NEGATIVE PRESSURE WOUND THERAPY (NPWT)
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NEGATIVE PRESSURE WOUND THERAPY (NPWT)
Introduction
Negative Pressure Wound Therapy (NPWT) is also known as topical NPWT, Vacuum sealing
technique,  sealed surface wound suction,  Vacuum Assisted Closure (VAC) or Vacuum pack
technique.  This essentially  involves  the application  of sub atmospheric  pressure to a healing
wound with the help of a porous dressing and a drain attached to a vacuum device and covered
with a polyurethane film to make an air tight seal.
Since its introduction in the early 1990’s NPWT has become widely used in the management of
complex  wounds  of  various  etiologies  in  in-  patient  as  well  as  out-  patient  settings  with
significant success. It has now become gold standard of care for wounds like open abdominal
wounds and dehisced sternal wounds following cardiac surgery. It is also now being applied in
home care settings.
Principles and Mechanism of action of NPWT21
- Functional Principle
The  principle  of  NPWT  involves  extending  the  usually  narrowly  defined  suction  effect  of
drainage across the entire area of the wound cavity or surface using an open-pore filler that has
been fitted to the contours of the wound.
To prevent air from being sucked in from the external environment, the wound and the filler that
rests inside or upon the wound are sealed with an airtight adhesive polyurethane drape that is
permeable to water vapor, transparent, and bacteria proof. A connection pad is then applied over
a small hole that has been made in the drape and connected to a vacuum source by means of a
tube.
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Figure 20: Functional Principle of NPWT
MECHANISM OF ACTION
NPWT acts  in  different  ways  to  promote  wound  healing.  The  wound  is  subject  to  suction
pressure  that  is  propagated  through the  wound filler  to  the  wound bed.  This  suction  drains
exudate from the wound and creates a mechanical force in the wound edges that result in an
altered tissue perfusion, angiogenesis and the formation of granulation tissue.
The following effects of NPWT on the wound healing and the affected tissue are considered to
be the clinically significant mechanisms of action for this therapy.
-  Creating A Moist Wound Environment 
A moist environment is vital in wound healing as it facilitates the re- epithelialisation. 
However, in an overly moist wound, exudate may cause infection and maceration, 
leading to damage to the wound edge. Removal of exudate is important to prevent the 
accumulation of necrotic tissue and slough that tend to continually accumulate in wounds
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and alter the biochemical and cellular environment. The accumulation of necrotic tissue
or slough in a wound promotes bacterial colonisation and hinders repair of the wound.
NPWT balances  these effects,  providing a moist  wound environment  while  removing
excess fluid.
-  Removal Of Edema 
Edema causes increased pressure on the wound tissue, which in turn compromises the
microvascular  blood flow, reducing the inflow of nutrients and oxygen. This  reduces
resistance to infections and inhibits healing.
NPWT causes compression of the tissue closest to the surface of the wound, which is 
believed to reduce interstitial edema
-  Mechanical Effects On Wound Edges 
NPWT mechanically stimulates the wound bed and produces a suction pressure on the
wound edges that push onto the wound and contract it. These mechanical effects lead to
tissue remodeling that may facilitate wound closure. It has also has been found that the
wound tissue and the filler material interact on a microscopic level to micro deform the
tissue. These mechanical deformations affect the cytoskeleton of the cells and initiate a
cascade of biological reactions that may accelerate the formation of granulation tissue
and  subsequent  wound  healing  by  stimulating  the  expression  of  angiogenic  growth
factors  and  receptors,  such  as  vascular  endothelial  growth  factor  (VEGF),  VEGF
receptors and the angiopoietin system receptors.
-  Change in perfusion 
NPWT has been shown to improve perfusion in a few studies.
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NPWT exerts compression on the tissue leading to an anti- edematous effect that might
promote perfusion. It has also been speculated that the negative pressure causes a force in
the tissue that opens up the capillaries, increasing flow.
-  Change in bacterial count, bacterial clearance and immunological effects 
NPWT offers a closed system for wound healing, as the adhesive drape provides a barrier
against secondary infection from an external source and has been suggested to reduce the
bacterial load in the wound. However it must be emphasized that the degree of bacterial
colonisation is unrelated to the success or failure of NPWT. It is exceedingly important to
perform  proper  debridement  between  dressing  changes  to  mechanically  remove  the
microorganisms.
-  Molecular mechanisms in wound healing 
It is hypothesized that the NPWT device induces the production of pro-angiogenic factors
and promotes the formation of granulation tissue and healing.
NPWT is also associated with an up-regulation of basic fibroblast growth factor (bFGF)
and extracellular signal-regulated kinase (ERK) 1/2 signaling, which may be involved in
promoting the NPWT-mediated wound healing response.
NPWT also influences the local expression of pro inflammatory cytokines in tissue or 
fluid from acute infected soft-tissue.
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Figure 21: Proposed mechanism of action of NPWT
Advantages of NPWT
- Handling
o Hygienic wound closure—bacteria proof wound dressing for sealing the wound so
no external bacteria can enter the wound and the patient’s own wound bacteria are
not  spread.  This  is  particularly  important  in  the  event  of  contamination  with
problematic bacteria, as in patients with methicillin-resistant Staphylococcus
aureus (MRSA)-infected wounds. Thus, it also reduces the risk of cross-infections
and development of resistance within the hospital
o Transparent dressing permits continuous clinical monitoring of the surrounding 
skin through the film with which the wound has been sealed.
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o Odorless and hygienic dressing technique; constant seeping through the dressing
onto the patient’s clothing and bedding can be avoided, reducing demands on the
nursing staff
o Reduction in the number of required dressing changes (only necessary every two
to three days), which reduces nursing time requirements, particularly in patients
with exudating wounds.
o Compared with other  forms of  wound dressing,  NPWT is easier  to  tailor  and
maintain  in  position.  Almost  every  configuration  of  wound,  including
circumferential extremity wounds and wounds located in proximity to orthopedic
fixation frames, can be managed with relative ease.
- Patient comfort
o Easy  and  early  patient  mobilization:  Accelerated  wound  healing  with  NPWT
significantly reduces the time to wound closure in diabetic patients, returning
these patients to baseline more quickly and improving quality of life.
o Visually appealing dressing method due to clean, exudate-free dressing 
conditions, even during mobilisation.
Treatment Goals with NPWT22
- Provide a temporary wound cover
- Manage wound fluid and edema
- Accelerate patient mobility
- Improve pain management
- Prevent wound progression
- Increase dermal and wound perfusion
- Stimulate formation of granulation tissue
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- Enhance wound bed epithelialization
- Improve matrix material availability
- Reduce bacterial load
- Provide moist wound environment
- Influence expression of genes involved in wound healing
Treatment variables & Precautions
There are a number of different treatment variables related to NPWT that might affect wound
healing. The level of negative pressure, the wound filler material (foam or gauze), the presence
of wound contact layers, the pressure application mode (continuous, intermittent, or variable), or
instillation of fluid may be chosen according to patient needs, disease, wound type and shape.
- Pressure level/ suction strength
Low pressures may be ineffective in draining the exudates while high pressures may be
painful  and  have  a  negative  effect  on  the  microcirculation.  The  preferred  range  of
pressure is between – 50 to -150 mm Hg. However there is no significant evidence to
suggest  an ideal  clinical  range and it  is  suggested that  the negative  pressure may be
adjusted  according to  the circumstances.  For  example  in  patients  with pain it  can be
appropriate to choose a suction setting less than −125 mmHg. Pressures as low as −40
mmHg may be used for the treatment of sensitive, poorly perfused tissue where there is a
risk for ischemia, for example in the case of circumferential dressings, vascular disease,
diabetic foot ulcers (DFUs) and thin skin transplants.
High output wounds will require high pressures for initial few days which may be 
lowered once the exudate lessens.
- Vacuum Source
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Traditional  NPWT  systems  use  an  electrically  powered  pump  to  generate  negative
pressure at the wound bed. Recently portable devices that deliver NPWT without the use
of  an  electrically  powered  pump  have  been  introduced.  These  smaller  light-weight
devices are either battery powered or mechanically  powered, and generate continuous
sub-atmospheric pressure level to the wound bed between −75 and −125 mmHg. These
devices allow the patient to be mobile and independent from hospital’s wall-suction on
the ward and to be treated by NPWT in the home care setting.
Some battery-powered NPWT units use an electronically controlled feedback system that
ensures the maintenance of the selected pressure level even in the presence of small air
leaks, guaranteeing the effectiveness of NPWT. Additionally, audiovisual alarms alert the
staff and the patients to large air leaks (loss of seal),  blockage of the tubing and full
canisters. These therapy units are designed to reduce complication and allow faults to be
promptly recognized.
- Intermittent or continuous mode
NPWT can  be  applied  in  an  intermittent  or  continuous  or  variable  mode.  The  most
common  mode  used  currently  is  the  continuous  mode.  Intermittent  mode  involves
repeatedly switching on and off (usually 5 minutes on to 2 minutes off), while variable
NPWT provides a smooth cycling between two different levels of negative pressure.
NPWT with intermittent suction has been shown to be of benefit for wound healing. This
mode produces a mechanical stimulation of the wound bed (a massaging effect) and a
greater circulatory stimuli, oxygenation and angiogenesis, and presumably a lower risk of
occurrence of ischemic damage. Intermittent mode is however known to cause more pain.
New pressure cycles, without going to 0 mmHg suction, but only lowering the suction, to
50 % for example, should be able to maintain the highest degree of blood vessel
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formation  and  also  a  significant  decrease  in  pain  compared  with  the  traditional
intermittent group.
Under  special  wound  conditions,  when  the  wound  involves  structures  such  as  the
peritoneum, between toes, in tunneling injuries, in sternotomies, in the presence of high
levels of exudate and when using NPWT on grafts or skin flaps, the continuous mode is
the option of choice.
- Wound fillers
The choice of wound filler has been shown to have significant effect on wound healing.
The commonly available material for this include foams and gauges with different pore
sizes, stability etc.
The pressure distribution is similar for gauze and foam in dry wounds and the differences
in performance are related to the structure of the material and its mechanical effects in the
wound. The degrees of micro- and macrodeformation of the wound bed are similar after
NPWT regardless of whether  foam or gauze is used as wound filler.  The increase in
blood flow following NPWT is also similar with all wound fillers.
Wound  contraction  is  more  pronounced  with  foam  than  with  gauze.  Wound  fluid
retention is lower in foam, while more fluid is retained in the wound when using bacteria
and fungus-binding mesh.
Thus NPWT may be tailored to the individual wound type to optimise the effects and
minimize the complications by choosing different wound fillers. The choice of filler may
be made with regard to the morphology of the wound, the wound characteristics,  the
patient feedback, possible infection and scar tissue formation.
o  Wound morphology
Different wounds have different shapes and depths and thus require different fillers.
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Foam may fit better into a wound with a uniform shape, while gauze may be easier to
apply in wounds that have an irregular shape, or with undermining edge since it can be
better  manipulated  to  the  shape  of  the  wounds.  Different  wound  fillers  can  also  be
combined. Over a thin graft or a wound sleeve, the gauze will give a more appropriate
cover. Foam may be advantageous for ‘bridging therapy’ since the foam compresses to a
greater extent and thereby contracts the wound and speeds up the closure.
o Exuding wounds
In heavily exuding wounds, foam at a higher pressure (−120 mmHg) may be useful, 
Since foam is less dense than gauze and a higher level of negative pressure drains the 
wound quicker.
o Wounds at risk of ischemia
In circumstances where there is a risk of ischemia, a lower pressure (−40 to −80 mmHg) 
and using gauze may be considered.
o Infected wounds
Various wound fillers designed for infected wounds include foam with silver, gauze that
is impregnated with PHMB, gauze that is impregnated with silver, bacteria and fungus
binding mesh.
Instillation techniques allow the irrigation of the wound with antiseptic solutions.
o Tendency to form excessive granulation tissue
Areas such as joints, where movement of the skin and the underlying tissue occur, may
benefit from the use of gauze. Foam allows rapid growth of granulation tissue and may be
a better  choice in wounds where large amounts of granulation tissue is  desirable, for
example, postsurgical wounds such as sternotomy wounds.
- Wound contact layers
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Removal  of  foam used  as  fillers  in  NPWT can  lead  to  pain  and  trauma.  When  the
clinician anticipates such complications,  a non- adherent wound contact  layer such as
paraffin or silicon may be placed over the wound bed beneath the wound filler. A wound
contact  layer  also may be placed over vulnerable  structures  such as blood vessels  or
nerves. This wound contact layer may reduce the pain during dressing changes.
However keeping the quality of wound healing in mind, it is important to use wound
contact layers only when there are structures to protect, in order not to slow healing.
- Protection of tissues and organs
NPWT can at times be the only option available to the treating clinician for managing
highly infected and large wounds. However of late there have been reports of serious
associated complications of NPWT.
The  risk  of  right  ventricular  rupture  and  bypass  graft  bleeding  following  NPWT of
mediastinitis is estimated to be between 4–7 % of all cases treated. Severe bleeding of
large blood vessels such as the aorta has also been reported in several patients receiving
NPWT. The incidence of NPWT-related bleeding in patients with exposed blood vessels
or vascular grafts (such as femoral and femoral-popliteal grafts) in groin wounds were
relevant in some studies. Severe bleeding has also been reported in patients receiving
NPWT for burn wounds.
FDA has thus stated that NPWT is contraindicated in certain types of wounds: those with
necrotic tissue with eschar, in non- enteric and unexplored fistulas, where malignancy is
present, in wounds with exposed vasculature, anastomotic sites, exposed nerves, exposed
organs and untreated osteomyelitis.
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It has been suggested that exposed sensitive structures need to be protected either through
the  interposition  of  autologous  tissue  (muscle  flaps)  or  with  heterologous  material
(dermal substitutes) or a number of wound contact layers.
- Pain treatment
Pain is a significant issue associated with NPWT and may affect patient adherence.
In patients that are neuropathic or paraplegic, where the pain is not of a significant nature,
the filler can be used efficiently. In patients with low adherence, especially children and
the  elderly,  and on painful  lesions  (such as  pyoderma gangrenosum, burns,  PUs and
infected wounds), gauze, tends to be better tolerated.
Another option to reduce pain due to NPWT is to prepare the patient by infiltration of the
wound filler with saline solution or local anaesthetics before dressing change.
Clinical Applications5
- Acute wounds
Acute wounds are often traumatic but can also be due to surgical debridement of infected or
necrotic tissue. Wounds requiring extensive surgical debridement often present a wound dressing
challenge due to anatomic location (eg, Fournier's gangrene), the size of the tissue defect, or the
patient's body habitus.
- NPWT dressings can be applied immediately following operative debridement,  which
simplifies postoperative wound care. The ability of the foam and adhesive dressing to
conform to almost any wound contour, shape, or size contributes to the success of NPWT
in these complex wounds.
- NPWT can also be used in conjunction with skin grafts or flaps, which are frequently
needed to cover tissue defects.  NPWT has been used instead of traditional  bolstering
methods to provide skin graft fixation. The NPWT dressing distributes negative pressure
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uniformly over the surface of the fresh graft, immobilizing the graft with less chance of
shearing leading to improved qualitative skin graft take and quantitative improvements in
skin graft success.
- For acute  open wounds,  NPWT is associated  with a  reduced time to wound closure.
NPWT has  also  been  used  to  manage  acute  wounds  resulting  from lower  extremity
fasciotomy,  degloving  injury,  open  amputation,  and  complex  traumatic  wounds  with
exposed tendon, bone, or orthopedic hardware.
- NPWT may also have a particular role in the treatment of burn wounds.
- Chronic wounds
- NPWT may improve the healing of some types of chronic wounds/ulceration, such as
diabetic foot ulcers, pressure ulcers, and open abdomen, provided that the wounds are
well vascularized
- Post-sternotomy mediastinitis is an uncommon but devastating complication of cardiac
surgery which may require the use of NPWT to manage the open wound while awaiting
sternal closure.
- Prophylactic use
Evidence suggests that there may be a prophylactic role for NPWT in reducing the rates 
of surgical site infections.
Indications of NPWT
Trauma & Orthopedic Surgery
- Acute trauma & dermatofasciotomy wounds
NPWT is a tool in the treatment of traumatic wounds and high-risk incisions after surgery in
- contaminated acute wounds (open fractures, penetrating injuries) and wounds with tissue 
defects requiring a step wise procedure.
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- amputation stump resulting from a guillotine-like marginal zone amputation.
- management of motorcycle spoke heel injury
- perineal trauma-related wounds.
- Periprosthetic infections of the hip and knee joint
NPWT is a useful option in the management of early or delayed infections following 
implantation of an endoprosthesis.
- Osteomyelitis and surgical site infection
- Exposed tendon, bone and hardware
NPWT is the treatment of choice in such conditions where plastic surgery procedures 
cannot be used to provide immediate cover to exposed structures.
Acute burns and scalds
The management of burns with their associated high-fluid exudate following burn excision and
skin grafting has always posed a challenge in burn wound care. The ideal dressing should protect
the wound from physical damage and microorganisms; be comfortable and durable; allow high
humidity  at  the  wound;  and  be  able  to  allow  maximal  activity  for  wound  healing  without
retarding or inhibiting any stage of the process. NPWT fulfils all these criteria.
Plastic and reconstructive surgery
NPWT  has  produced  a  change  of  paradigms  within  the  treatment  algorithms  in  traditional
reconstructive surgery.
NPWT may represent a valid alternative to immediate reconstruction in selected cases of acute
complex traumas of the lower limb.
NPWT is a valid tool for reliable  fixation of skin substitutes,  such as tissue-engineered skin
substitute and split skin grafts in all severe traumatised wounds and is associated with improved
graft survival.
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Abdominal Surgery
NPWT is one of the techniques used for temporary abdominal closure (TAC) in laparotomies
with packing, ACS or severe septic intra-abdominal complications.
Cardiovacscular Surgery
While NPWT is the method of choice for management of poststernotomy mediastinitis, its use is
also being advocated over closed sternal incisions to reduce the incidence of deep sternal wound
infection.
Vascular surgery
- Infected blood vessels and vascular grafts
For high-risk surgical patients with a fully exposed infected prosthetic vascular graft, NPWT
along with aggressive debridement and antibiotic therapy is an effective alternative to current
management strategies.
- Lymphocutaneous fistulas
NPWT can be used for the management of lymphocutaneous fistulae resulting from axillary and 
inguinal lymph node dissections as well as surgery of the infra-inguinal vessels.
Non-healing wounds
- Leg ulcers
Although NPWT has been used extensively in non healing leg ulcers, there is limited rigorous
RCT evidence available concerning the clinical effectiveness of NPWT in the treatment of leg
ulcers.
- Pressure ulcers
NPWT is increasingly being used to manage PUs, most likely due to its flexibility, which allows 
the caregivers to insert it into a more complex and articulated therapeutic strategy.
- Diabetic foot ulcer
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The  rationale  for  adopting  NPWT in  DFUs  is  related  to  its  capacity  of  removing  exudate,
protecting the wound from exposure to the environment, reducing odor and helping debridement.
Complications of NPWT5
NPWT  is  generally  safe  and  well  tolerated.  Some  of  the  important  complications  include
bleeding, infection, pain, organ damage, and possibly death. Such complications are most likely
to occur when NPWT is applied to patients whose wounds have devitalized tissue or exposed
vital structures (eg, organs, blood vessels, vascular grafts).
- Bleeding
Bleeding is the most serious complication of NPWT and can occur in hospitals, long-term
care facilities, and at home. Minor bleeding during dressing changes due to granulation
tissue at the base of the wound is common and is best managed with firm pressure to the
wound surface.
Severe hemorrhage can occur during removal of foam that has become adherent to the
granulation tissue below, especially in patients who are anticoagulated, or in patients with
exposed  vessels  or  vascular  grafts.  In  patients  with  severe  bleeding,  direct  pressure
should be applied and emergency services contacted. Surgery may be needed to control
bleeding.
- Infection
Infection related to the use of NPWT is often due to prior wound infection that  was
inadequately controlled prior to initiating NPWT. When infection is suspected the NPWT
dressing in discontinued, the wound irrigated and debrided, wound cultures obtained, and
empiric antibiotics initiated.
- Enterocutaneous fistula
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Contraindications of NPWT21
- Risk of bleeding
NPWT should not be applied in wounds with manifest bleeding or a risk of bleeding, as suction 
could result in a significant blood loss.
Additionally the bleeding can clot the foam and therefore stop any function of the NPWT device.
- Exposed vessels and vascular prostheses
- Necrotic wound bed
Necrotic tissue acts as a barrier to new tissue growth. The use of NPWT must therefore be 
preceded by radical debridement.
- Untreated osteomyelitis
Due to the deep extension of a potential osteomyelitic focus, simple surface treatment is unlikely
to  be  successful.  In  this  case,  treatment  must  include  the  radical  removal  of  the  focus  of
infection.
- Malignant wounds
NPWT is known to promote granulation tissue growth and is therefore used for the purpose of
improving tissue perfusion and enhancing granulation tissue formation.  As a consequence,  it
should not be used in the presence of malignant neoplastic tissue.
NPWT can be useful as a purely palliative measure in inoperable cases. When used as a purely
palliative measure, it allows wounds to be covered in a hygienic and clean manner.
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