Wound Therapy

Negative Pressure Wound Therapy, or "NPWT", is a "topical treatment used to promote healing in acute and chronic wounds by applying negative pressure to the wound bed."[1]  Negative pressure simply means that the pressure inside of a contained area (in our case the wound) is made less than the atmospheric pressure outside of the contained area; this is obtained by using suction to remove air and fluid from the contained area.

Negative pressure has been used for over a century in healthcare.  For much of the 20th century it was used in the form of open suction to remove fluid and debris from wounds and surgical incisions.  It wasn't until the 1980s that closed suction was researched and used in treatment. This is an important distinction, because closed suction means that the wound is actually sealed (through dressings or other means) around the suction tube so that a true negative pressure environment occurs. In the 1980s, several important studies were done to show the efficacy of negative pressure in helping wounds to heal.  Russian research teams published studies describing the use of funnel or chamber type devices, encircling the wound, and applying suction to the wound at specific time intervals. This funnel would sit over the wound and create a seal between the healthy skin and the funnel.  During this same period another team of researchers was experimenting with surgically inserting drainage tubes into the wound and then sealing the wound (surgically) which created a negative pressure environment.

The results of these early studies showed that using negative pressure after wound debridement helped to eliminate bacteria growth and infection in the wound, and thus helped the healing process.  These trials also showed that using too low of a negative pressure could result in tissue edema and separation of adjacent muscle fibers.

In 1997 the most comprehensive study was published regarding the use of NPWT, by Morykwas and Argenta (two researchers from Wake Forest University). They published a study using an open cell foam instead of gauze; they sealed the wound with an adhesive drape and applied negative pressure. Although this is similar to at least one earlier study, it was a landmark study because they were not only testing the clearance of bacteria from the wound but they were also testing the effects of NPWT on blood flow, granulation tissue formation, and nutrient flow with the use of negative pressure (these will be covered in more detail in a later section).

This study showed that negative pressure positively affects many areas of wound healing above and beyond the clearance of bacteria and exudate from the wound.  After the publication of this study several companies began offering NPWT products for use in hospitals and nursing homes (see Competitors section).

Because this therapy is still incredibly young, many studies are still being done to fully understand all of the mechanisms involved in Negative Pressure Wound Therapy and to understand the best ways to use this treatment.  These studies done in the 80's and 90's were the landmark studies that demonstrated the efficacy of NPWT.

Treatment of the Wound

The basic treatment steps are usually the same no matter the wound.  The wound bed must be prepared for healing, then the wound must be kept in bacterial balance, the wound space should be filled and covered as soon as possible, and new tissue formation should be promoted to prepare the wound for healing or surgical closure.  These last two steps are where NPWT resides in the treatment regimen.

Wound Bed Preparation

The first thing a care provider will have to look for is necrotic tissue. Necrosis happens when cells and tissue die prematurely.  Necrotic tissue does not send the same chemical signals to the immune system as normal cells, so the body will not take care of these cells itself.  Medical intervention is usually necessary to remove necrotic tissue.  This is very common in many types of wounds.

It is important to remove the necrotic tissue so that healthy tissue can begin the healing process.  Removal of the necrotic tissue requires the care provider to debride the wound (removal using surgical, mechanical, chemical, or other means) to expose the healthy tissue below.  Once it has been debrided, the wound will be irrigated and cleansed to remove any bacteria that could affect the healing process.

Dressings

The dressings used with a Negative Pressure Wound Therapy (NPWT) device are what allow the device to function and they are what help the wound to heal. Dressings fill the wounds to retain structure, they provide a protective barrier, maintain a moist environment, and they can deliver antimicrobials.  However, incorrect use of dressings can cause more harm than good, and can render a course of NPWT treatment ineffective.  There are two types of dressings currently used in NPWT: gauze based dressings, and foam based dressings.  Both have been proven clinically effective in helping to treat wounds, and each has their benefits and their disadvantages.

Comparison

The foam based dressings are easier to install in the wound for treatment; the gauze based dressings are more complex and require more steps to install.  Although the foam based dressings can be used with a non-adherent lining, this is not the general practice, and the foam can cause some issues being in direct contact with the wound.

The foam can become attached to the wound as the healing process occurs, and when the foam is removed it can tear the surface of the wound that had healed.  Smith and Nephew recommends that the wound be irrigated with saline for 15-30 minutes before the gauze is removed to help try to prevent this from happening.  However, because the gauze dressings use a non-adherent lining they are able to be removed with relative ease and without damaging the wound bed.

The non-adherent gauze lining also allows for more options when treating the wound, because it is in direct contact with the wound.  Dressings with silver alginates have been shown to moisten the wound while also providing an antimicrobial layer.  These linings can also be used with foam dressings, but it is much less common.

[1] Miller Michael, M. (2005) New Microvascular Blood Flow Research Challenges Practice Protocols in Negative Pressure Wound Therapy, Wounds, 17, (10), 290-294