maple
Canadian Medical Alliance for the Preservation of the Lower Extremity
Negative Pressure Wound Therapy (NPWT)
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Negative Pressure Wound Therapy (NPWT) is a specialized
technique using a vacuum pump on the wound, in order
to speed healing.
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The technique begins with a specialized open-cell foam
dressing, like those below, which is cut and shaped to fit
the contours of the wound. It is then applied directly to the
wound. Sometimes gauze (below right) is used instead of foam.
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To the right we see an example of the black foam
applied to a wound.
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The foam dressing is then covered by an occlusive
plastic seal that adheres to the skin. A drainage
tube extends from the dressing to a container,
where the waste from the wound from the wound
is deposited.
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The vacuum is powered by an electric pump unit,
such as those seen below. When the vacuum is
turned on, a negative pressure is created typically
at -125mmHg, providing suction.
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The suction may be applied continuously or, more commonly, intermittently.
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NPWT is sometimes used with instillation--introducing fluids to flush through the wound. A variety of instillation products have been tried, such as antibiotic solutions, though there is some evidence that instillation with simple saline works just as well.
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The dressing is changed about 2-3 times per week, more frequently if there is a lot of drainage or if the seal is damaged.
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NPWT is generally considered useful for chronic wounds, acute wounds, traumatic wounds, burns, ulcers, flaps and grafts, and wound dehissence (where the wound edges are pulling apart).
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NPWT is not used in the presence of malignancy, osteomyelitis (bone infection), necrotic tissues, over blood vessels, organs, nerves or unexplored fistulae.
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How Does A Vacuum Help A Wound?
The vacuum has at least eight positive effects on wounds.
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The vacuum sucks away exudate, the fluid produced by the
wound. This fluid is commonly filled with dead white
blood cells, pus, and other cellular debris that may inhibit
wound healing.
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Along with the exudate, the vacuum removes bacteria,
which are always present in a wound, continually trying to
spread into the body.
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The vacuum decreases edema and stagnant interstitial fluid (the fluid present between our cells). When there is a lot of this fluid, it can interfere with how well arterial blood, required for healing, is able to reach the wound.
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With the exudate gone from the wound, new blood flow is drawn into the wound space by the negative pressure. Blood supplies the wound with oxygen, fresh cells, and the building blocks for proper wound healing.
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Wounds that are too dry don't heal well, and the vacuum promotes a moist environment conducive to healing.
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NPWT creates local changes at a microscopic level. The foam applied to the wound is made up of a firm, open cell material (a magnified example is seen below left). This foam bears down on the red, wound bed. When suction is applied through negative pressure, the segments of wound bed between the black foam struts is drawn upwards (below right). The budding effect created by NPWT is known as microdeformation.
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Microdeformation results in new capillary growth into the wound, (granulation tissue), promoting cell growth to fill the void of the wound.
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The suction of the vacuum not only draws the wound bed and circulation vertically up into the void of the wound, it also draws the wound edges together horizontally across the wound, gradually decreasing the circumference of the ulceration. This process is known as macrodeformation, an example of which is below.
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​​​Evidence for NPWT
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A recent (2018) Cochrane review, (produced by a group of non-biased individuals without a financial conflict of interest in the topic being researched), was performed with a desire to determine if the number of patients healed with NPWT and the speed of healing were improved with NPWT. They examined eleven randomized controlled studies of 972 patients. While critical of imprecision and potential bias of some of the studies reviewed, they concluded that NPWT "may increase the proportion of wounds healed and reduce time to healing."
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A 2019 British review looked at seven studies produced between 2008 and 2018. As with the Cochrane review, the researchers had criticisms of the studies in the literature, such as small sample sizes, and that many studies did not use the preferred reporting trials. However, they found that "All the included studies reported that NPWT led to better clinical outcomes when compared to standard treatment."
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Our Bottom Line on Negative Pressure Wound Therapy
We at Canadian MAPLE like, use, and recommend NPWT.
NPWT appears to speed the healing of wounds, and we have found it particularly
helpful for difficult, non-responsive wounds, where patients have had difficulty in
healing through traditional means. In these patients, NPWT often seems to kick
start a healing response that is difficult for a patient's body to initiate on its own.
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One small downside is that while the motors of the bedside NPWT units are not
loud, they are not silent, and they can be distracting for some patients, particularly
on the intermittent setting, where the motor is repeatedly turning off, then kicking
back on. Some patients use music or white noise to mask the sound of the NPWT
unit.
In our experience, NPWT is most useful in a hospital setting, where patients are
kept relatively sedentary and the negative pressure seal can be monitored by
nursing staff.
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There are home versions of NPWT, an example is seen to the right, that employ
a unit that has no motor. Instead, suction is initiated by pulling a plunger. For
some patients, particularly patients with wounds on the top and side of the foot,
locations that are less likely to be disrupted by weight bearing, these units can
be very effective.
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However, some may find the seal of the NPWT to be difficult to maintain when
they are at home, walking, and carrying on with normal activities. This is
particularly true when patients are more active, and especially when the ulcer
is located on the bottom of the foot, where the seal is most likely to be disrupted
by walking.
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There are literally thousands of studies on negative pressure wound therapy, and hundreds for its use in the neuropathic foot. Suffice it to say that the vast majority of studies on NPWT show positive effects. An early example from the prestigious Lancet found more wounds healed with NPWT than with standard therapy, and the wounds healed faster, too.
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In fact, there are enough studies out there on NPWT that we can look at many studies at once to draw conclusions over larger patient populations.
As one example, a 2017 review looked at 691 studies, chose what they felt were the best eleven, all randomized controlled trials, totaling 1,044 patients. They found that compared with standard dressing changes, NPWT had a higher rate, shorter healing times, greater reduction of wound area and wound depth, and patients suffered fewer amputations. They also found NPWT to be more cost-effective than standard dressing changes.
"A vacuum is a hell of a lot better than some of the stuff nature replaces it with."
--Tennessee Williams
As the black foam struts hold down the red wound bed, the vacuum draws up buds between the black foam struts, promoting new capillary growth. This is known as microdeformation.
In the photo below we see a chronic ulcer of several years duration that had been non-responsive to earlier treatments.
Here we see the left and right wound edges beginning to be drawn together from vacuum created by Negative Pressure Wound Therapy.
The wound is approaching closure. The drawing together of the wound edges is known as macrodeformation.
Liu Z, Dumville JC, Hinchliffe RJ, Cullum N, Game F, Stubbs N, Sweeting M, Peinemann F. Negative pressure wound therapy for treating foot wounds in people with diabetes mellitus. Cochrane Database Syst Rev. 2018 Oct 17;10:CD010318. doi: 10.1002/14651858.CD010318.pub3. PubMed PMID: 30328611; PubMed Central PMCID: PMC6517143.
Wynn M, Freeman S. The efficacy of negative pressure wound therapy for diabetic foot ulcers: A systematised review. J Tissue Viability. 2019 Aug;28(3):152-160. doi: 10.1016/j.jtv.2019.04.001. Epub 2019 Apr 10. Review.
PubMed PMID: 31056407.
Armstrong DG, Lavery LA: Negative pressure wound therapy after partial diabetic foot amputation. Lancet 2005; 366:1704-1710.​
Liu S, He CZ, Cai YT, Xing QP, Guo YZ, Chen ZL, Su JL, Yang LP. Evaluation of negative-pressure wound therapy for patients with diabetic foot ulcers: systematic review and meta-analysis. Ther Clin Risk Manag. 2017 Apr 18;13:533-544. doi: 10.2147/TCRM.S131193. eCollection 2017. Review. PubMed PMID: 28458556; PubMed Central PMCID: MC5403129.
"It is just the feeling of a vacuum,
a void waiting to be filled."
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--Eleanor Catton
The Rehearsal
Kim PJ, Attinger CE, Oliver N, Garwood C, Evans KK, Steinberg JS, Lavery LA. Comparison of Outcomes for Normal Saline and an Antiseptic Solution for Negative-Pressure Wound Therapy with Instillation. Plast Reconstr Surg. 2015 Nov;136(5):657e-64e.
doi: 10.1097/PRS.0000000000001709. PubMed PMID: 26505723.
Above is a wound before starting
Negative Pressure Wound Therapy
This page written by Dr. S A Schumacher
Podiatric Surgeon
Surrey, British Columbia Canada
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Unless otherwise indicated, all clinical images on this page are owned and provided by Dr. S A Schumacher.
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The clinical images may be reproduced for educational purposes with attribution to
Dr. S A Schumacher Surrey, BC Canada
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