PBLI is a major cause of death in military conflict and terrorist attacks on civilian populations. However, the detailed damage process and mechanisms of PBLI are not well understood. To study the complex pathophysiological changes of PBLI, a standardized animal model must be established at first.
Chai et al developed a type of real explosive device, in which a compression explosive column was used as the source of the open-field explosion. By this device, they established blast injury model in rats. Wu et.al  observed pathological changes in the rabbits caused by the blast wave, which caused by detonator explosion. The real-time signal was recorded by a pressure sensor attached to the rabbit’s right chest. These modeling methods need to be carried out outdoors in a wide area and have strict requirements on the site. In addition, it is necessary to protect the animal's body, otherwise it is easy to cause complex injuries instead of PBLI. These types of test also need higher expenses, lots of manpower and material resources, and pathophysiological test is inconvenient to be performed at site [11, 12].
With the development of science and technology and the deepening of the research in this field, it was found that shock waves were generated mainly by detonating explosives outdoors and was the main factor causing blast injury. The researchers began to build the blast injury model by a single shock wave. The model induced by compressed air instead of explosive air to generate shock wave, without heat and debris generation, with advantages of single injury, good repeatability and safety, and easy data collection. This kind of model is widely used at present [9, 13, 14]. Vikas Mishra1 et.al developed a 9-inch square cross section, 6 meters long shock tube instrumented with pressure sensors. This shock tube could reproduce complex shock wave signature after a single shot was fired. However, this device has a large size which doesn't match the size of a normal laboratory. Hou et.al  self-designed a smaller shock tube stimulation device, which induced blast injury by using compressed air to form a shock wave produced by cracked an aluminum foil directed at a certain part of the mouse body.
Our device used in this study was modified from Hou’s  shock tube simulation device in two main ways. Firstly, added a metallic firing pin in the device, which can pierce the aluminum foil directly when the burst pressure reaches at preset value. The multi-layer aluminum foils were pierced by the computer-controlled pricking pin at preset value actively, instead of being cracked passively. This method will not be affected by the thickness of multilevel aluminum foils and is more accurate to reduce the error between the release pressure and the preset pressure. Secondly, mice were placed on a thick rubber plate with holes, allowing a specific body part of the mouse to be exposed to the shock wave, and the rest parts of body were isolated. Under the rubber plate, there is a pressure sensor that measure the real-time pressure record at the moment of blast. This can help us screen the experimental data that the actual burst pressure is consistent with the preset pressure. So, we have successfully developed a mini shock tube simulation device which could induce PBLI in mice and is very suitable for indoor laboratory environment. Compared with previous devices, it has advantages of higher accuracy and smarter controls.
In this study, our unique animal model of PBLI showed that 24-hour survival of mice depended on the magnitude of blast overpressure (from 0.5 bar to 2 bar). With the increase of the peak pressure of shock wave, the mortality rate of mice increased gradually. The research results showed that it could cause severe, sometimes fatal, blast-related acute lung injury when blast overpressure exceed 0.5 bar. Under the condition of 0.5 bar overpressure, all 10 mice survived and exhibited marked lung damage. The injured mice died within 24 hours after the blast and then the mice gradually recovered. Moreover, in the initial stage of blast lung injury, the mice would have symptoms of shortness of breath and even cardiac apnea. Some of the injured mice would bleed in the eyes, ears, mouth and nasal passages. Our findings are consistent with the characteristics of PBLI reported in previous studies and clinical observation .After simulated blast injury, diffuse small hemorrhagic spots appeared on the lung tissue of mice, especially obviously at 6 h after injury. Obvious pulmonary edema occurred in the model group, and the highest degree of the edema was found at 6 h as showed by increased lung coefficient and vascular permeability. Pathological examination of lung tissue showed the ruptured alveolar septal and the infiltration of inflammatory cells in the lung tissue of the injured mice. The recruitment of inflammatory cells in lung tissue indicates the occurrence of inflammatory response in lung tissue from the perspective of pathological injury. The degree of pathological injury of lung tissue reached the peak at 6 h, and then gradually decreased after 12 h. Blood cells also appear in the interstitial lung significantly 6 h after injury, suggesting the occurrence of intrapulmonary hemorrhage. All these results suggest symptoms of pneumothorax and internal bleeding at this time, which are closely consistent with those seen in clinical patients with PBLI and in laboratory animals in other studies. The reasons for these changes are that the volume of the mouse chest immediately decreased after suffering the blast shock wave and instantly rebounded when the external force was eliminated. During this process, the intrathoracic pressure rises initially and then falls sharply, resulting in the rupture of the alveoli and the pulmonary blood vessels. The permeability of alveoli-capillary membrane increased significantly, and a large number of proteins and cells exude through the blood-gas barrier [11, 16–18]. As seen in this study, shock waves induced by our self-made device could cause mice lung damage, which may lead to a variety of pathophysiological changes, including hemorrhage, edema, and inflammatory cells infiltration in the lung.
In this study, the expression of IL-1β, IL-6 and TNF-α in the mice lung tissues was significantly increased in the model group, and peaked within 2 and 6 h, respectively. The expression levels of inflammatory factors in the mice lung tissues were significantly changed only in the earlier acute phase of PBLI, and returned to normal at 24 h after injury. Combined with pathological observation and molecular level exploration, it was not difficult to find that the lung tissue, as the target organ of the shock wave, occurred obvious inflammatory response. The increase of inflammatory factors in this inflammatory response is positively correlated with the degree of recruitment of inflammatory cells. Inflammatory cells number and recruitment degree are closely related to the degree of lung tissue damage. Previous researches suggested that when lung tissue were damaged by shock waves, the damaged cells released a large number of cytokines, inducing various inflammatory factors [9, 19]. Previous studies have reported that PBLI involves an immediate autonomic response, followed by intrapulmonary hemorrhage and histopathological injury, and finally the production of cytokines associated with inflammation . Our findings are basically consistent with the previous results, except that the inflammatory response appears earlier, which is more consistent with the clinical manifestations of critical illness. It indicated that our PBLI animal model could be used in the diagnosis and treatment of PBLI and other related studies in the pre-hospital phase.
Clinical studies have shown that the initial signs of PBLI are cough, hemoptysis, and other pulmonary symptoms [20, 21]. Some of them have hemothorax and/or combined pneumothorax. The victim at the scene of the explosion often have various degrees of visceral injury without significant surface damage, a minority of patients developed rapidly progressive pneumonia leading to acute respiratory distress syndrome (ARDS). The main cause of death is shock, respiratory failure, septicemia, even ARDS. We wondered if there was relationships between hemorrhage, inflammation and other pathological processes in PBLI. How do various pathological processes develop? Are there a key link that connect the various pathologic processes and determine the patient's outcome?
In 2004, Brinkmann reported in The Journal Science that neutrophils produce an extracellular fibrous network of DNA, histones, and granulocins such as elastase when activated by cytokines or cytotoxins to capture bacteria . This network of extracellular fibers known as neutrophil extracellular traps (NETs), shows significant bactericidal activity. The formation of NETs starts with neutrophil activation followed by ROS production and calcium mobilization. Recently, the role of NETs in the occurrence and development of a variety of bacterial infections or non-infectious diseases has been observed. COVID-19’s impact on the respiratory system has gained much attention. Now, reports have been pouring in of the disease’s effects throughout the body, many of which are caused by clots . In COVID-19 pneumonia patients, NETs in plasma were found to be significantly higher than those in healthy . NETs play important roles in inflammation and coagulation involved in the development of in COVID-19 pneumonia. In addition, Hongji Zhang found that NETs mediated systemic changes in blood hypercoagulability in liver ischemia-reperfusion injury and leaded to distal organ injury by promoting microvascular immune thrombosis . The role of NETs in PBLI has not been observed in past studies. In our research, we found that NETs levels in the mice lung tissues peaked at 6 h after blast injury and recovered over time. That was consistent with inflammatory responses and expression of other injury indicators. These results suggested that NETs maybe involved in PBLI and contribute to the progression of this disease. NETs composition closely matched the trends of inflammatory factors, suggesting that NETs also mediates inflammatory responses in PBLI. However, the specific role of NETs in PBLI mechanism and weather NETs could be a key link between inflammation response and other pathological process in lung after PBLI need further study.