Page 54 - JSOM Spring 2018

FIGURE 5 Three lengths of pipe and a collapsible tube. of transmural pressure (the internal fluid pressure minus the
external ambient pressure) is decreased downstream, the wall
tends to collapse inward. In the Venturi effect, a corollary
of the Bernoulli principle (Daniel Bernoulli [1700–1782], a
Swiss mathematician, detailed a theorem of phenomenon of
internal pressure reduction with increased flow velocity in a
fluid), flow velocity is increased locally within the constric-
tion. The velocity increase is a consequence of the principle of
mass continuity as the mass of water remains in continuous
flow without gaps. On the other hand, the pressure loss is
a consequence of the principle of conservation of mechani-
cal energy. These velocity–pressure changes are linked and
balanced. Readers may recall that deaths resulted from a
1982 military parachuting operation in California, when a
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Venturi effect occurred as winds accelerated through a gap
between mountains near the drop zones. Similarly, atheroscle-
Image of the collapsible tube (red tube at the bottom with length of rotic plaque and arterial stenoses also produce Venturi effects,
150mm) and three pipes (three yellow pipes at the top) in the three and an area reduction of about 95% is sometimes considered
lengths used (5mm, 20mm, and 70mm, top to bottom).
critical. The observed change from laminar (smoothly stream-
differences among mean flow rates were not statistically sig- lined) flow to turbulent flow may be clinically useful because
nificant (p > .05, all three). Unsurprisingly, the mean flow rate of its potential relevance to bleeding. Turbulent flow might be
of the control group was greater than that of the study group. easier to control than laminar flow, in part because the intra-
luminal pressure is lower and thereby more susceptible to an
Preliminary analyses did not reveal an interaction between the externally applied compression downstream.
effects of degrees of compression and the effects of lengths of
compression. A closure of 97% was necessary to reduce flow rate to 95%.
Such a magnitude, 97%, of closure needed to control flow
is an actionable finding to guide further study and educate
Discussion
first aid students. According to the best information available,
The major finding from this simulation study is that arterial an artery has to be closed about 97% to control the bleed.
compression significantly reduces flow rate in a curved fash- Because blood flow through veins is not pulsatile yet is lami-
ion. Within this study, the luminal area had to be reduced by nar, the degrees of closure needed as tested here may be clini-
81% to provide a 40% flow reduction and by 97% to provide cally relevant to control of venous bleeding. The experimental
a 95% flow reduction. Other collapsible tube studies have had setup may also work adequately for assessing vein-like tubes
similar results based on both measurements and mathematical or testing wraps, manual compression, or other mechanical
equations. 7–10 Before we conducted the present study, we spec- interventions. However, further model development is justified
ulated that the area–flow curve would bend nearly straight for possible future setups such as for collateral blood flow or
down when area reduction was nearly 80%, but this actually vessel networks. For example, various degrees and lengths of
did not occur until area reduction was nearly 95%. Our cur- compression may be studied in a vein-like tube.
rent thinking is now accommodating this new major finding
so that a future study may be planned to delimit a reasonable As expected, measured data agreed well with calculated data.
threshold such as an area reduction of 95% that may suffice The agreement offers reasonable validity for developing new
for caregivers. These points discussed here may help people to knowledge with interventions such as external compression.
better understand challenging aspects of hemorrhage control The findings of this study have clinical applicability where ves-
such as optimal ways to stop bleeding. First aid instructors sels have lost the support given by the surrounding tissue of
may implement the finding actionable as, “To reduce bleeding their exterior wall. Such loss may occur in traumatic wounds
by 95%, an artery needs to be closed about 95%.” or during treatments such as surgery. The loss of support be-
tween the adventitia—the most external portion of the vessel
The secondary finding of the present study was that the length wall—and adjacent supportive tissue may also occur during
of compression did not have a significant effect on flow rate dissections in research or during interventions such as vessel
under the conditions tested, specifically among three lengths clamping in advanced caregiving in the field.
of compression applied to the external wall of a simulated
artery. A reason for this lack of association among the three The results of this study also have potential applicability to
lengths tested may be attributed to the collapsed tube remain- the biomechanics of wound packing with gauzes, dressings,
ing downstream to the compression itself in all tests with pads, or the like; use of tourniquets or vascular clamps; and
compression. The downstream luminal area was as small as use of compressors or wraps; recently, Kragh et al. published
that of the location compressed upstream. The collapse mim- a collapsible tube study that compared two types of wound
icked a long length of compression and may be related to a packing. Furthermore, the applicability of how vessels are
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phenomenon called the Venturi effect (named after Giovanni compressible may help inform refinements in manikin design.
Battista Venturi [1746–1822], an Italian physicist who stud-
ied this) in which a fluid’s pressure drops as it flows through a After data collection and during our ongoing search for rel-
constriction, such as when water flows through a nozzle of a evant knowledge, we found a 1938 report by Mann and coau-
garden hose to exit. Pressure loss is not regained downstream. thors that validated our basic strategy and reasoning. In their
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Because the collapsible tube is easily deformed and its gradient study, Mann et al. studied blood flow several ways and found

50 | JSOM Volume 18, Edition 1/Spring 2018