Wound VAC (Negative Pressure) Therapy
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Direct answer
Suction at minus 50 to minus 150 millimetres of mercury (typically minus 125), applied through reticulated foam sealed under an adhesive drape, does four things at once: removes exudate, reduces oedema, draws wound edges together (macrostrain) and stretches cells into proliferation and angiogenesis (microstrain), all inside a moist protected environment. Dressings change every 48 to 72 hours. The headline indications are diabetic foot and pressure ulcers, dehisced sternotomy and laparotomy wounds, open abdomen management and skin-graft bolstering; the absolute contraindications are malignancy in the wound, untreated osteomyelitis, necrotic eschar, unexplored fistula and exposed vessels or anastomoses without protective cover. Negative pressure over the wrong wound is not neutral — it is harmful.
What you must remember
- Settings: minus 75 to minus 125 mmHg for most adult wounds; continuous mode for the first 48 hours, then intermittent cycles (about 5 on, 2 off) to accelerate granulation.
- Foam choice: black polyurethane foam (hydrophobic, most aggressive granulation) as default; white polyvinyl alcohol foam (hydrophilic, gentler) for tunnels and fragile tissue; silver-impregnated foam for high bioburden.
- Change interval: every 48–72 hours in contaminated wounds, twice weekly once clean; topical lidocaine instillation eases painful removals.
- Mechanism vocabulary: macrostrain (wound contraction and edge drawing) and microstrain (cytoskeletal stretch driving proliferation and VEGF-mediated angiogenesis) — the viva words beyond "it sucks fluid".
- Indication highlights: diabetic foot ulcers (trial evidence of better healing and fewer amputations), pressure injuries, sternal wound dehiscence, open abdomen with delayed fascial closure, graft and flap bolstering, exuding surgical wounds.
- Absolute contraindications: necrotic eschar and unexplored fistula (debride and define first), malignancy in the wound, untreated osteomyelitis, exposed major vessels, anastomoses and organs without interposed protection.
- Instillation variant: NPWTi — cycles of saline or antiseptic instillation and dwell — for heavily contaminated and exuding wounds.
- Indian context: proprietary kits cost thousands of rupees per change; improvised bottle- and wall-suction negative pressure systems, validated in Indian units, extend the same physics to ward patients who cannot pay for disposables.
A worked fortnight on a diabetic foot wound
A 55-year-old diabetic, post-debridement, has a 6 by 4 cm plantar forefoot wound with exposed tendon, offloaded in a cast. Day 1: black foam trimmed slightly smaller than the wound, a non-adherent contact layer over the tendon, adhesive seal, minus 125 mmHg continuous. Day 3: first change — oedema visibly reduced, granulation brighter; switch to intermittent mode. Week 2: granulation covers the tendon, so a split-skin graft is planned with VAC as the bolster for three to five days — take on a plantar convexity is markedly better under negative pressure than with classic tie-overs. The counter-example from the same clinic: an identical-looking wound with slough and devitalised bone at its base — VAC placed on necrotic tissue achieves nothing and hides osteomyelitis; debridement and bone biopsy come first, the machine second.
Where students slip
Applying VAC to unsuitable wounds is the planted error in every exam stem: necrotic eschar (nothing grows on dead tissue), an unexplored fistula (suction pulls enteric content continuously), malignancy in the wound (bleeding and tumour propagation). The second slip is foam laid directly on exposed vessels or a fresh anastomosis without an interposed protective layer — erosion and catastrophic haemorrhage are documented. When the viva asks how it works beyond fluid removal, the winning words are macrostrain, microstrain and mechanotransduction-stimulated angiogenesis; "vacuum dries the wound" is the losing answer.
Frequently asked questions
What suction pressure and mode are standard in VAC therapy?
Typically minus 125 mmHg (range minus 50 to minus 150), continuous for the first 48 hours and then intermittent cycles to drive granulation.
How often are VAC dressings changed?
Every 48–72 hours in contaminated wounds, stretching to twice weekly as the wound cleans; removal pain is blunted with topical lidocaine.
What are the absolute contraindications to negative pressure wound therapy?
Untreated osteomyelitis, necrotic eschar, malignancy in the wound, unexplored fistulae, and exposed major vessels or anastomoses without protective interposition.
How does VAC help split-skin grafts?
As a bolster it holds the graft uniformly onto irregular or convex surfaces, evacuates haematoma and serum, and improves take — especially over mobile, exuding beds.
Is low-cost NPWT feasible where commercial kits are unaffordable?
Yes — improvised bottle- and wall-suction systems reproduce the physics of commercial devices and have been used successfully in Indian wards when disposables are out of reach.