Traditional negative pressure technology involves the use of a medical grade foam that is attached to a non-collapsible evacuation tube attached to a vacuum source. The foam can be reticulated polyurethane black ether foam (Granufoam, 3M + KCI), foam impregnated with silver, dense hydrophilic white foam, or perforated foam. The packing may not be a foam but an antimicrobial packing product. The reticulated foam dressing serves both to absorb wound exudate and to distribute evenly the negative pressure over the entire wound. An adhesive drape consisting of a transparent film is placed over the foam dressing and the tube, extending 3 to 5 cm beyond the edges of the wound, and is affixed to intact periwound skin, creating an airtight compartment over the wound bed. A perforated contact layer or petrolatum-impregnated gauze can be used as contact layers in more shallow wounds such as chest wounds. Antimicrobial medical honey, collagen dermal templates, hydrolyzed collagen powder/gel, amniotic-based products, or bilayer cellular dressings can be packed into the wound before closure. The evacuation tube is connected to a collection vessel, which is connected to an adjustable vacuum pump. The pump creates a negative pressure that pulls effluent from the wound into a collection vessel. Typically, the vacuum pump can be programmed to provide various amounts of negative pressure on an intermittent or continuous basis depending on the wound type.
Traditional NPWT is indicated for a variety of wounds, especially wounds with significant tissue defects, edema, and exudate production. The advantages of a traditional unit include adjustable pressure, ability to instill antimicrobial and cleansing agents, and intermittent mode, which in some studies has shown increased efficacy due to intermittent hyperemia around wound edges. There are some disadvantages to these units: in-hospital inpatient use leading to longer hospital stay, bulky equipment limiting mobility, limited parent–child interactions due to cumbersome equipment, painful removal and initiation of certain models, and difficulty in obtaining outpatient reimbursement.
Portable NPWT single-use devices have changed the way we manage vacuum-assisted closure (VAC), from lack of internal fillers to canister-less equipment allowing freedom of movement, earlier discharge home, and shorter hospital stays. These units work in part by exudate absorption but mostly evaporation; thus, they should be prescribed for mild to moderately exudative wounds (between 300 and 500 mL). The adhesive materials are often atraumatic (silicon) while the inside absorptive layers are often made up of special technology.
Last month we talked about the PICO (Smith and Nephew Healthcare, Watford, UK) negative pressure system, with its special configuration of an internal absorptive pad. Outer high moisture vapor transmission rate film allows a high rate of evaporation, while the middle absorbent layer absorbs exudate and supports evaporation (20% absorption/80% evaporation); the inner airlock layer allows even distribution of negative pressure across the dressing. Different models can function continuously between 1 and 2 weeks, transmitting 80 mm Hg continuously. This is the most common portable single-use NPWT model we use in my hospitals. The wounds vary from acute trauma, dehisced surgical wounds, pressure injuries, severe extravasations, and wounds related to malignancies to high-risk wounds benefiting from incision support.
Other companies have modified their internal materials to include an absorptive gelling internal layer, such as the Avelle NPWT system by ConvaTec (Deeside, UK). On contact with exudate, the hydrofiber technology gel locks bacteria and exudate, contours to the wound shape to minimize dead space, and provides a soft interphase for the outer wound layer. The middle layer has special fenestrations, designed to augment evaporation, covered by the foam designed to distribute the negative pressure, and finally film that is also breathable, silicone made and continuing into the perforated border that is atraumatic upon removal. The post entry is soft, the connecting tubing is flat, and the working time is up to a month of continuous use and up to 80 mm Hg negative pressure.
These devices combine traditional mechanisms of action of VAC with evaporative capability.