How does negative pressure wound therapy work, and what are its contraindications, complications, and alternatives?

How Negative Pressure Wound Therapy Works

Negative pressure wound therapy (NPWT) applies continuous or intermittent subatmospheric pressure (typically 50-125 mmHg) through a sealed wound dressing connected to a suction pump, which mechanically stimulates healing by increasing blood flow, removing inflammatory exudate, promoting granulation tissue formation, and contracting wound edges. 1, 2

Mechanism of Action

NPWT operates through four primary physiologic mechanisms:

• Macrodeformation: The negative pressure physically contracts wound edges and reduces wound surface area 3, 2
• Fluid evacuation: Active drainage removes wound exudate, inflammatory mediators, and reduces tissue edema—with volumes up to 800ml frequently evacuated 1, 4
• Microdeformation: Mechanical forces at the cellular level stimulate angiogenesis and granulation tissue formation 3, 1
• Enhanced perfusion: Increased local blood flow improves tissue oxygenation and antibiotic delivery to the wound bed (after adequate debridement) 5, 4

The sealed adhesive film prevents evaporative fluid loss, maintains a moist wound environment, reduces malodor, and prevents external bacterial contamination 1

Technical Components

The system consists of three essential elements:

• Wound filler material: Foam (various densities/pore sizes) or gauze placed within the wound cavity—foam is advantageous for large defects while gauze may reduce scar formation and pain in smaller wounds 4, 2
• Semiocclusive barrier: Adhesive film that seals the wound and directs all fluid into the collection system 3, 1
• Suction apparatus: Pump delivering continuous or intermittent negative pressure connected to a fluid collection canister 2, 6

Pressure Settings and Application Modes

Use continuous pressure of 75-80 mmHg for most applications, with pressures as low as 50 mmHg for vulnerable patients (those with exposed tendons, previous anastomosis, or dilated bowel). 1, 5

• Pressures up to 120 mmHg increase fluid drainage but risk excessive desiccation and reduced bowel blood flow in abdominal applications 1, 7
• Never use intermittent or variable pressure modes as they severely compromise wound splinting and moisture regulation 1, 7
• Preclinical data suggests maximal biological effect often occurs at -80 mmHg, though clinical confirmation is lacking 4

Contraindications

Absolute contraindications that must be respected:

• Wounds with residual necrotic tissue or uncontrolled infection—complete surgical debridement extending into healthy tissue is mandatory before NPWT application 5, 8
• Chronic non-surgical diabetic foot ulcers—the IWGDF provides a strong recommendation against NPWT use due to lack of evidence showing benefit over standard care 5, 1
• Purulent wounds—debridement must be performed first 5

Additional contraindications include exposed blood vessels without protection, malignancy in the wound bed, and untreated osteomyelitis 1, 2

Complications

Potential adverse effects requiring vigilance:

• Wound maceration from excessive moisture 1, 5
• Retention of dressing materials in the wound 1
• Paradoxical wound infection 1, 5
• Bleeding, particularly with exposed vessels 3
• Pain during dressing changes 3
• Toxic shock syndrome (rare) 3
• Fistula formation if applied to inadequately debrided wounds 5

Special Considerations for Exposed Structures

When tendons, bone, or other vital structures are exposed:

• Mandatory: Place a large, fenestrated non-adherent interface layer directly over the exposed structure to prevent desiccation and damage during dressing changes 5
• Reduce pressure to 75-80 mmHg (lower than standard 125 mmHg) 5
• Never apply foam directly to exposed tendon without the protective interface layer 5

Clinical Applications Where Evidence Supports Use

Post-operative wounds: NPWT may be considered despite methodological limitations in supporting studies showing benefit in time to healing and proportion healed 1

Open abdomen management: NPWT serves as first-line therapy after complete debridement, effectively managing fluid, preventing contamination, and facilitating delayed primary closure 1

Split skin grafts: Some evidence suggests improved graft take, though study quality is limited 1

Practical Management

• Dressing changes every 2-3 days, with an average of 5 changes required for optimal granulation 5, 8
• Measure evacuated fluid volume for fluid replacement calculations and early identification of blood or fecal contamination 1
• NPWT stimulates healing but does not result in complete epithelialization—it prepares the wound bed for definitive closure 1

Alternatives to NPWT

When NPWT is contraindicated or unavailable:

• Standard moist wound dressings with frequent changes
• Passive drainage systems (e.g., Bogota bag for open abdomen, though significantly less effective at fluid management) 1
• Advanced wound care protocols including appropriate debridement, offloading, and infection control 1

Critical caveat: NPWT requires skills, organization, and cannot replace necessary surgical procedures—it is an adjunct to, not a substitute for, proper wound debridement and management 1, 2