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What is NPWT?

Terminology

How does NPWT work?

Key principles of NPWT

What is NPWT? 

Terminology

 

Negative Pressure Wound Therapy (NPWT) has many names: sub-atmospheric pressure device (SPD), topical negative pressure (TNP), vacuum-assisted closure (VAC), vacuum sealing technique (VST), and sealed surface wound suction (SSS). Note that the term VAC is a registered trademark for products from KCI Medical and should, strictly speaking, be reserved for those products. If we want to appear super knowledgeable, you should actually write it like this: VAC™. 

 

In recent years there has been a general agreement to use the term NPWT, and it is this term you will find used mainly in literature. Throughout our website, we also use the term NPWT.

 

You will sometimes encounter strange abbreviations like MPNPWT, which stands for mechanically powered negative pressure. That is when we use mechanically powered pumps ( for example, by a spring mechanism) rather than an electrical pump. Other common abbreviations are tNPWT ( traditional NPWT meaning regular pumps) instead of sNPWT ( single-use disposable pumps). Are you slightly confused by these abbreviations? Don`t worry; you are not alone. 

As the technology matured, new ways of using NPWT were developed. Sometimes NPWT is used on closed surgical wounds - incisional NPWT, abbreviated iNPWT. At other times we use pumps that can irrigate the wound with saline or an antibacterial solution and let the fluid dwell there for some minutes - irrigation and dwell NPWT, abbreviated NPWTid.

What is NPWT?

NPWT involves placing a medium - either gauze or foam- into a wound. This is then covered with a self-adhesive, occlusive plastic drape to create an airtight atmosphere. A tube is connected to this, which leads to a suction pump, thus creating a vacuum in the wound area. 

 

Most pumps are designed to deliver a negative pressure of about minus 80-120 mmHg. This is quite an intense vacuum pressure. If you apply a blood pressure cuff to your arm and pump it up to 120 mmHg, you will appreciate how strong this pressure is - although, with NPWT, it is negative pressure. 

 

The pump must be connected to the wound continuously. It is quite common to change the dressings about every third day; sometimes, longer shift intervals are acceptable. NPWT treatment often lasts for 2-3 weeks. ære koblet til hele tiden. Ofte skifter en bandasjen hver 3. dag. Behandlingen varer ofte i 2-3 uker.

NPWT is generally used to promote granulation tissue formation in a wound. It is essential to understand that NPWT does not promote skin cells to migrate over the wound. Once a wound has filled nicely with granulation tissue, the NPWT is discontinued, and other treatment modalities are then used. 

Despite the advent of numerous new techniques and technologies, negative pressure wound therapy (NPWT) remains a cornerstone of managing complex wounds. In our experience, there is no other modality in wound care treatment that has the same potency to turn a wound in the right direction. 

Figure 1 Once a wound has granulated as nicely as shown in the image above; there is no need to continue the NPWT treatment. 

Wounds with little necrosis that show some signs of granulation tissue here and there often respond well to NPWT treatment. If there is exposed bone or exposed tendon in the wound bed, you will need some experience to use NPWT in these wounds. Tendons and bones do not respond well to NPWT treatment, and you can actually harm these structures if you are not careful. We will discuss these scenarios later on in the chapter.

When used incorrectly, NPWT can cause harm to the patient. One of the biggest dangers is turning off or disconnecting the suction pump for several hours. The airtight dressing, which an NPWT dressing is, will turn into a greenhouse for bacteria. Within a few hours, high concentrations of pathogenic bacteria can develop under the plastic film and, in some cases, cause severe infections and septicemia. 

There are several contraindications concerning the use of NPWT, and it is essential that you are aware of these before you start using NPWT. We will discuss the indications/contraindications later on in the chapter.

How does NPWT work?

When cells in the wound bed are exposed to a vacuum effect, they experience a mechanical deformation. The suction effect pulls on the cells, thus stimulating cell division. Under ideal conditions ( i.e., young patient and clean wound), we sometimes see granulation tissue developing at 5-10 x the speed of natural tissue formation. 

Studies show that NPWT stimulates the growth of new blood vessels in the wound bed ( angiogenesis). The suction also removes exudate and, in that way, keeps the wound cleaner. Many strains of bacteria do not thrive well in a sub-atmospheric environment, and the bacterial burden is often lower when using NPWT. 

Paradoxically, NPWT does not aid the epithelialization process. In the early years of NPWT, caregivers were not aware of this, and some patients used an NPWT pump for several months without achieving healing. Once the wound bed has filled up with granulation tissue, you will actually delay the healing if you continue with NPWT treatment! So the key to success is using NPWT in the right type of wound for the correct amount of time. 

When the wound has filled up with nice granulation tissue, it is time to stop the NPWT and use another treatment modality to encourage skin cells to migrate onto the granulation tissue. The exception to this rule is when we place a split-thickness skin graft onto the granulated wound. Using NPWT for about 5-7 days over the skin graft will help the graft attach to the wound bed. 

Video 1 This film by KCI/3M illustrates how the wound bed responds to the negative pressure.  copyright: 3M/KCI https://www.youtube.com/watch?v=4yPnWIrLKhg

Video 2 An interactive case-based discussion by HMP Education about different aspects of NPWT. This video is over an hour long, but is worth watching as it contains several perals of knowledge. Copyright: HMP Education https://www.youtube.com/watch?v=lPtU-rMg34w

Key principles of  NPWT  

  • Before you start with NPWT you need a plan. What do you want to achieve with the NPWT treatment? Do you want to " kick-start" a wound that seems to have stalled? Do you want to get the granulation tissue forming as quickly as possible? Do you want to use NPWT to control the amount of exudate that comes from a wound? 

 

  • Before you embark on the NPWT, you will also need a solid plan for the logistics. How often are you planning on changing the NPWT dressings, and who will do the dressing changes? What should the patient do when a blockage or leakage alarm goes off when the patient is at home? Who will check several times daily that the pump is working correctly and that the batteries of the pump are charged? Does the patient have access to electricity to charge the pump? If the dressing changes are to occur at your clinic: who will perform the NPWT treatments if you have a few days off work and you are the only one who has some knowledge about NPWT? 

 

  • NPWT will only help to promote granulation tissue. It will not help skin cells to migrate over the wound! 

 

  • NPWT should not be used in wounds with much necrotic tissue, have a high bacterial burden, or are infected, smelly wounds, in cases with untreated osteomyelitis, close to major blood vessels, wounds with fistulas to internal organs, or malignant wounds.

 

  • To make NPWT work, you have to place a medium into the wound; this can be a gauze material or a foam. You cannot simply put a plastic film over a wound and attach a tube to this and apply suction. If you try that, it will end in a disaster ( most likely a severe infection).  

 

  • It is essential to understand that the medium ( gauze or foam) is in contact with the entire wound cavity. If there are pockets where the medium is not in contact, exudate will not be sucked out of these pockets. This can lead to bacteria thriving in these areas and cause infection. This is a fundamental principle of NPWT. This is, for example, important in deep ulcerations like extensive pressure ulcers. You need to be vigilant and ensure that the medium is in contact with the whole wound cavity to avoid an infection. 

 

  • It is essential that the NPWT dressing is as airtight as possible. Some caregivers think that a small leakage does not matter, which is wrong. When the dressing is 100% airtight we see the best results, and there is usually less smell coming from the wound. Our hypothesis is that leakages give rise to a higher bacterial burden developing under the NPWT dressing. We also see more peri-wound skin maceration when the dressing is not entirely airtight. 

 

  • If an NPWT pump is switched off for several hours or is malfunctioning otherwise, this can lead to a severe infection, even septicemia. An NPWT dressing without active suction attached to it behaves like a greenhouse for bacteria. We have witnessed life-threatening septicemia in patients where a pump had been switched off for a whole day. Most producers, therefore, advise changing the entire dressing if the pump has been switched off for more than two-three hours. 

 

  • As technology advances some NPWT pumps have become tiny and user-friendly. However, be aware that the small pumps have limitations concerning what kind of wounds you can use them on. They are less forgiving with respect to leakages and may be more challenging to apply to difficult anatomical areas like the groin area, for example. 

  • We consistently see the best results when we use NPWT with simultaneous irrigation. This type of NPWT involves rinsing the wound bed with saline or antibacterial solutions at regular intervals. NPWT with irrigation allows us to use NPWT in wounds that are less clean or in wounds where we may be worried about infection. The pumps with this capability are significantly more expensive than regular NPWT pumps. We will discuss this technology later on in the chapter. 

Video 3  HMP Education has provided us with a number of excellent webinars concerning NPWT. This video discusses some of the scientific evidence behind NPWT technology. This presentation is 60 minutes long. Copyright: HMP Education https://www.youtube.com/watch?v=eifwxbOZiZ8

Video 4  HMP Education has provided us with a number of excellent webinars concerning NPWT. While this video is maybe more useful for experienced users of NPWT, it also contains some perals for beginners. This presentation is about 58 minutes long. Copyright: HMP Education https://www.youtube.com/watch?v=ZpcECExJFGM