preparation of a shocktube for modeling highly repetitive low-level blasts

Shocktube Method to Model Highly Repetitive Low-Level Blast in Mice

Low-level blast (LLB) exposure occurs when individuals or structures experience relatively low magnitudes of explosive force, typically arising from small industrial accidents, controlled demolitions, or certain military training activities. In contrast, high-level blast (HLB) exposure entails exposure to intense and potentially destructive magnitudes of explosive force, commonly encountered in military combat, terrorist attacks, or large-scale accidental explosions. The primary distinction between LLB and HLB therefore lies in the intensity of the explosive events and, by extension, the ability of exposed persons to tolerate repeated exposures before experiencing physical or functional injury. In this regard, the effects of HLB exposure tend to be more obvious than the effects of LLB exposure. Because of this, persons with significant LLB exposure may be at increased risk for slowly developing injuries or deficits that go undetected until their cumulative effects become discernable.
Ongoing research aims to enhance our understanding of how the properties of blast exposure, such as intensity or repetition, may cause injury so that we can better guide prevention and medical management. In military medicine, understanding the clinical implications of blast exposure is of paramount importance, and as a result, animal models capable of informing those outcomes are needed. Although animal models have helped elucidate the effects of HLB, the effects of LLB exposures remain largely understudied. Numerous modeling studies examine the effects of blast overpressures near or above 10 pounds per square inch (psi) peak pressure1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18, but few reports focus on pressure levels ranging from 1 to 7 psi19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36, which are more common in military training environments37,38,39,40 and fall near the historical threshold of 4 psi for safe environmental exposure. Thus, broader dissemination of methods for the study of frequently used peak pressures of LLB may help catalyze rapid clinical insights for application to military medicine and force optimization.
A significant association between the occupational risk of LLB and diverse clinical diagnoses is emerging from epidemiological investigations of military LLB41,42,43,44. These studies support a poorly defined dose-dependent relationship, with repetitive LLB exposures demonstrating heightened risks41. This suggests that increasing cumulative blast exposure plays a crucial role in shaping clinical outcomes in military settings.
Previous animal modeling studies of LLB under 10 psi have primarily used explosives or shocktube systems to investigate the effects of exposure. Although these models typically examine the effects of one to three exposures, they have nonetheless contributed to a growing understanding of the mechanistic19,20,30,31, neuropathological29,31,33, and behavioral consequences19,20,23,25,32,34, associated with low-intensity blast exposures that are typical of the military training environment.
Studies examining single LLBs generated by open-field explosives have reported evidence of subtle brain pathologies and behavioral changes frequently associated with posttraumatic stress. Woods and colleagues24 were unable to detect microscopic brain injury at 2.5-5.5 psi, but they did detect quantitative changes in brain tissue glycosphingolipids by mass spectrometry. Using the same peak pressures and experimental design, Rubovitch and colleagues25 observed behavioral changes following blasts that occurred with a similar lack of brain pathology when measured by light microscopy. However, in subsequent pathological investigation, unambiguous ultrastructural damage to brain myelin, mitochondria, neurons, and neurovasculature was identified by electron microscopy29,30,31,32,33 in 6.7 psi LLB-exposed mice. Interestingly, several LLB studies using open-field explosives with pressures of ~10 psi and less report approximately 3-8% mortality after a single exposure25,36.
Similar results have been previously noted by several studies using laboratory shocktubes. In studies examining single LLBs produced by shocktubes, evidence has been found of neural cytoskeletal injury and changes in neuronal firing patterns developed after exposure to a single 1.7 psi blast22. At 4 psi, corpus callosum dysfunction was reported to accompany neurobehavioral deficits in LLB-exposed rats23. Compared to the blast duration measured in air, Chavko and colleagues27 found that the positive phase duration of the blast overpressure was significantly longer in the brains of rats exposed to 5.8 psi. Biosignatures of similar injury responses may be supported by a study in mice following 7.5 psi exposure in which Ahmed and colleagues35 report detectable changes in serum levels of specific inflammatory, metabolic, vascular, and neural injury proteins up to a month after exposure. Interestingly, this study also reported 4.5% mortality at 24 h following exposure.
In studies examining three shocktube LLBs over a single 20 min exposure session, LLBs between 1.4 and 8.7 psi caused psi-dependent increases in intracranial pressure (ICP) in rats, with observable ICP changes taking longer for lower psi20 and resulting in cognitive changes19,20. Using pigs, the same group determined that three 4 psi LLB exposures from a variety of military equipment were sufficient to cause histological neuropathology when the animals were placed in gunner positions simulating human use of the equipment21.
These studies collectively illustrate the diverse effects of LLB exposure that may occur under conditions of limited exposure and recovery periods. Repetitive LLB exposure appears to induce persistent cognitive and behavioral deficits, emphasizing the need for a nuanced understanding of the cumulative effects so we can better determine when those effects may become clinically significant; this is particularly relevant for military trainees who are exposed to high levels of repetitive LLB. To achieve this, new studies are required since the current literature does not adequately model the clinical experiences of routine military training exposures that exceed one to a few blasts over the course of a few days.
Special Operations Forces (SOF) may endure significant and highly repetitive LLB during routine exposures. A recent study estimates the representative exposure anonymized across all positions in an explosive entry breaching team to be as high as 184 cumulative peak psi over the course of one training week42. This is based, in part, on a conservative estimate of 6 breaching charges used per day, with an average of 4 psi peak pressure each, as measured by personnel-mounted blast gauges; it does not account for flashbangs and other devices45. A routine training cycle may last several weeks. To facilitate the study of clinical LLB experiences, such as those of training SOF members, we present a laboratory-shocktube model of highly repetitive LLB exposure. The method, based on established pneumatic shocktube systems46,47,48, allows for highly reproducible investigations of pressures of 2 psi and higher. The procedure is not dependent on external factors such as weather, results in no observed mortality, and is lab-based. As a result, the method enables sustained, daily repetitive LLB exposures in the same subjects for studies lasting weeks to months, facilitating the high-fidelity investigation of military training.

Author Spotlight: Deciphering the Long-Term Effects of Low-Level Blast Exposures in Mice

Modeling Highly Repetitive Low-level Blast Exposure in Mice

Our research focuses on the long-term effects of repeated low level blast exposure in mice. Our goal is to determine the long-term effects of this exposure during military training For animal research, the effects of low level blast are subtle, and research teams will often use techniques like electron microscopy to detect differences between blast and sham mice. Our work suggests increased blast repetition makes these differences easier to detect using more accessible microscopy techniques.There's currently very limited literature on both low level blast exposures, which are also called low intensity blasts, and the models that use these exposures over multiple days. We designed our protocol to include multiple low level blasts over many days to address these gaps.

Preparation of a Shocktube for Modeling Highly Repetitive Low-Level Blasts

preparation-shocktube-for-modeling-highly-repetitive-low-level

Research

JoVE Journal

Neuroscience

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