Limited Access Dressing (LAD): The Concept and Applications
The Limited Access Dressing is a combination of intermittent negative pressure (for 30 minutes) and a moist wound dressing (for 31⁄2 hours without negative pressure) that is covered with a transparent polythene material (a total of 3 hours moist dressing and 21 hours negative pressure dressing in a 24- hour period). Negative pressure (up to -30 mmHg) is applied through tubing connected to a suction machine that is then placed under a polythene wound cover.
The material that contacts the wound LAD is classified into 2 groups:
Hydrocolloid material contacts the wound (LAD I, Figure 1)
Polythene sheet contacts the wound (LAD IB and LAD II, Figure 2)
Some may be skeptical as to whether a hydrocolloid material encountering the wound or negative pressure (LAD I) makes a difference. Liquefied hydrocolloid materials blocking the tube, and poor wound floor visibilityfor the initial few days until the suction removes any liquefied material, are two additional problems.
To avoid doubts and problems associated with LAD I, in LAD IB (Figure 2) a sterile polythene sheet separates the wound along with tubes from hydrocolloid and in the Hydrocoll® (Hartmann, Heidenheim, Germany) a central hole was made to improve visibility. LAD I and LAD IB was used for smaller wounds (up to 10 cm x 10 cm given that the maximum size of Hydrocoll was 15 cm x 15 cm).
In LAD II (Figure 3) wounds are covered with larger polythene sheets, polythene tubes, and polythene bags after placing tubes (as in other LAD designs) and when sealing is achieved with pieces of Hydrocoll and the adhesive polyurethane film (OpSite™, Smith & Nephew, Largo, Fla).
If required in critically ill patients, after taking out intravenous and other lines through the LAD polythene bag that covers the extremities, the puncture site may be sealed effectively.
There is a general belief among most physiologists that true interstitial fluid pressure in loose subcutaneous tissue is slightly less than atmospheric pressure (average value of this pressure is negative in relation to atmospheric pressure and is approximately -3 mmHg).
When the skin cover is absent in wounds,the pressure will rise to 0 mmHg (ie, equal to atmospheric pressure). These increases in interstitial tissue pressure from -3 mmHg to 0 mmHg will also cause the lymph flow to increase 20-fold and the re-absorption of fluid to increase through capillaries. Hence, the chances bacterial invasion and absorption of chemicals (toxins) through venules and lymphatics increases when edema increases the interstitial tissue pressure or produces an open wound that is not sutured.
If the interstitial tissue pressure is slightly more negative than -6 mmHg, the lymph flow is slight 35 and consequently, the absorption of interstitial fluid will be negligible. As a result, the risk of sepsis due to bacterial invasion and the risk of systemic inflammation syndrome (SIRS) due to absorption of pro inflammatory cytokines will reduce considerably at -6 mmHg.
Surgeons generally believe that after wounding the wound remains contaminated for 6-8 hours, after which bacterial invasion occurs. If negative pressure (more than -30 mmHg) is applied every 31⁄2 hours for 30 minutes chances of invasive wound sepsis and SIRS will be reduced considerably—the author determined this schedule by a trial and error method and through daily observation of changes in wound granulation appearance after applying the LAD in more than 1000 cases while generating a maximum -30 mmHg negative pressure through the suction machine.This level of pressure (-30 mmHg) not only appears to be safe for most of the tissues, but also produces a desirable negative pressure effect even if slight leakage occurs. Magnitude of negative pressure is directly proportional to the pain and discomfort produced.
The intermittent negative pressure (cycle of 30 minutes suction and 4 hours rest) produces a leech effect.The leech effect is beneficial in the following ways:
Intermittent negative pressure of LAD can possibly retain local, beneficial effects of cytokines (inflammation, fibroplasia) during the rest period, but drains it out during the suction period; thereby, the leech effect removes the deleterious systemic effects of proinflammatory cytokines (SIRS). It also most likely removes chemicals producing progressive capillary thrombosis and, hence, the chances of progressive necrosis requiring repeated debridement after a crush injury, for example, is reduced.
Intermittent negative pressure prevents bacterial invasion, removes cytokines, and toxins along with the wound secretions.
Intermittent negative pressure helps relieve venous congestion (like the leech effect) and increases tissue oxygenation.
Effect on Renal Failure. High concentrations of proinflammatory cytokines have been reported to correlate with the prognosis of sepsis and the development of multiorgan dysfunction syndrome. In critically ill patients with ARF, proinflammatory and anti-inflammatory cytokines are markedly elevated in the presence or absence of sepsis and associated with significant increase in the risk of death. A preliminary study (unpublished data) indicates that use of LAD dressing helps in recovery from ARF associated with wound sepsis- related SIRS. The following case reports support the evidence of the leech effect.
Wounds. 2008;20(2):49-59. © 2008 HMP Communications
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Cite this: Limited Access Dressing - Medscape - Feb 01, 2008.