Modeling Highly Repetitive Low-level Blast Exposure in Mice

Abstract

Exposure to explosive blasts is a significant risk factor for brain trauma among exposed persons. Although the effects of large blasts on the brain are well understood, the effects of smaller blasts such as those that occur during military training are less understood. This small, low-level blast exposure also varies highly according to military occupation and training tempo, with some units experiencing few exposures over the course of several years whereas others experience hundreds within a few weeks. Animal models are an important tool in identifying both the injury mechanisms and long-term clinical health risks following low-level blast exposure. Models capable of recapitulating this wide range of exposures are necessary to inform acute and chronic injury outcomes across these disparate risk profiles. Although outcomes following a few low-level blast exposures are easily modeled for mechanistic study, chronic exposures that occur over a career may be better modeled by blast injury paradigms with repeated exposures that occur frequently over weeks and months. Shown here are methods for modeling highly repetitive low-level blast exposure in mice. The procedures are based on established and widely used pneumatic shocktube models of open-field blast exposure that can be scaled to adjust the overpressure parameters and the number or interval of the exposures. These methods can then be used to either enable mechanistic investigations or recapitulate the routine blast exposures of clinical groups under study.

Figure 1:. Procedural steps for the shocktube model of repeated murine LLB.

Following both the preparation of the shocktube (Steps 1–10) and the animal preparation stages (Steps 11–18), mice are exposed to one or more LLBs (Steps 19–32), before being removed from the tube (Step 33). Mice are then placed on their backside onto a warmed heating pad (Step 33). The amount of time it takes the animal to flip over onto their ventral side is recorded as the righting time (Steps 34). Abbreviation: LLB = Low-level blast.

Figure 2:. Representative pressure-time curves for exposures near 4 psi.

(A) Additive stacks provide linear peak pressures across the range of 2–4.5 peak psi. Representative pressure versus time (milliseconds) profiles averaged from 3–6 shocktube blasts (red) as compared to the idealized Friedlander curves for (B) 1 sheet, (C) 2 sheets, (D) 3 sheets, and (E) 4 sheets.

Figure 3:. Intersubject Interval.

Set up and execution of a single blast requires on average 9.8 ± 1.9 min (mean ± standard error of the mean (sem)). Additional blast exposures require an additional 1.7 ± 0.4 min per event (mean ± sem). Dots represent results from individual animals.

Figure 4:. Daily righting times during 3 weeks of highly repetitive LLB exposures.

Graph represents the sham-normalized righting times over 3 weeks of LLB exposure. LLB mice were subject to 6 daily blast exposures for a total of 90 total LLB exposures occurring over 15 days. Mean overpressure characteristics were (± sem) 3.05 ± 0.07 peak psi, 0.94 ± 0.04 positive phase duration, and 2 ± 0.1 psi * msec impulse. p-values reflect results from 2-way ANOVA. Abbreviation: LLB = Low-level blast.

Figure 5:. Effects of the laboratory shocktube LLB model on animal attrition following highly repetitive LLB exposures.

Attrition rates for sham (N = 24) and LLB mice (N = 32) from the first LLB exposure (day 1) through all study exposures (ending day 19) and following a 6-month recovery period (day 199). There was no statistically significant difference between the attrition rates of sham and LLB groups over the observed period. LLB mice experienced an average of 62 exposures at an average 4.78 ± 0.01 peak psi and 3.16 ± 0.023 psi·ms impulse. Exposures were administered to mice 5 days per week (i.e., Monday–Friday) for 3 consecutive weeks to model recently reported SOF overpressure exposures during routine breaching training. Abbreviation: LLB = Low-level blast; SOF = Special Operations Forces.