) were randomized to groups BABT by 7.62 Â 51 mm NATO-type bullets (mean velocity 803 m/s) to a military grade ceramic plate armor (n ¼ 7) or control (n ¼ 5). Modified double lumen tracheal tubes provided respiratory dynamics in the lungs separately/intermittently for two hours, with alveolar recruitment after one hour. Results: Venous admixture increased 5 min after BABT (p < .05) and correlated with increased cardiac output. Static compliance decreased 5 minutes after BABT (p < .05) and further by recruitment (p < .005). Physiological dead space decreased 5 minutes after BABT (p < .01) and further by recruitment (p < .01), while not in the contralateral lung. V 0 A/Q 0 decreased 5 minutes after BABT (p < .05), also shown in phase III volumetric capnography (p < .05). Most effects regressed after one hour. Conclusions: High velocity projectile BABT caused hypoxia by a severe and transient decrease in V 0 A/Q 0 to <1 and increased venous admixture in the exposed lung. Alveolar recruitment was hemodynamically and respiratory tolerable and increased V 0 A/Q 0 . Body armor development should aim at ameliorating severe pulmonary consequences from high projectile velocities which also needs to include further understanding of how primary and secondary effects are distributed between the lungs.

Pulmonary hypoxia and venous admixture correlate linearly to the kinetic energy from porcine high velocity projectile behind armor blunt trauma

“…To validate the BABT-simulator to comparable effects as real high projectile velocity BABT, we calibrated the speed and size of the projectiles to create a similar back-face signature (impression distance) of 24 mm, and maximum impression speed of the thoracic wall of 24-34 m/s, as high velocity projectile BABT from a 7.62 mm bullet impacting a ceramic plate armor at 800 m/s. [8][9][10] The back-face signature correlated linearly to the E k with a low R 2 =0.20. The low R 2 suggests that a back-face signature threshold alone may be inadequate when assessing lethality of high velocity BABT.…”

“…14 High velocity projectile BABT caused an immediate and severe hypoxia in swine which peaked at 5 minutes. 9 The hypoxia was related to a severe and transiently increased venous admixture (Q's/ Q't) in the exposed lung, 10 and we hypothesized that the gas exchange would deteriorate in relation to E k . Measuring the absorbed E k is methodologically challenging, and investigations of porcine chest trauma by bolt guns have not assessed graded injury.…”

“…In the animals selected for 7.62 mm BABT, a focally severe pulmonary contusion was induced, as previously described [8][9][10] by a gunshot perpendicular to a body armor panel fixed on the mid-lateral right thorax with two 3-cm wide girdles (Figure 3A-C). The armor protection consisted of a 300 × 255 mm ceramic ballistic plate (weight 3 Kg) and 14 additional 0.24 mm aramid layers placed behind the plate, (Åkers Krutbruk Protection Inc.).…”

“…3,[6][7][8] We have shown that high velocity projectile BABT in swine results in an immediate and severe hypoxia, 9 which is attributed to a severe and transient decrease in ventilation perfusion mismatch (V' A/Q') to <1 and increased venous admixture (Q's/Q't) in the exposed lung. 10 BABT injury mechanisms are complex and differ from other types of thoracic trauma, e.g. traffic accidents, and may include characteristics of primary blast injury.…”

“…The BABT-simulator was validated to inflict similar back-face signature and impression velocity, when fired directly at the naked chest as a high velocity projectile assault rifle 7.62 mm bullet impacting a ceramic plate armor placed on the thorax at 800 m/s. [8][9][10] Using the BABT-simulator, we investigated the relationship between impact parameters (E k ) and immediate and potentially life-threatening pulmonary consequences. Quantitative estimates of the effect of oxygenation and venous admixture (Q's/Q't) on gas exchange was measured in relation to the E k absorbed by the thorax.…”