NPWT settings and dressing choices made easy
25/05/10 | Complex wounds | Malmsjö M, Borgquist O
Gauze as interface
Gauze is often used because of its conformability and ease of application to large and irregular wounds . Gauze has become popular among some plastic surgeons for wound-bed preparation before grafting . Gauze may also be a good choice when the cosmetic result is of greater importance, or in cases where scar tissue may restrict movement, for example over joints. During NPWT, the wound filler is pushed into the wound and it is suggested that the technique of using gauze under negative pressure can tamponade superficial bleeding .
Gauze is a good wound filler, especially when circumstances are extreme. The use of gauze in NPWT is described by Jeffery et al 2009, when treating wounds to military personnel caused by landmines and other explosive devices . There are no reported problems with ingrowth of granulation tissue into gauze in NPWT  and therefore no wound contact layer is needed. It is important to note that nearly all gauze used in NPWT has been a particular type of cotton gauze (Kerlix®, AMD) , which may provide antimicrobial control since it is impregnated with polyhexamethylene biguanide (PHMB).
HOW OFTEN SHOULD THE NPWT DRESSING BE CHANGED?
The frequency of dressing changes required will depend on the type of dressing used but also the wound type. The usual recommendations are to change foam dressings every 48 hours [47,48]. This is because the foam needs to be changed before ingrowth becomes a problem .
For gauze or a non-adherent wound contact layer, ingrowth is unlikely and dressing changes can probably be less frequent. It is currently recommended that gauze dressings are changed two or three times a week .
WHAT LEVEL OF NEGATIVE PRESSURE SHOULD BE USED?
What pressure is the gold standard?
There are currently no detailed clinical guidelines regarding the adjustment of negative pressure levels to suit the individual wound. The most common pressure level used (-125mmHg) is based on a limited study on pigs carried out in 1997 . These high levels of negative pressure can sometimes cause pain and therefore need to be reduced [48,37].
We know from preclinical studies that the maximum biological effects on the wound edges, in terms of wound contraction , regional blood flow  and the formation of granulation tissue [21,34], are achieved at -80mmHg. In addition, clinical studies have shown that negative pressure levels below -125mmHg have resulted in excellent wound healing . A series of clinical cases found that wound healing was similar when using -125mmHg and -75mmHg .
What pressure may be used if there is pain or a risk of ischaemia?
Blood flow is known to be decreased in the most superficial layers of the wound bed tissue (0.5cm from the wound edge) and increased in deeper layers of the tissue (2.5cm from the wound edge) [25,26,51,53] (Fig 2). The decrease in blood flow is a result of the wound dressing being pushed into and compressing the tissue . In a wound that is not at risk of ischaemia, it is probable that the combination of increased and decreased blood flow is advantageous for the wound healing process. Increased blood flow leads to improved oxygen and nutrient supply to the tissue, as well as improved penetration of antibiotics and the removal of waste products. The reduction in blood flow stimulates angiogenesis, which will promote granulation tissue formation.