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COMMENTARY
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Negative Pressure Darwinism:
Survival of the Fittest Paradigm
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Michael Miller, DO; Farhad Bybordi, MEdM
WOUNDS 2009;21(7):192–197
Abstract: The use of negative pressure for wound healing has been
based on a set of parameters and devices that until recently were combined into a single paradigm. Despite historical and more recent evidence providing viable alternative considerations, it is only recently
that this paradigm and its tenets have come into question. As the
understanding of the limits of the current paradigm and specific
instances of its benefits and drawbacks are identified, shifts in the
paradigm must take place if the therapy is to evolve, develop, and continue to be efficacious. The pertinence of the concept of survival of
the fittest is used to explore the need for a paradigm shift in negative
pressure wound therapy.
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Address correspondence to:
Michael Miller, DO
The Wound Healing Center
2900 16th Street
Bedford, IN 47421
Phone: 812-798-2091
E-mail: [email protected]
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From the Wound Healing Center,
Bedford, Illinois
he field of negative pressure wound therapy (NPWT) has had a dramatic expansion in terms of technologic reproduction, but there is a
question as to whether there has been a definitive and identifiable
change in the conceptualization, applications, and outcomes as newer versions are considered and enacted.
In 1997, the first commercial incarnation of NPWT entered the market.
With it came concepts within this new paradigm that at first were met with
skepticism. These included: using an open cell polyurethane foam inserted
into a non-sterile wound for a period exceeding 23 hours without changing
it; sealing the wound with a plastic film that “prevented the air from getting
to the wound”; attaching a tube to the sealed foam/wound that allowed for
a direct connection with the outside; keeping the drainage in an attached
container for more than 24 hours without emptying it, and many other then
controversial considerations. Despite these early concerns, the success of
the technology and paradigm(s) it created led to major advances in wound
healing and subsequent commercial success. It can be shown that there
were earlier recorded uses of negative pressure; however, the creation and
acceptance of this particular paradigm was due to its multi-factorial considerations such as the level of suction, wound contact medium, method of
application, and of course, the pump technology, which led to improved
healing outcomes.
An upstart company, led by a 2004 legal victory, forced the door open
and allowed a limited juxtaposition of technologies and a challenge to the
established paradigm. Since there is now a variation on an established technology, the terms “gold standard” and “perfect” must be evaluated and con-
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The Wound Contact Medium
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The first component of the paradigm to be considered
with respect to the aforementioned tenets of a perfect
NPWT product is what I call “the wound contact medium,” or in the case of the current paradigm, open cell
polyurethane foam. At the time of the writing of this article, there are two companies with NPWT paradigms and
products based on this wound contact medium (Kinetic
Concepts, Inc. San Antonio,Tex; Innovative Therapies, Inc.
Gaithersburg, Md).
The difference between open-cell and closed-cell
polyurethane foams is that these two types of
polyurethane foams have different R-values, permeability,
strength, and costs.2 Interestingly, the scientific jargon
used to legitimize the use of foam as a wound healing
adjunct can also explain other aspects and uses “…the
influence of pneumatic damping caused by friction
between the gas within the open-celled foam and matrix
polymer.”3 A continuous shape function is introduced to
characterize the piecewise continuous stress-strain characteristic of flexible, open-cell foam. The new model is
able to predict the dynamic performance of a seat cushion with fidelity. This statement pertains not to wound
healing, but to the performance of an automobile seat
cushion.3
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sidered. For a technology and/or paradigm to be perfect
it must have no shortcomings, no faults—in essence it is
a one size fits all technology. In contrast, the term gold
standard recognizes the temporal advantages and benefits of a given paradigm. It recognizes that at a given
time, in a given place, under a given set of circumstances, that the paradigm was the best available. It
allows for a change in any of these variables, but more
importantly, recognizes that the changes in these and
other variables allow for changes in the paradigm with
an equal potential for improvement or worsening.
With these concepts in mind, the progression of scientific technologies must follow the basic tenets of
British economist Herbert Spencer who transitioned
from Charles Darwin’s biologic theories of natural evolution to that of sociologic, and subsequently economic
events. It was he who coined the phrase “Survival of the
Fittest” in his book Social Statics. Technology must be
adaptable. “The more adaptability, the greater the viability.”1 The question before us is simple: Is the initial and
prevalent NPWT paradigm perfect or the gold standard?
If “perfect,” then why should there be any other viable
alternatives expected to survive? If the “gold standard,”
then it is mandatory to consider, explore, create, and test
alternatives to all aspects of the paradigm. It would be
unrealistic to preconceive which of them could/would
remain unchanged, hence, “perfect.”
The first consideration must look at the conundrum
of perfect versus the gold standard in NPWT devices and
technologies. In order to be perfect, the paradigm would
have to consist of the following:
• Be universally available
• Easy to apply
• Easy to remove
• Cause no pain
• Be adaptable to all patients
• Be appropriate for all wounds
• Be cost effective
• Allow for variations in all wound and patient parameters
• Have no complications
• Have universally complete healing outcomes.
Since achieving this is tantamount to reaching infinity,
the next best thing and the obvious conclusion that can
realistically be reached is that the current paradigm is at
best the gold standard. The considerations now shift to
identifying those aspects of the gold standard paradigm
that can be changed potentially for the better with the
ultimate goal of reaching perfection.
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Paradigm Safety
The ongoing evaluation of paradigm safety is always a
concern, and there are several with the current gold standard. The Materials Safety Data Sheet (MSDS) on opencell foam clearly states, Eye Contact: Flush eyes with
water for at least 15 minutes. Consult a physician. Skin
Contact: Wash affected area with soap and water. If irritation persists, consult a physician. Inhalation: Remove
to an uncontaminated area; administer oxygen if necessary. If victim has stopped breathing, begin CPR. Get
medical attention. Ingestion: If swallowed, give water or
milk and induce vomiting. Get immediate medical attention. Precautions to be taken in handling: Avoid eye contact and prolonged or repeated skin contact. Workers
should thoroughly wash hands with soap and water
prior to eating, drinking, smoking, and using lavatory.4
A review of the Manufacturer and User Facility Device
Experience Database (MAUDE) reports on foam used in
NPWT identified 22 incidents reported from 1/1/08 to
8/22/08.These included bleeding, retained foam, foam so
intimate it required hospitalization and surgery to
remove, hospitalizations due to pain, bleeding, need for
exploration, etc.4 It is important to recognize that these
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Risks and Benefits
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The ultimate questions must identify the risks and the
benefits of using foam in a given patient on a given
wound. Is a paradigm shift necessary? What else can be
considered to improve on the gold standard?
I have adopted the term “porous, non-adherent wound
contact layer” to define and identify those materials and
dressings potentially suitable for use as an interface with
negative pressure. Presently, there is a trend toward using
AMD (polyhexamethylene biguanide [PHMB]) gauze due
to its availability, ease of use, overall familiarity in the
wound care field, and studies documenting its antimicrobial efficacy.6 MAUDE database has no reported incidents
on gauze. The MSDS for AMD gauze identified the following regarding Respiratory Protection, Hand
Protection, Eye Protection, and Body Protection:“No specific measures necessary.” The only caveat was that in
case of eye contact, “flush eyes with plenty of water for
1–2 minutes” but no specific eye protection was warranted with use. Is this evidence sufficient to consider a
paradigm shift?7
• Remove edema and restore normal fluid balance in
tissues reducing tissue hypoxia in early stages of
inflammation
• Promote increased lymphatic and venous drainage
through changes in pressure on lymphatics and colloid osmotic pressure
• Remove healing inhibitory “waste products” (in
wound drainage) in contact with wound
• Negative pressure brings tissue together (coaptation), which facilitates natural tissue adherence and
increases healing
• Cellular membrane distension causes intracellular
elements to send an injury current pattern to the
cell nucleus causing secretion of pro-healing factors
including angiogenic factors.8
At a minimum, these characteristics define the presence of NPWT. Therefore, any system that meets the criteria of this paradigm must be considered to be a negative pressure-generating wound therapy system. Since
wounds do not have the ability to discern the differences
between NPWT technologies, their response is based
solely on the presence of this force. Thus, the paradigm
of having only one entity with the ability to create the
necessary negative pressure to promote healing is nonviable.
Timing of therapy. The timing of negative pressure
application is another consideration in the spectrum.
Even if one is to consider 24 hours as the gold standard,
as a defined unit to compare other therapy timing regimens to, this allows for unique alterations:
• Constant: The machine operates for a 24-hour
period (standard)
• Periodic: The machine operates for a period < 24
hours (6–8 hours) or more than one treatment
period per 24 hours9–11
• Intermittent: The suction alternates between on
and off or lower/higher pressures
• Continuous: A single pressure is maintained during a given treatment period.
This allows for 4 unique NPWT alternative timing
schemes: constant/continuous, constant/intermittent,
periodic/continuous, and periodic/intermittent. These
have all been used to an extent either purposefully or
accidentally (based on patient compliance and
device/dressing reliability factors, among others).
Additionally, the case evidence is sufficient to show that
these timing combinations have resulted in wound healing.12–31 The deciding factor as to which will result in the
“best” healing is unlikely to ever be identified; however,
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events have appeared numerous times in prior databases
despite more than 11 years of the paradigm’s gold standard status. Moreover, the US Food and Drug
Administration (FDA) has mandated that there must be a
non-adherent, interpositional layer between the foam
and any viscera, and that the foam cannot be placed on
any exposed vessels.5
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Differences in Technologies
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Some of the hardest fought and yet simplistic arguments pertain to the differences in the suction that the
various technologies produce. For example, the argument that suction generated from one device promotes
wound healing, while suction from another device simply removes drainage is one of illogical thinking. A scientific explanation to support these claims has yet to materialize and cannot be found in the literature.
NPWT characteristics. To make the argument understandable, there must be defining characteristics for
NPWT in which all paradigms should fit. Characteristics
that are generally accepted:
• Promote moist wound environment
• Increase capillary and venous engorgement bringing fresh blood flow into wound
– An increase in growth factors
– The promotion of white cells and fibroblasts
within the wound
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review found discrepancies between available articles
regarding which pressures were best for the foam-based
NPWT technologies. Additionally, when the foam deformation pressures were taken into account, it appeared
that lower pressures were better for non-foam application (ie, less detrimental effects). The review also noted
that negative pressure levels should reflect the types of
tissue being treated, or higher pressures (comparatively)
should be used to treat denser tissues such as muscle or
fascia. Nonetheless, the foam incarnations have clung to
the same gold standard pressures recommended for all
tissues.37
Conclusion
Robert Burton, a Scientist and Philosopher from the
1600s stated, “A dwarf standing on the shoulders of a
giant may see farther than a giant himself.”38 Despite marketing hype, ego, and braggadocio, there is clearly a place
for both foam and non-foam applications of NPWT with
each technology having proven efficacy under multiple
circumstances, albeit not by double blind randomized
studies. The conundrum facing practitioners now is not
simply to identify those wounds and conditions that
respond to the present gold standard, but rather those
that do not. We may never achieve perfection but it is
incumbent upon us to recognize that for each success of
a given technique another failure awaits, as does another
paradigm.
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the understanding of different and yet beneficial alternatives suggests that it is unlikely that the gold standard
will be universally successful and the “best” in all situations. In this case as well, variability increases the potential for improved viability of a newer paradigm.
Pressure intensity. The paradigm must consider not
just the timing of the applications of NPWT but also the
intensity of the pressure.The variables regarding the optimal pressure to use have been oversimplified in the current gold standard of -125 mmHg to -150 mmHg when
other factors are considered. The literature from early
Russian scientists Usupov and Yepifanov32 using a rabbit
wound model demonstrated that a pressure of -75 mmHg
provided beneficial healing results, reduction in bacterial burden, and other parameters. They also identified tissue hemorrhage of previously coagulated vessels with
negative pressures more than -120 mmHg to -125
mmHg.32 Additionally, Larichev reviewed all aspects of
the Russian negative pressure therapy experience and
propounded the need for these lower pressures during
therapy as well as other currently novel ideas.33 In contrast, Morykwas et al34 identified pressures of -125 mmHg
to -150 mmHg as ideal for their technology. This became
an integral feature of the gold standard paradigm based
on numerous factors, which include not just successful
healing, but also marketing efforts.34
In 2004, Wackenfors et al35 evaluated varying negative
pressures using an inguinal pig wound model. They used
open-cell polyurethane foam as the wound contact medium and reported that different pressures should be used
to obtain ideal healing for soft versus dense tissue
injuries. They reported that the ideal pressures to promote healing and minimize adverse effects were 60
mmHg–80 mmHg for soft tissues and 80 mmHg–100
mmHg for denser tissues. They also reported that low
negative pressure during treatment may be beneficial,
especially in soft tissue, to minimize possible ischemic
effects.35
Timmers et al36 evaluated the effects of high negative
pressure. They evaluated the effects of polyurethane and
polyvinyl chloride (PVC) open-cell foams on forearm
skin of healthy volunteers. This limited study found a significant increase in cutaneous blood flow with both
foams using -300 mmHg. The obvious criticisms of these
findings surround the application of the foam to intact
skin and the use of healthy volunteers.36
A 2005 literature review37 juxtaposed findings regarding NPWT techniques and other related factors, as
reported in the previously mentioned studies.32–36 The
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