Page 33 - Journal of Special Operations Medicine - Winter 2016

used to compare pressure changes per second between values are for loss from the fast component and from
the conditions of different constraints, tourniquets, com- the slow component; so the closer pressure (y) becomes
positions, completion-pressures, and circumferences. to the plateau.

Graphing and statistical analyses were performed using Pressure-loss curves for SWATT applications are also
GraphPad Prism version 5.02 for Windows (GraphPad well-described with two-phase decay equations. However,
Software Inc.; www.graphpad.com). Statistical signifi- curves from SWATT 20% ballistic gel and arm applica-
cance was set at p ≤ .05. tions at 262mmHg had many combinations of parameter
values that led to equally good curve fits (Table 2).

Results Pressure-loss curve equation parameters, goodness-of-
fit values, and times to <5mmHg and <1mmHg from
Arterial Occlusion plateau are shown in Table 2. Complete equations are
Thigh C-A-T and RMT applications with 262mmHg shown in Table 3. The values of greatest clinical inter-
completion-pressures did not maintain arterial occlu- est in Table 2 are thigh and arm plateau pressures and
sion. All other thigh and all arm tourniquet applications times. Plateau pressures indicate pressure losses are
maintained arterial occlusion. greater with higher completion-pressures and least with
the SWATT. The times for thigh and arm pressure losses
Completion-Pressures, Friction-Pressures, to become within 5mmHg or 1mmHg of plateau suggest
Mechanical Advantage Use 5 or 10 minutes as a reasonable reassessment time.
Table 1 shows completion-pressures achieved with each
tourniquet application. Table 1 also shows C-A-T and Tourniquets, Compositions,
RMT friction-pressures (i.e., the pressures exerted with Completion-Pressures, and Circumferences
the strap secured but the mechanical advantage system not Pressure-loss curves were highly affected by tourniquet
yet engaged). Friction-pressure does not affect the pres- design (Figures 1 and 2); considerably less and slower
sure at which occlusion occurs but does affect mechanical pressure loss occurred with the elastic SWATT. The
advantage system use and thereby affects strap bunching, C-A-T and RMT had statistically significantly different
which affects tissue (or gel) under the tourniquet. 9 decay equations despite both using a 3.8cm-wide non-
elastic strap. The blood pressure cuff, despite containing
Pressure-Loss Curves an elastic bladder, had pressure-loss curves more similar
Pressure losses occurred under each tourniquet (Fig- to C-A-T and RMT curves than to SWATT curves.
ures 1 and 2). Pressure-loss curves for all C-A-T, RMT,
and blood pressure cuff applications are well-described Pressure-loss curves for each tourniquet were affected
mathematically with two-phase decay equations: by the composition of the material (10% gel, 20% gel,
thigh) on which they were applied (Figures 1 and 2).
y = Plateau + SpanFast × e (−Kfast × x) + SpanSlow × e (−Kslow × x) Pressure-loss curves were also affected by completion-
pressures (Figures 1 and 2). Plateau loss values (loss
This means each curve can be characterized as having after an infinite time interval) were of greater absolute
a maximum pressure loss after infinite time defined as magnitude on 10% ballistic gel than 20% ballistic gel,
the plateau value, and the rate of pressure loss can be and were of greater absolute magnitude with higher
characterized as having a fast and slow half-life occur- completion-pressures.
ring concomitantly. In other words, the pressure (y) at
any given time (x) is equal to the sum of the plateau (a Thigh pressure-loss curves (Figure 1C and Figure 2C)
negative number representing the total pressure loss at did not match those of the 10% or 20% ballistic gel (Fig-
infinity) plus the amount of loss at time (x) from the fast ure 1A and Figure 2A). On each gel, during at least the
process plus the amount of loss at time (x) from the slow first 300 seconds, the blood pressure cuff curves tended
process. The time (x) is multiplied by each rate constant to have the greatest absolute magnitude pressure losses
(KFast and KSlow) to determine the negative exponent (all pressure losses hereafter are discussed as absolute
to which e (2.718, the base of the natural logarithm) magnitude unless otherwise indicated). On the thigh,
is raised in each portion of the equation. SpanFast and the blood pressure cuff curves had lesser pressure losses
SpanSlow represent the portions of the pressure loss than did the C-A-T or RMT. The C-A-T and RMT also
from each half-life (SpanFast = (y0 – Plateau) × Percent- had much steeper pressure-loss curves through 60 sec-
Fast × 0.01, SpanSlow = (y0 – Plateau) × (100 – Percent- onds on the thigh than the gels. Additionally, the times
Fast) × 0.01, PercentFast = the percentage of the span for pressure losses to approach plateau were longer for
from y0 to Plateau accounted for by the faster decay applications on either gel than for applications on the
rate). The larger the time (x), the smaller the positive thigh.

Tourniquet Pressure Loss 17