Is High Pressure Better Than Low Pressure For NPWT?
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.