Negative pressure wound therapy

Continuous/intermittent application of subatmospheric pressure to system

Exact mechanism is somewhat unclear

Advantages

• Significantly reduce the number of dressing changes required for complex or dirty wounds - decreasing pain
• Easier to tailor and manage in position
• Particularly pronounced effect in diabetic patients

Disadvantages

• Have to carry pump
• ?Cost - however consider that amount of wound care is actually drastically reduced

Contraindications

• Exposed vital structures (organs, blood vessels, vascular grafts) - leads to tissue erosion
  • Need to wait for granulation layer to form, or tissue graft/flap is done
  • Can MAYBE use barrier dressings sometimes but experienced hands only
• Presence of malignancy

Relative contraindications

• Ischaemic wounds - no benefit demonstrated, may worsen ischaemia
• Ongoing infection or devilatised tissue - debride and treat infection first
• Fragile skin (age, corticosteroid use, etc)
• Adhesive allergy

Indications

• Elective and emergency laparotomy wounds for high-risk patients
• Acute surgical wounds eg debridement - can be applied immediately post op
• After grafting - associated with significant reduction in loss of graft area
• Acute open wounds - faster wound closure
• Burns - maybe
• Traumatic wounds - primary advantage is ease of application/dressing changes
• Chronic wounds - provided wound is well-vascularised - check pulses, and if not present, should evaluate arteries prior to placement
  • DFU
  • Pressure ulcer
  • Open abdomen
  • Venous stasis ulcers - particularly useful in preparing wound bed for graft and managing exudate
• Prophylactic use on large surgical wounds to prevent SSI and dehiscence - more RCTs needed
  • PREVENA and PICO probably prevent SSI and dehiscence for emergency wounds - however more research needed
  • Moderate certainty that these also reduce SSI in elective setting. The effect is likely more pronounced where other risk factors are present, such as diabetes, smoking, obesity and prolonged operating time.

Placement:

1. Trim foam to fit - don't extend beyond edges of wound
2. Secure beneath adhesive sheet
3. Cut hole in adhesive
4. Suction port placed and connected to pump
5. Pressure between -50 and -175 (commonly -125)

Direct effects

• Moist and warm environment, ideal for wound healing
• Fluid removed - first from wound, then from interstitial space
• Positive pressure to wound surface
• Foam contracts, drawing edges together or bolstering grafts/flaps down

Indirect effects

• Reduced perfusion due to positive pressure. Ischaemia leads to increased granulation - more prominent with intermittent or variable pressure
• Diminished inflammatory response
• Altered bacterial burden
• Changes in biochemistry

Complications

• Wounds often become malodourous however infection is actually rare
  • Try adding Aquacel silver interposition layer, or hydrotherapy at time of dressing change, or withholding VAC for a day or two, or using some topical metronidazole
• Pain with dressing change - dry saline soak off or soaking in LA without adrenaline
• Bleeding - severe haemorrhage can occur during removal of foam adherent to granulation tissue
• Infection - often due to pre-existing inadequately treated infection
  • Stop NPWT, irrigate and debride, wound cultures, empiric Abx

Specific systems

PREVENA

• 125mmHg pressure, captures effluent into separate canister

PICO

• 80mmHg, absorbent pad instead of canister (capacity 200ml)