Page 46 - Journal of Special Operations Medicine - Spring 2015

Table 3 Time to Effectiveness, Pressure, and Blood Loss specific environmental exposures, such as diurnal heat
Results by Exposure exposure, humidity changes, or longer durations of ex­
Group (No. of tests) posure. Some grades of nylon eventually degrade upon
certain environmental exposures through oxidative and
Heat No Heat
Exposure Exposure thermal degradation, resulting in reduced physical prop­
Variable (300) (300) erties such as tensile strength. Polymers that make up
Mean portions of a tourniquet may or may not be engineered
Time effectiveness, s 17.5 21.8 to be heat stabilized; such stabilization in other nylon
Pressure, mmHg 209 207 products in the marketplace improves resistance to deg­
Blood loss, mL 131 160 radation due to exposure. Indirectly, the current policy
recommendation to stow the individual tourniquet in the
(p = .07; Table 3). The mean pressure was 6mmHg less first­aid kit until needed remains prudent in that this may
for the more experienced user (p < .0001). offer protection from some exposures, such as ultravio­
let light. Only one model, the SOFTT­W, had damage,
The mean blood loss was 29mL less in the heat­ exposure and this was of only one component: The time label fell
group (p < .0001; Table 3). The mean blood loss was off the strap of six of 10 devices (three of the six were
13mL less for the more experienced user (p = .003). in each group). The mechanism of label­strap separation
for the SOFTT­W appears to be independent of heat ex­
Statistical Analyses by a Generalized posure; it may simply be due to mechanical shear or peel
Linear Model Procedure and a Mixed Procedure back. This separation was the only quality­assurance is­
In a comparison of time to effectiveness by model of sue found, and it was seen in only one tourniquet model.
tourniquet, the effect of tourniquet model on time was
statistically significant (p < .0001). In a comparison of The second major finding was that minor outcome re­
pressure by model of tourniquet, the effect of model sults (time to effectiveness, pressure, and blood loss)
on pressure was statistically significant (p < .0003); were mixed: Some results were significantly associated
the mean time of the study group was 4 seconds faster. with heat exposure, while others were not. In any event,
In a comparison of blood loss by model of tourniquet, the differences were small and unlikely to be clinically
the effect of model on blood loss was statistically signifi­ significant. The minor outcomes were continuous vari­
cant (p < .0001); the mean loss in the study group was ables and not binary variables like damage or effective­
29mL less. ness. The learning effect of the user with increasing
experience in repetitive testing confounded the results,
Results also varied by user. The magnitude of the mean and the statistical model was able to account for much
differences for the two users was small; therefore, a user of the effects as resulting from the learning, a user effect,
and model effect was implemented as random effects in and not being from the heat exposure. The variables
the statistical model to account for such confounding. 12 that responded to learning (time, blood loss) were statis­
tically significant, and the variable that did not respond
When the mixed procedure was used to adjust for the to learning (pressure) was not significantly associated.
effects of user and model, the comparisons of time to ef­ Since learning increased with test number (a measure of
fectiveness and of total blood loss were still statistically user experience) and the heat­exposure group was tested
significant (p < .0001), but the comparison of pressure second by the users, the two variables, test number and
still was not (p = .0613). The CAT and RMT had similar heat exposure, were confounded. Likely, the learning
times (p = .11), but the SOFTT­W had longer times (p < was the reason for the confounding. The statistical mod­
.0001). The blood loss results were least for CAT, inter­ els were able to account for this confounding well. No­
mediate for RMT, and greatest for SOFTT­W (CAT ver­ tably, learning and damage were not associated.
sus RMT, p = .0013; RMT versus SOFTT­W, p < .0001;
CAT versus SOFTT­W, p < .0001). The minor findings of the present study were that time
to hemorrhage control and blood loss remained statisti­
cally significant even after a mixed procedure was used
Discussion
to adjust for the effects of user and tourniquet model.
The first major finding of the present experiment was The time–blood­loss association is linear in the manikin
that the critical outcome, damage, was not associated if there was no hemorrhage control, but if there was par­
with exposure to heat of 54.4°C for 91 days. This find­ tial hemorrhage control (eg, by intermediate compression
ing suggests that the type of exposure studied may not as in a venous tourniquet use), the relationship became
be the culprit of the damage rates reported from tour­ complex and was no longer a simple, linear equation. In
niquet users in the field and reported by previous inves­ a prior study, we showed that with increasing user ex­
tigators. Future investigators should focus on other perience, blood loss by partial hemorrhage control may
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36 Journal of Special Operations Medicine Volume 15, Edition 1/Spring 2015