Vacuum assisted closure (VAC) has been used in human medicine to manage a variety of wounds that are typically labeled "difficult to treat." Wounds involving pressure points, diabetic patients, and chronic nonhealing conditions may respond to VAC therapy. The method of treating wounds has been expanded to other uses, including securing free grafts to a wound be and managing septic peritonitis. This presentation will discuss the basics of VAC therapy and its use in veterinary patients.
Negative pressure applied to a wound (vacuum) has been shown to result in rapid formation of granulation tissue and minimization of peri-wound edema. Experimental data suggests the best effects are with intermittent, controlled vacuum. This form of negative pressure can be achieved with specialized, regulated vacuums that are commercially available, but somewhat more expensive than continuous negative pressure. The cessation and reapplication of negative pressure is also associated with a level of discomfort in people and animals. This discomfort is understood and therefore tolerated in human patients, but not veterinary patients.
Our veterinary patients usually undergo sedation for bandage changes, and concurrent analgesic medications are still active when the vacuum is first applied, thus eliminating the initial application of negative pressure to the wound bed. This eliminates the pain associated with the vacuum, and constant vacuum is then tolerated by dogs and cats. If the vacuum remains intact, pain is minimal. Small vacuum pumps may be taken outside during leash walks for dogs; the pump is maintained on battery power until the patient returns indoors and the power cord reconnected to the electrical outlet. If a small pump is not used, the tubing connected to the pump may be obstructed with a clamp to maintain negative pressure for short duration disconnection from the pump for walking outside. Some of the negative pressure may be lost, but usually not so much as to result in pain upon re-application of the pump and vacuum.
Benefits of the VAC include wound coverage, active drainage of exudate in a closed system, facilitation and improvement of second intention healing or secondary closure, adherence of skin grafts, and perhaps management of severe septic peritonitis. We have investigated a technique of vacuum assisted peritoneal drainage (VAPD) in dogs at the University of Georgia.
Some of the advantages of the system include a simple, easy to manage bandage, the ability to trend the gross character and quantity of drainage, and improved and more rapid healing of wounds. The typical modified Robert Jones bandage is not required with the VAC. The adhesive drape material excludes the environment, and the negative pressure obviates the need for an absorptive layer. Thus the patient has improved mobility and is not hindered by bulky bandage material, which may increase patient comfort during the healing process. The active drainage and collection of wound fluid eliminates the need for frequent bandage changes and decreases the potential for contamination that is present during "strike through" with a modified Robert Jones bandage. Active drainage allows for bandaging of areas typically not easily bandaged with the modified Robert Jones system as well as trending of wound fluid character and volume. The benefit of cytologic examination of the fluid, however, has not been proven. The problems with cytologic examination likely result from the fluid accumulation outside of the body in a cool environment and the duration of collection allows for bacterial growth in the collection system.
The VAC has been associated with decreased bacterial numbers in the wound bed, however. It also is well known that VAC therapy encourages granulation tissue ingrowth. Other, unproven benefits are minimization of dead space and allowing small areas of wound debridement by altering the contact layer of the VAC bandage.
The mechanism of increasing wound healing likely relates to mechanical and biochemical factors. Mechanical stress on the tissue alters its healing, much like that of the Ilizarov technique of encouraging bone formation and lengthening or replacing diaphyseal bone. Granulation ingrowth is followed by fibrous connective tissue, cartilage, and bone formation in that model. The mechanical stress of the VAC is used for soft tissue healing, and closure is by second intention, secondary closure, or skin flap or graft application. Stress on the tissue alters its DNA transcription of cytokines, which will have an effect on wound healing as well.
Diminution of peri-wound edema likely plays a role in the improved healing seen with the VAC. Mechanically removing interstitial fluid due to the application of negative pressure to the wound occurs in the tissue surrounding the wound. Less interstitial fluid allows capillaries to open and flow freely. This restoration of flow, increased capillary blood velocity, and stabilization of capillary membranes has been shown experimentally. There is also removal of wound fluid and the associated cytokines within that fluid. This changes the fluid present in the wound, and with the alteration of DNA transcription related to the mechanical stress may result in the improved formation of granulation tissue.
The VAC bandage requires the following: 1) contact layer 2) means of applying negative pressure to the contact layer 3) seal to exclude the environment and allow the formation of a vacuum in the wound 4) connection to a regulated pump for proper maintenance of the right level of negative pressure in the wound. A commercially available system is manufactured by KCI® Animal Health (San Antonio, TX).
Prior to application of the VAC, the wound must be aseptically clipped of hair in a wide margin around the wound edges, debrided, and lavaged as normal. Allowing at least 24 hours of healing after surgical debridement is necessary to avoid continual hemorrhage when negative pressure is applied to the wound bed. The wound should no longer be undergoing necrosis prior to the application of the VAC, which requires normal wound care with surgical debridement and wet-to-dry bandages for debridement as normal prior to granulation of the wound. The skin will require repeated clipping of hair with a #40 blade if time has passed for debridement with a wet-to-dry bandage or if the VAC is used for a number of days, as hair growth will block the ability of an adhesive drape to remain in intimate contact with the skin. Lifting of the adhesive drape will cause loss of vacuum under the bandage, which will decrease wound healing, cause accumulation of fluid in the wound, and may result in significant environmental contamination.
The contact layer is made of open cell foam in which every pore is connected with its neighbor. Thus, negative pressure applied to one pore is transmitted to every part of the foam, which is imperative for even application of vacuum to the entire wound surface. If the KCI system is not used, open cell foam may be purchased from www.thefoamfactory.com as speaker/filter foam, which is autoclavable. Its pore sizes are not ideal per experimental data, but may be used if necessary.
The commercially available system supplies adhesive drape material to exclude the environment and allow the formation of negative pressure under it. If the system is not used, IobanTM drape material (3MTM, St. Paul, MN, USA) can be applied over the foam to create a seal. Connection of the foam to the pump requires tubing and contact with the foam. The commercially available system has a connection that is made by cutting a small piece of the adhesive drape upon which an adherent pad with tubing connects to the regulated pump. The tubing is supplied with a clamp for cessation of negative pressure if desired. If the commercially available system is not used, a large bore red rubber feeding tube is tunneled into the foam and connected to suction tubing which is then connected to a pump. The adhesive drape should completely encircle the red rubber catheter to 1) eliminate the entry of air along the length of the tubing (which would destroy the negative pressure of the wound bed) 2) decrease the pain associated with pressing the tube onto the skin of the patient. The suction tubing connects to the red rubber catheter via an adapter.
The suction tubing or commercially available tubing is connected to a regulated pump that has a collecting reservoir. The pump used MUST have a regulator and should be set to 125 mm Hg for wounds or 75 mm Hg for skin grafts or VAPD. The commercially available pump is easily set to the desired pressure. It is beneficial in that the pump is only active when needed and senses the pressure within the dressing, automatically shutting off when the pressure is met. This automated system is quieter and very desirable in a busy practice. Other regulated pumps used for surgery can be set to a desired pressure but remain "on" at all times, which can be a bit noisy.
Once the pump is connected to the bandage, the foam should become small and firm, indicating good vacuum under the adhesive layer. Listen to the bandage carefully for air entering the site - this "leakage" will interfere with the ability to establish vacuum across the entire wound bed. Air entry can be treated with application of more adhesive drape material or medical adhesive to sites of leakage. Management of the bandage is simple and merely requires checking the foam for firmness and small size and ensuring no air leaks and adequate maintenance of the desired pressure. The vast majority of our VAC patients are maintained in the ward and not in the intensive care unit.
We change the bandage within the first 24 h of application, then as needed (every 2–3 d). Skin grafts held in place with the VAC should NOT be changed until at least 3–5 days after application. Premature graft evaluation and manipulation will disrupt fibrin adhesion and vascular anastomoses that are vital to graft survival.
Contraindications to VAC therapy include malignancy, exposed vascular structures, active hemorrhage, coagulopathy, presence of necrotic tissue, and osteomyelitis. Small areas of debridement can be done by placing 4x4 gauze sponges moistened with saline to areas that require debridement - the VAC is NOT made for debridement, as the open cell foam is largely non-adherent. The VAC is NOT made for eliminating dead space, but we have found that sequentially decreasing the size of the foam can accomplish some of this function.
Veterinary reports of VAC use include distal limb wounds and burn wounds. A new report on VAPD is pending publication. Important concepts to remember include: 1) avoid pressure of the vacuum tubing along the skin 2) do not contact foam with intact skin, as granulation tissue will grow through the epithelium 3) clip the hair repeatedly if hair growth interferes with adhesion of the drape to the skin.