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                     INTRODUCTION TO NPWT
The management of difficult to heal wounds has always been a cause of concern for the treating
clinicians.  There  has  been a  tremendous  increase  in  the  number  of  patients  presenting  with
difficult to heal wounds. The conventional techniques have been in use since a long time for the
management of these wounds, yet desired results are not achieved always. Thus a newer novel
technique which might be useful in the difficult to heal wounds and delivering at par or better
results as compared to the conventional techniques is the need of the hour. Despite numerous
advances,  chronic  and other  difficult  to  manage  wounds  continue  to  be  a  challenge  for  the
clinicians. The relatively newer technique negative pressure wound therapy (NPWT) is very
promising and also useful in the management of difficult to heal wounds. 
The use of controlled  levels  of  negative pressure application  has  been shown to accelerate
debridement and promote healing in various types of wounds. This optimum level of negative
pressure appears to be around -125 mmHg below ambient and there is evidence that this is most
effective if applied in a cyclical fashion. Earlier studies used more conventional methods such as
a wall suction apparatus or surgical vacuum bottles for creating the negative pressure.  However,
there were multiple problems present in the use of these conventional methods such as non-
regulated  pressure,  high  protein  loss,  cross  contamination  from  infected  patients,  also  the
mobility of the patient was restricted. 
Hence we introduced the commercial  system for promoting negative pressure wound therapy
also  known  as  vacuum  therapy,  vacuum  sealing  or  topical  negative  pressure  therapy.  This
equipment  called  CCNPWT Wound  Management  system  was  designed  to  overcome  the
problems associated with conventional methods for the creation of negative pressure. The heart
of the system is a microprocessor-controlled vacuum unit that is capable of providing controlled
levels of continuous or intermittent sub-atmospheric pressure ranging from 
-20mmHg to -220 mmHg. 
NPWT is an efficacious & cost-effective management approach for almost all wound etiology
such as pressure ulcers, trauma wounds, acute & chronic wounds, diabetic foot ulcers, venous leg
ulcers, burns, post-surgical wounds, pre & post-op flaps/grafts. 
MECHANISM OF ACTION
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 hermetically sealed
with an airtight adhesive polyurethane drape that is permeable to water vapour, 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 (Fig 1). 
Fig 1. Mechanism of action of Negative pressure Wound Therapy
Two main theories prevail regarding the mechanism of action of NPWT used in conjunction with
reticulated open cell foam: 
 The first is based on the theory that tissue strain caused by NPWT has a stimulatory
effect on cellular proliferation. 
 This theory is supported by the fact that tissues have been shown to undergo a 5-20%
strain when subjected to NPWT. 
 This level of strain is assumed to proactively cause cell division and angiogenesis. 
 The second theory is focused on the effects  of NPWT on the mechanical  removal  of
extracellular fluid and oedema. 
 By actively removing fluid at the wound bed it is felt that the local micro-circulation is
improved and secondary necrosis is reduced. 
This hypothesis is supported by studies which have shown reduced need for debridement at the
time of ‘second look’ for wounds treated with NPWT. It is also supported by in vivo studies
using a porcine model which have demonstrated acceleration in capillary formation and increase
in luminal area in wounds treated with NPWT
      
   
Fig 2. Dressing application technique
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.  Some  of  the
mechanisms of action have been demonstrated experimentally and clinically. 
The effects of NPWT on open wound can be summarised as follows:
 Reduction of the wound area due to negative pressure acting on the foam, pulls together
the edges of the wound (wound retraction)
 Stimulation of granulation tissue formation in an optimally moist wound environment; in
several situations even over brady trophic tissue such as tendons and bone NPWT was
able to stimulate granulation tissue formation
 Continuation of effective mechanical wound cleansing (removal of small tissue debris by
suction)
 Effective biochemical reduction of the fluid concentration of wound healing-impairing
proteases (such as elastase)—in the first days
 Reliable,  continuous  removal  of  wound  exudate  (and,  consequently,  fewer  dressing
changes) within a closed system
 Pressure-related  reduction  of  interstitial  oedema  with  consecutive  improvement  of
microcirculation, stimulation of blood flow and oxygenation
 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  meticillin-resistant  Staphylococcus  aureus (MRSA)-infected  wounds.
Thus, it
 also reduces the risk of cross-infections and development of resistance within the hospital
 Transparent  dressing  permits  continuous  clinical  monitoring  of  the  surrounding  skin
through the film with which the wound has been sealed
 Odourless 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
 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.
Patient comfort
• Easy and early patient mobilisation
• Visually appealing dressing method due to clean, exudate-free dressing conditions even during
mobilisation.