Page 33 - JSOM Fall 2018

FIGURE 6 (A) Complete acceleration profile of trial 2 (mannequin-mounted sensor). (B) Section of the acceleration profile of trial 2
(mannequin-mounted sensor).
(A) (B)

The variability in litter orientation, due to the unavoidable would experience a maximum acceleration below a value be­
maneuvering necessary to meet the demands of the terrain, tween 2.4g and 3.7g. Influencing the conditions of evacuation
prevented postrun calibration of this type of accelerometer to could substantially reduce the peak acceleration experienced
the effect of gravity. As a result, this force of 1g could apply by a patient transported on a litter. Judicious route planning,
fully to any one of the three axes at a given point, or partially decreased rate of movement, and evacuation­specific training
in lesser amounts distributed along each axis, depending on could result in substantial reductions of mechanical shock on
the litter orientation. the patient.
Although the exact physiologic effects of these forces is difficult
Discussion
to determine, the data can be compared with other modes of
The mechanical impacts experienced during military dis­ transportation from previous studies, with the acknowledge­
mounted evacuation have not been described previously in ment that differing protocols will introduce variability. When
the literature to our knowledge; therefore, we emphasized the choosing how and when to evacuate, Operators can use this
development of an adequate method for characterizing these information to compare the risk of each method to make in­
forces. Because training exercises of evacuation either use a formed battlefield decisions. One study measured a mean peak
live actor or mannequin as the patient, we sought to test the acceleration of 0.8g for 9­second intervals of an ambulance
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validity of affixing the accelerometer to either the litter or driving at 35 mph on a paved road. Unsurprisingly, accelera­
mannequin during protocol development. Although the litera­ tion during this evacuation under pristine conditions was sig­
ture suggests affixing the accelerometer to the litter is the most nificantly lower than the peak accelerations measured during
common technique for evaluating forces to a patient during Sked evacuation. Another study used data collected from the
evacuation, our study deemed a similar protocol less suit­ US Army Aeromedical Research Laboratory, which provided
able when using a flexible stretcher. The recorded forces for vibration profiles for helicopter and MRAP evacuation. These
the Sked­mounted trial were substantial, and although they profiles contained a maximum acceleration of approximately
should have resulted in moderate injury, there was no signif­ 1g for helicopter and approximately 3g for MRAP evacua­
icant discomfort or injury delivered to the Soldier. Therefore, tion during approximately 1­minute intervals. As expected,
6
we surmised that when analyzing an evacuation method using helicopter evacuation introduces lower magnitude forces than
a flexible litter similar to the Sked, the accelerometer should does Sked evacuation. Compared with the MRAP 1­minute
not be placed directly on the litter. interval, the 75­minute, mannequin­mounted trial from this
study included peak accelerations approximately twice as
In addition to contributing to protocol development, the data large in magnitude; however, the 90th percentile confidence
collected from the mannequin­mounted trial were evaluated interval of the 1­minute intervals of this trial encompasses the
to help guide decision making when faced with evacuation MRAP benchmark. If it is possible to avoid the handful of
scenarios in a mountainous environment. Although the av­ extreme acceleration changes during the movement through
erage maximum acceleration for the 75 1­minute intervals additional training, it would result in mechanical shock less
was 1.9g, the global maximum of the entire trial, 5.5g, poses than or approximately equivalent to evacuation in an MRAP.
the most significant risk for a patient being evacuated. The Future studies may further investigate the physical events lead­
GEV model allows us to better understand the possibilities of ing to the larger acceleration changes and determine if they are
maximum mechanical impact on a patient. The model esti­ avoidable.
mated a 90th percentile confidence interval of between 2.4g
and 3.7g for the distribution of maximum acceleration points Conclusion
over repeated 1­minute intervals. This means that for an in­
finite population of 1­minute evacuation intervals, only 10% This study characterized one possible permutation of the fac­
would have a maximum acceleration over a value between tors considered under the mission, enemy, terrain, troops,
2.4g and 3.7g. Articulated another way, by avoiding the 10% time, and civilian considerations heuristic and demonstrated
of 1­minute intervals with the greatest accelerations, a patient the magnitude of peak accelerations during a dismounted

Mechanical Forces During Movement in Mountainous Terrain | 31