Page 57 - JSOM Spring 2020

FIGURE 5 Unconstrained and constrained bladder versus chamber pressures sigmoid curves.

(A) (B)

On each panel, the y- and the x-axes have the same scale and show y = 1 ✕ x + 0 is shown on each graph. Calculated sigmoid curves are shown
gauge pressure (absolute pressure – atmospheric pressure). Individual as lines in the same colors as their respective individual data points.
data points are shown in the indicated color for each inflated blad- (A) Unconstrained bladder. (B) Constrained bladder.
der pressure (legend on each graph). A line of perfect agreement with

pressure, applied pressures above 400mmHg to 550mmHg are conductive material separated by a compressible dielectric ma-
potentially outside the system limits of linearity and may there- terial and may have better sensitivity, repeatability, and stabil-
fore be underestimates of actual applied pressures. Using vari- ity than resistive tactile pressure sensing technology. 21
ants of this system on live humans, occlusion and completion
pressures with 3.8cm-wide, nonelastic, emergency-use-limb Unlike tactile pressure sensors, the inflated bladder-based
tourniquets are sometimes greater than 400mmHg 9-12,14 and system only provides the average pressure applied to the en-
sometimes greater than 550mmHg. 9-12 Completion pressures tire inflated bladder. When the bladder is within an air-filled
with elastic limb tourniquets (10.2cm- and 15.2cm-wide Es- chamber such as the Erlenmeyer flask of this study, the entire
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marks and 10.4cm-wide Stretch Wrap And Tuck Tourni- bladder surface is experiencing a uniform applied pressure.
quets 9,12 ) are also sometimes greater than 400mmHg. 6,9,12 When the bladder is under a tourniquet on a live limb or a
limb model, the applied pressure is not uniform. The output
At this time, we are unaware of live human pressure results pressure of the inflated bladder-based system is an average of
from nonpneumatic emergency use limb tourniquets with the all the discrete, nonuniform pressures applied to the bladder.
use of any systems other than variants of the one described
herein. Occlusion pressures with 4.5cm and wider pneumatic If a portion of the inflated bladder is not completely contained
limb tourniquets on humans have been measured using the between the tourniquet material and the underlying surface,
bladder of the tourniquet as the source of the pressure data: that bladder portion will be able to increase in volume as the
approximately 50 to 430mmHg for tourniquet widths of 4.5 tourniquet-applied force increases. Until the bladder ruptures,
to 80cm. 15-17 this will result in average bladder pressures lower than would
be the case for a bladder completely contained between the
Other pressure measuring systems exist that could be used to tourniquet and the underlying surface (Boyle’s law: Pressure
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noninvasively monitor tourniquet pressures on live humans. ✕ Volume = Pressure ✕ Volume ). Neonatal blood pressure
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Tactile pressure sensors offer the ability to not only moni- cuff design does not require bladder construction that can
tor average applied pressures under tourniquets but to also withstand the expansion pressures to which an unconstrained
assess the location and magnitude of peak pressures and to bladder would be exposed if part of the bladder is not con-
visualize pressure gradients under tourniquets. Two major tained between the tourniquet and the underlying surface. In
technologies for thin, flexible tactile pressure sensors are re- other words, use of an unconstrained neonatal blood pressure
sistive and capacitive. The resistive technology involves detect- cuff bladder to attempt to measure pressures under such tour-
ing pressure-induced changes in electrical resistance through niquets as 2 or 4 wraps of ½-inch soft rubber tubing or the
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piezoresistive ink and has been used to look at the interface Rapid Application Tourniquet System would be more likely
pressure between tourniquets and silicone limb-segment mod- to result in bladder rupture than occlusion and completed ap-
els (HapMed Tourniquet Trainers). 18,19 The resistance response plication pressure data. In our experience, the unconstrained
to applied pressure requires calibration specific to the compli- No. 1 neonatal cuff-based system requires limb tourniquet
ance of the interface materials, drifts over time, drifts under widths of 3.8cm or greater for the bladder to be completely
load, is affected by temperature, and changes sensitivity with contained between the tourniquet material and the underlying
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exposure to higher forces. The capacitive technology involves surface. If it is to be used to measure pressures under narrower
Pressure Measuring System | 53