Recently, Mohan et al.
1 reported a study of blast effects on the retina and optic nerve, using a small, custom-built, enclosed “blast chamber.” This chamber was 50 cm long and 33 cm wide. Two parts of this chamber were separated by a Mylar membrane. The side of the chamber away from the test subject was pressurized until the Mylar membrane burst, thereby reportedly generating a blast wave.
The only pressure measurement reported by Mohan et al.
1 was the pressure of the pressurized side at which the Mylar membrane burst (20 ± 0.2 pounds per square inch [psi]; 137.8 ± 1.3 kilopascals [kPa]). However, this is insufficient information to characterize the pressure/shock wave experienced by the mouse subjects. Was a pressure transducer of sufficient bandwidth placed near the target position of the blast (exposure) side of the chamber to directly measure the magnitude and shape of the overpressure wave generated in this chamber? To properly characterize overpressure loading conditions and enable comparison of results between studies, it is important to report conditions at or near the test subject. Various studies have used a variety of methods to generate blast loads, including shock tubes, air guns, and live explosives. To translate blast wave exposures produced by explosives into experimental methodologies appropriate for the laboratory and to develop computer simulations of the blast exposure, a complete pressure-time history is necessary.
The mouse subject was described as being located at a distance of 30 cm from the Mylar burst membrane. However, the illustration in
Figure 1B
1 seems to indicate a much closer distance. The test setup as described may have generated a reasonable blast exposure environment, but without a pressure-time history, complications may have occurred that cannot be dismissed, as follows:
It is not clear from the illustration of the blast chamber in
Figure 1 of the study
1 if there was any damping mechanism or pressure relief mechanism to prevent the pressure waves from reflecting off of the walls of the chamber and returning to the target location. If this was so, the mouse subjects likely experienced a very complex, turbulent exposure to multiple blast waves, complicating the interpretation of the study's results. Once again, direct measurement of the pressure wave at the target position would have answered this concern.
A free-field blast wave
4 generated by an explosive event generally has the form shown in
Figure 1. This figure shows an idealized measurement of the overpressure by a pressure transducer located at a fixed distance from the blast source. The pressure shows a nearly instantaneous increase to a peak positive pressure value (the shock), followed by a rapid decay back to the ambient pressure. The duration of this positive peak is on the order of a few milliseconds or less. Following the positive pressure peak, there is a negative pressure trough, in which the pressure is less than the ambient pressure.
To facilitate comparisons between studies, a standardized characterization of the blast wave is necessary. A graph showing a typical pressure wave profile should always be presented. Furthermore, for blast waves of the form shown in
Figure 1 it is recommended that the following parameters be reported:
If the pressure profile is not that typical of a shock wave, then in addition to the peak positive pressure and duration of the positive pressure, the impulse (in kPa-ms) should be reported.