These authors say high pressure NPWT can positively affect edematous wounds or unstable wounds, and works well with hydrophilic dressings or barrier dressings.
By Quan Ngo, MBBS(Hons), and Anand Deva, BSc(Med), MBBS(Hons), MS, FRACS
Over a decade ago, Fleischmann, Morykwas and their respective colleagues first introduced negative pressure wound therapy (NPWT).1,2 Their early work confirmed the effectiveness of NPWT in enhancing healing in both human and animal models.
Fleischmann and colleagues used NPWT in combination with antiseptic solutions in 27 orthopedic wounds and were able to obtain a healthy wound amenable to primary or skin graft closure within seven days of treatment.1 Pressures ranged from 20 to 80 kPa or -150 to -600 mmHg, and were generated via a vacuum pump or wall suction device. Truncal wounds and acute traumatic wounds are subject to higher pressures while wounds in the limbs, especially where perfusion was poor, are subject to the lower pressures. There was no clear justification for the range of pressures they used and the parameters may well have been influenced by the range of pressures available from the vacuum generators they used.
Morykwas and co-workers defined various parameters in the application of NPWT that remain the standard today.2 They utilized a porcine model and examined the effects of pressure and intermittent cycles versus constant pressure on blood flow, granulation tissue formation, bacterial load and skin flap survival.
Up until recently, the use of NPWT in Australian hospitals was virtually synonymous with the VAC therapy wound dressing system (KCI). More recently, Smith and Nephew introduced a similar wound dressing system, V1STA, which also uses negative pressure. For VAC therapy, KCI recommends a pressure setting of -125 mmHg for normal use in the majority of wounds.3
This recommendation was based on the original work by Morykwas and colleagues, who looked at negative pressures from 0 to -400 mmHg in 25 mmHg increments.2 Their model applied NPWT to 2.5 cm2 artificially created defects down to deep fascia overlying the spine. When they measured underlying blood flow with a Doppler, there was a bell-shaped curve response over a range of NPWT. The maximal flow was four times the baseline and occurred with -125 mmHg pressure. When the pressure was above -200 mmHg, blood flow began to decrease.4
How Strain And Biomechanical Forces Can Affect Pressure Settings
In clinical applications, the pressure setting may also be influenced by several other factors. First, the denser or thicker the dressing, the greater the pressure may have to be in order to draw fluid out of the wound. Hydrophilic dressings such as polyvinylchloride (PVC) foam or gauze may retain fluid more strongly than hydrophobic foam (e.g. polyurethane), thus requiring a higher vacuum setting.
Consider cases in which NPWT aids in stabilization of a wound, such as in a dehisced sternotomy wound. In such cases, maintenance of higher pressure may be beneficial in minimizing motion, thereby reducing the pain associated with rubbing of wound edges.
Work by Saxena and colleagues showed that NPWT induced microdeformational changes at the wound-dressing interface with a range of tissue strains induced.5 Computer 3-D modeling analyzed the wound surface in relation to several factors including negative pressure exerted, wound compliance and foam architecture. Wound surface strain was maximal at the foam struts or edge of foam pores, and decreased toward the center of the pores. At -110 mmHg, a wound would experience peak strain of 125 percent (i.e. it is stretched to 25 percent of its resting length). The average strain, however, is between 5 and 20 percent across the wound.
As pressure increased in the study by Saxena and colleagues, the peak and average strain increased accordingly.5 As the wound became stiffer with time, (e.g. due to edema or fibrosis), strain induced by negative pressure began to decrease. This showed that in order to maintain the same level of strain at the wound surface in chronic, edematous wounds, the pressure ought to be raised.
The positive impact of mechanical force transmission on tissue growth occurs in orthopedic injuries or tendon repairs where researchers have found a certain degree of load bearing to be important for tissue healing.6,7
Ingber and colleagues showed that biomechanical forces influence cellular development through their influence on cell and nuclear shape changes.8 At a microscopic level, microtubules, microfilaments and extracellular matrix (ECM) are important elements, which function in a complementary way to support mechanical loads. Specifically, tension generated within actin-myosin filaments in the cytoskeleton control cell and nuclear shape. Disruption of these elements can inhibit cellular growth.
However, it still remains unclear as to what the optimal strain level should be to minimize tissue trauma and maximize healing at the same time. The micro-deformation modeling has shown that higher pressures may be required to counteract the edema and increased tissue stiffness in chronic wounds to produce the same level of tissue strain at the dressing interface.5
Can A Barrier Layer Influence The Delivery Of Pressure?
In many situations, one does not apply a foam dressing directly to the wound surface but via an intervening dressing barrier such as Jelonet™ (Smith and Nephew) or Mepitel® (Molnlycke Healthcare). One would employ this practice in skin grafts to prevent graft lifting at the first dressing change, and for many patients with highly sensitive wounds where ingrowth of granulation tissue into the foam can lead to a very painful dressing change process. KCI also recommends having a barrier between the foam and delicate tissues such as nerves, vessels and bowels.3
The use of such a barrier layer can adversely affect the delivery of intended negative pressure to the wound surface, as shown by Jones and co-workers.4 The study consisted of 40 healthy individuals with researchers placing microprocessor pressure transducers at the skin-dressing interface and recording pressure measurements at three-second intervals. They found that the Jelonet dressing was capable of reducing pressure delivery up to -42 mmHg while Mepitel dressings introduced an average variation of -5.59 +/- 7.2 mmHg in pressure. In such circumstances, the pressure may need to be higher in order to maintain the same microenvironment at the wound dressing interface that one could otherwise achieve without a barrier dressing.
Indeed, a number of reported clinical studies have used NPWT with higher pressures than the recommended -125 mmHg with good outcomes. Jeschke and co-workers combined NPWT with fibrin glue in order to hasten the take of Integra (Integra Life Sciences).9 The study found NPWT with pressure of -150 mmHg reduced the take period from an average of 24 days down to 10 days. Meanwhile, the success rate of Integra increased from 78 to 98 percent.
On the other hand, excessively high pressure can at times create discomfort for the patient. In patients with very sensitive wounds, such as venous ulcers, pressure tolerance can improve by reducing pressure incrementally to -100 mmHg, -75 mmHg or even -50 mmHg. Once the patient is accustomed to the stimulation, pressure can slowly increase again.2
Despite NPWT having been in clinical use for over a decade, there remains controversy as to the optimal application of the treatment. In regard to pressure setting, clinicians currently use settings that are founded based on the early experimental works in the animal model. Although these experiments remain an important foundation for the application of NPWT, there are several confounding factors that one should take into consideration when determining the optimal pressure setting. When using NPWT with barrier dressings, edematous wounds or an intervening barrier dressing, the use of higher pressure is justified.
Dr. Ngo is affiliated with the Surgical Infection Research Group at the Australian School of Advanced Medicine at Macquarie University in Sydney, Australia.
Associate Professor Deva is the Head of the Cosmetic Plastic and Reconstructive Surgery and Co-Director of the Surgical Infection Research Group at the Australian School of Advanced Medicine at Macquarie University in Sydney, Australia.
1. Fleischmann W, Russ M, Westhauser A. The vacuum sealing as a carrier system for selective local drug application to wound infections. Unfallchirurg 1998; 101(8):649-654.
2. Morykwas MJ, Argenta LC, Shelton-Brown EI, McGuirt W. Vacuum-assisted closure: a new method for wound control and treatment: animal studies and basic foundation. Ann Plast Surg 1997; 38(6):553-562.
3. Kinetic Concepts, Inc. V.A.C.® Therapy™ Clinical guidelines, 2006.
4. Jones SM, Banwell PE, Shakespeare PG. Interface dressings influence the delivery of topical negative-pressure therapy. Plast Recon Surg 2005; 16(4):1023-1028.
5. Saxena V, Hwang CW, Huang S, et al. Vacuum-assisted closure: microdeformations of wounds and cell proliferation. Plast Reconstr Surg 2004; 114(5):1086-96.
6. Urschel JD, Scott PG, Williams HT. The effect of mechanical stress on soft and hard tissue repair: a review. Br J Plast Surg 1988; 41(2):182-186.
7. Olenius M, Dalsgaard C, Wickman M. Mitotic activity in expanded human skin. Plast Reconstr Surg 1993; 92(6):1202-3.
8. Ingber DE, Prusty D, Sun Z. Cell shape, cytoskeletal mechanics and cell cycle conrrol in angiogenesis. J Biomechanics 1995; 28(12):1471-1484.
9. Jeschke MG, Rose C, Angele P. Development of new reconstructive techniques: use of integra in combination with fibrin glue and negative-pressure therapy for reconstruction of acute and chronic wounds. Plastic Recon Surg 2004; 113(2):525-30.
For further reading, see “Is Foam More Effective Than Gauze With Negative Pressure Wound Therapy?” in the July 2009 issue of Podiatry Today or “Examining The Role Of NPWT In Limb Salvage” in the December 2009 issue.
While there is no true consensus regarding the appropriate pressure settings to achieve maximum results, this author says the literature reveals benefits to low pressure NPWT that range from increased uptake with grafts to reduced pain and tissue damage with dressing changes.
By Ryan Fitzgerald, DPM, AACFAS
Negative pressure wound therapy (NPWT) systems provide a significant addition to the wound specialist’s armamentarium in the management of complex soft tissue wounds. The advent of these systems has largely revolutionized the way we manage this challenging patient population.
The literature is rife with numerous studies demonstrating that NPWT can rapidly improve the development of granulation tissue and speed up coverage of exposed tendon, exposed hardware and bone to provide improved wound healing outcomes. With the use of NPWT, granulation tissue often occurs in days to weeks rather than taking months as with the use of more conventional wound therapies.
Numerous studies have demonstrated through both animal models and clinical trials that NPWT can reduce the bioburden in contaminated wounds, provide exudate management with reduction of interstitial edema and increase capillary blood flow, all of which lead to increased formation of granulation tissue.1
This increase in granulation tissue can be dramatic. In one study, patients in the NPWT group demonstrated faster healing rates and a higher proportion of healed wounds.2 Patients in the NPWT group reached a level of 76 to 100 percent granulation tissue coverage of the wound bed twice as fast in comparison to patients treated with conventional wound therapies.
Apart from the development of granulation tissue, NPWT reduces wound surface area by the traction force of negative pressure.3,4 This increases the mitosis of periwound tissue and promotes decreased wound size and progressive wound closure.3,4
Does Low Pressure NPWT Facilitate Improved Uptake With Skin Grafts And Bioengineered Alternative Tissues?
In addition to the generation of granulation tissue, NPWT has demonstrated great effectiveness in the promotion of skin graft uptake. Historically, buttress dressings ensured split thickness and full thickness grafts remained in contact with the wound bed throughout the phases of wound healing.5
More recently, however, clinicians have found great success utilizing NPWT to promote graft uptake for split-thickness skin grafts (STSG) and incorporation of bioengineered alternative tissues (BAT) such as the Integra™ bilayer matrix (Integra Life Sciences).6
There is substantial research to support the use of NPWT in this manner. Morykwas and colleagues demonstrated increased STSG uptake with decreased graft failure rate when utilizing NPWT systems in conjunction with graft placement.7 Moisidis and colleagues confirmed this in a prospective, blinded, randomized controlled study, which also demonstrated a decreased STSG failure rate when researchers utilized NPWT.8
Scherer and colleagues demonstrated a 4.5 cm STSG loss among control patients, whereas patients in the NPWT arm demonstrated no loss of graft.9 They also reported that VAC Therapy (KCI) patients required fewer repeat grafts versus standard therapy with a bolster dressing (3 percent versus 19 percent). The significant reduction in graft loss provided by NPWT translated into significantly shorter hospital stays and a significant cost savings.
For grafts or bioengineered tissue applications, physicians usually use a lower setting (<-100 mmHg).10
Can Low Pressure NPWT Result In Less Pain And Tissue Damage?
Despite the great successes demonstrated by the use of NPWT, this modality does demonstrate shortcomings, namely pain at dressing changes and pain with the maximum recommended setting of -125 mmHg.11 Additionally, lower negative pressures and intermittent therapy were associated in earlier studies with improved microvascular blood flow in porcine wound models and with reduced pain in patients in comparison to those patients in higher pressure study arms.12
While many patients with diabetic wounds demonstrate signs of peripheral neuropathy, which can provide some element of relief from the pain associated with this therapy, many other patients, such as those who have suffered significant burns, relate significant discomfort both during the therapy as well as at dressing changes.13 Indeed, many of the gauze-based NPWT device manufacturers recommend pressure settings of -60 to -80 mmHg. Studies frequently cite these devices/regimens as being less painful and minimizing the risk of bleeding and tissue damage.12
Several studies have demonstrated the efficacy of STSG uptake in burn patients utilizing a low-pressure negative pressure wound therapy (LP-NPWT) system at subatmospheric pressure levels of -75 mmHg and a low adherence dressing.12,14 For these patients, authors noted graft uptake to be comparable to higher pressure settings.14 The numerical pain score significantly improved, both during the active therapy as well as during dressing changes, and pain levels tended to decrease as therapy progressed. In this study, the authors noted little or no trauma on dressing removal, and no signs of infection. In all cases, all wounds healed after clinicians applied STSGs and followed with four days of LP-NPWT.
Limiting tissue damage is vital, particularly in the development of new granulation tissue and during the inosculation phase of STSG uptake. Numerous studies have demonstrated that NPWT settings at lower pressure values (-60 to -100 mmHg) can provide the beneficial effects of wound bed preparation while reducing the subsequent tissue trauma to the neodermis that has been created.14,15
Commonly, when one utilizes higher-pressure settings, significant macrodeformation occurs and consequently leads to significant tissue ingrowth into the contact media, either with a sponge or gauze. This new tissue tears away during the dressing change and subsequent removal of the gauze or sponge contact media.3
The use of NPWT has revolutionized wound care and provides wound care specialists with a means to provide for significant, rapid development of granulation tissue and promotion of graft uptake in this challenging patient population. What remains unclear, however, is a consensus on the appropriate subatmospheric pressure settings to provide the greatest overall efficacy in wound healing while also providing the patient with the greatest level of comfort.
Commonly, physicians utilize a standard setting of -125 mmHg. However, this often is a rote device setting with little understanding by the clinician of the available evidence that supports the use of lower pressure settings. There is a reason that lower pressure settings are available on the NPWT devices. Initiation of therapy should not simply be a matter of placing the occlusive dressing and then setting the device to “stun” at the highest pressure setting tolerable.
It is true that there are some indications to utilize higher pressure settings and it is important to individually tailor the treatment regimen to the patient. However, the simple reality is that the use of higher pressure settings in NPWT does not improve the likelihood of wound healing nor does it guarantee faster development of granulation tissue.
Indeed, the literature demonstrates numerous instances of increased comorbidity associated with the use of higher-pressure settings. Most commonly, these included patient pain — described both during the therapy as well as during dressing changes — as well as tissue damage that most commonly occurs during dressing changes. Tissue damage is of particular concern, both in the context of STSG or BAT graft placement as well as in newly formed granulation tissue. This is because a compromise of tissue can lead to wound chronicity and the potential for wound propagation.
To avoid these potential issues, wound care specialists can safely utilize NPWT at lower pressure settings unless higher settings are truly indicated.
Dr. Fitzgerald is in private practice at Hess Orthopaedics and Sports Medicine in Harrisonburg, Va. He is an Associate of the American College of Foot and Ankle Surgeons.
1. Weed T, Ratliff C, Drake DB. Quantifying bacterial bioburden during negative pressure wound therapy: does the wound VAC enhance bacterial clearance? Ann Plast Surg 2004; 52(3):276-9; discussion 279-80.
2. Armstrong DG, Lavery LA. Negative pressure wound therapy after partial diabetic foot amputation: a multicentre, randomised controlled trial. Lancet 2005; 366(9498):1704-10.
3. Borgquist O, Gustafsson L, Ingemansson R, Malmsjö M. Micro- and macromechanical effects on the wound bed of negative pressure wound therapy using gauze and foam. Ann Plast Surg 2010; 64(6):789-93.
4. Liguori PA, Peters KL, Bowers JM. Combination of negative pressure wound therapy and acoustic pressure wound therapy for treatment of infected surgical wounds: a case series. Ostomy Wound Manage 2008; 54(5):50-3.
5. Senchenkov A, Petty PM, Knoetgen J 3rd, et al. Outcomes of skin graft reconstructions with the use of Vacuum Assisted Closure (VAC) dressing for irradiated extremity sarcoma defects. World J Surg Oncol 2007; 5:138
6. Espensen EH, Nixon BP, Lavery LA, Armstrong DG. Use of subatmospheric (VAC) therapy to improve bioengineered tissue grafting in diabetic foot wounds. J Am Podiatr Med Assoc 2002; 92(7):395-7.
7. Morykwas MJ, Faler BJ, Pearce DJ, Argenta LC. Effects of varying levels of subatmospheric pressure on the rate of granulation tissue formation in experimental wounds in swine. Ann Plast Surg 2001; 47(5):547-51.
8. Moisidis E, Heath T, Boorer C, Ho K, Deva AK. A prospective, blinded, randomized, controlled clinical trial of topical negative pressure use in skin grafting. Plast Reconstr Surg 2004; 114(4):917-22.
9. Scherer LA, Shiver S, Chang M, Meredith JW, Owings JT. The vacuum assisted closure device: a method of securing skin grafts and improving graft survival. Arch Surg 2002; 137(8):930-3; discussion 933-4.
10. Llanos S, Danilla S, Barraza C, Armijo E, et al. Effectiveness of negative pressure closure in the integration of split thickness skin grafts: a randomized, double-masked, controlled trial. Ann Surg 2006; 244(5):700-5.
11. Borgquist O, Ingemansson R, Malmsjo M. The effect of intermittent and variable negative pressure wound therapy on wound edge microvascular blood flow. Ostomy Wound Manage 2010; 56(3):60-7.
12. Ahearn C. Intermittent negative pressure wound therapy and lower negative pressures - exploring the disparity between science and current practice: a review of the literature. Ostomy Wound Manage 2009; 55(6):22-8.
13. Fitzgerald RH, Bharara M, Mills JL, Armstrong DG. Use of a Nanoflex powder dressing for wound management following debridement for necrotising fasciitis in the diabetic foot. Int Wound J 2009; 6(2):133-9.
14. Nease C. Using low pressure, negative pressure wound therapy for wound preparation and the management of split-thickness skin grafts in three patients with complex wounds. Ostomy Wound Manage 2009; 55(6):32-42.
15. Long MA, Blevins A. Options in negative pressure wound therapy: five case studies. J Wound Ostomy Continence Nurs 2009; 36(2):202-11.