Chest wall velocity as a predictor of nonauditory blast injury in a complex wave environment.

Previous blast injury prediction criteria have been based on exposure to classic Friedlander or ideal blast waves. An ideal waveform is characterized by an instantaneous rise to a peak overpressure that decays exponentially to ambient pressure followed by a negative phase. The prediction criteria did not address injuries resulting from exposure to complex blast waves. It was difficult to establish a simple relationship between the two because complex blast waves typically consist of multiple shocks with variable frequency content and intensity that may be superimposed on a slow rising quasistatic pressure pulse. This paper deals with the application of a single degree of freedom mathematical model, originally developed to measure the response of the thorax to Friedlander waves, to calculate chest wall velocities resulting from various complex blast loads. Experimental results with sheep, exposed to complex blast waves in enclosures, demonstrated that there was a good relationship between the Adjusted Severity of Injury Index (which includes injury to the lungs, upper respiratory tract, gastrointestinal tract and solid intraabdominal organs) and the calculated peak inward chest wall velocity. In addition, there was a good correlation between these results and previously established Friedlander injury prediction curves. The velocity of complex blast waves was nearly the same as that of Friedlander waves for a given degree of injury: 3-4.5 meters/second for threshold injury, 8-12 meters/second for an LD1, and 12-17 meters/second for an LD50.

Nonauditory injury threshold for repeated intense freefield impulse noise.

Exposure to impulse noise is an important occupational health concern. The risk of injury to auditory structures is well recognized and provides the cornerstone for present safety standards. For freefield impulse noise, nonauditory injury is dependent on peak pressure, positive phase duration (or impulse), and number of exposures. Trivial laryngeal petechiae are shown to precede nonauditory injury to more critical organs (ie, pulmonary and gastrointestinal systems). This study identifies the critical impulse noise thresholds causing trivial laryngeal petechial changes resulting from exposure to 5, 25, and 100 repetitions of specific levels of impulse noise. Because of anatomical differences, sheep should be slightly more susceptible to impulse noise laryngeal petechial changes than man; therefore, it seems reasonable to set the absolute limits for human occupational exposure levels below those causing laryngeal petechiae in sheep for persons wearing adequate hearing protection. This study does not address human auditory injury that may occur above or below these exposure limits even with proper hearing protection.

Physical correlates of eardrum rupture.

Eardrum (tympanic membrane) rupture in humans and animals in relation to various blast pressure-time patterns was reviewed. There were few systematic studies on eardrum rupture as a consequence of blast overpressure. Most reports did not describe the area of the eardrum destroyed. The peak overpressures required to produce a 50% incidence of eardrum rupture (P50) were summarized. Most of the animal data pertained to dogs. The highest P50 for dogs, 296 kPa, was associated with smooth-rising overpressure. For complex wave patterns occurring inside open shelters subjected to nuclear blasts, the P50 was 205 kPa. For fast-rising blasts in a shock tube it was 78 kPa, and 105 kPa for statically applied pressures. The duration of the overpressure was not a factor unless it was very short. The influence of the orientation of the head to the oncoming blast was demonstrated. An ear facing the blast may receive reflected overpressures several times that for one side-on to the blast. An ear on the downstream side of the head was exposed to about the same overpressure as the side-on ear. A P50 for humans of 100 kPa and a threshold of 35 kPa has been used widely in blast criteria. A recent study suggests a threshold (P1) of about 20 kPa, and gives the overpressures required to produce minor, moderate, and major eardrum ruptures. These data were presented in the form of curves showing the overpressures as a function of duration required to inflict a P1 and a P50 of eardrum rupture of the three levels of severity.

Cloth ballistic vest alters response to blast.

Ballistic wounds have been and will remain the principal cause of casualties in combat. Cloth ballistic vests (CBV) play an important role in limiting critical wounds from fragments and small-arms fire. There is an increased risk of primary blast injury on the modern battlefield. In a previous study, volunteers were exposed to short-duration blast waves of low peak pressure (18.6 +/- 0.8 kPa). Pressure measurements made in the distal esophagus as an estimate of intrathoracic pressure (ITP) were significantly higher (p less than 0.05) when the standard U.S. Army ballistic jacket was worn (8.7 +/- 1.2 kPa) than when fatigues alone were worn (7.4 +/- 0.7 kPa). In this study 58 sheep were exposed to nominal blast levels of 115, 230, 295, and 420 kPa peak pressure in groups of 12, 18, 16, and 12, respectively. Half of each group was fitted with a CBV. Lung weight index (LWI), lung weight expressed as a percentage of body weight, was used as a measure of blast injury. Use of the CBV was associated with a significant increase in LWI (p less than 0.05) which averaged 21% for the two middle exposure groups. At the 420 kPa level, two of six non-CBV animals died as opposed to five of six animals wearing the CBV. Intrathoracic pressure was generally higher in the CBV group. Likely mechanisms of injury enhancement include an increase in target surface area and an alteration of the effective loading function on the thorax. This information may be useful in the triage and treatment of casualties exposed to intense blast environments.

The influence of clothing on human intrathoracic pressure during airblast.

Exposure to airblast can result in injury to the lungs and other gas-containing organs. The mechanism of lung injury is not clearly understood, but may be related to the rapid increase in intrathoracic pressure (ITP) which is produced when the blast wave strikes the chest wall. The purpose of this study was to determine if ITP during airblast would be influenced by several different types of protective clothing. Ten healthy young male volunteers were exposed to airblast while standing face-on and wearing 1) military fatigues (control condition); 2) fatigues with field jacket; 3) fatigues with ballistic armor vest; 4) fatigues with ceramic vest; 5) fatigues with ceramic vest over the ballistic vest. The incident blast waves simulated artillery muzzle blast. In each subject, an esophageal strain-gauge pressure transducer measured ITP during the blast. The pressure signal was analyzed for ITPmax, and maximum rate of rise of ITP (dP X dt max-1). In addition, the power density spectra of each ITP wave was computed and the peak frequency (fp) and centroid frequency (fc) were calculated. When the subjects wore the ballistic vest, the mean ITPmax was higher (p less than 0.05) than when they were exposed to airblast in fatigues alone. ITPmax was not influenced by the other clothing ensembles. The mean dP X dtmax-1 was not significantly different with any protective clothing ensemble. Clothing had no significant effect of fp, but with the ballistic vest, the mean calculated fc was higher (p less than 0.05) than that for the fatigues alone.(ABSTRACT TRUNCATED AT 250 WORDS)