Key Takeaways

• Traditional foam-based NPWT may exert significant positive pressure on the wound surface.
  Although commonly referred to as "negative pressure wound therapy" (NPWT), this speaker conveys that foam dressings compress under suction and can generate substantial downward force on the wound bed. According to the discussion, this pressure may exceed the device setting itself and could potentially reduce perfusion directly beneath the dressing, particularly in vulnerable tissues.
• Dressing material properties influence fluid removal and tissue perfusion.
  The speaker argues that conventional foam dressings can collapse under suction, narrowing fluid pathways and requiring higher pressure settings to manage exudate. In contrast, noncompressible polymer-based dressings may maintain open fluid channels, allowing effective exudate removal at lower pressure settings while potentially preserving blood flow to the wound bed.
• Pressure-sensitive wounds may benefit from further research into alternative NPWT approaches.
  Wounds associated with ischemia, vascular compromise, bony prominences, or pressure injuries may be particularly susceptible to the effects of sustained mechanical pressure. The presentation highlights what the speaker feels is the need for additional research evaluating how NPWT dressing design and pressure distribution affect perfusion, healing outcomes, and the management of hard-to-heal wounds.

Transcript

Please note: This content is a direct transcript, capturing the authentic conversation without edits. Some language may reflect the flow of live discussion rather than polished text. 

I'm Michael Schuler. I'm an orthopedic hand surgeon. My day job is with Athens Orthopedic Clinic in Athens, Georgia. My afternoon and evening job is cofounder and CEO of Clear Choice Therapeutics. 
 
To start off, the name “negative pressure wound therapy” is a bit of a misnomer. It's like saying “negative light” so you can't have negative pressure. You can have zero pressure and you can have a pressure difference between subatmospheric and other areas, but the term negative pressure is really not accurate. Additionally, with Newton's third law for every action, there's an equal and opposite reaction. So when you hook the suction up to your dressing, the foam doesn't expand like it would do in a vacuum. It actually compresses. And so that compression actually puts a positive force on the wound surface. And so what we looked at in our study was exactly what is that positive pressure being placed on the wound surface and it's surprisingly high actually. And so that's really what the main point of the paper was. 
 
I don't want to say that negative pressure wound therapy doesn't work because it certainly does. But I think the take home point is it works despite the foam as opposed to because of the foam. And so again, with foam, I like to describe it as it's similar to sucking a milkshake through a coffee stir. It's a vicious cycle when you put negative pressure or suction onto the foam. The foam collapses and so it collapses the very pathways that you want the fluid to be removed from. And so it requires higher pressures because the foam collapses. And so it's a vicious circle where you apply suction, the pathways collapse. So then you have to apply more suction, which then collapse the pathways further. 
 
And so again, Newton's third law for every action, there's an equal and opposite reaction. If you set the pump at 125 millimeters of mercury, you're putting at least 125 millimeters of mercury on the wound surface. And because foam collapses, it has recoil. And so just like the spring snake gag, when you open the jar and the spring snake pops out, when you disconnect the tubing with black foam, the foam expands back up. And so that compression of the foam actually magnifies that downward force. So it's not just 125. It actually comes up to about 80 to 190 millimeters of positive pressure. And as we all know, blood pressure is 120 over 80. And so there's a significant gradient where actually blood pressure or blood flow underneath the dressing doesn't occur. And so the traditional and seminal paper of 1997 that talked about minus 125 stimulating the most blood flow, they actually measured perfusion on the periphery of the wound not underneath the dressing.¹ And so when you get that increase in flow, it's not underneath the dressing, it's actually in the periphery. And so once you actually measure the pressures underneath the dressing, it actually ends up being something that results in ischemia. 
 
It turns out it's actually been described before. So there's two papers in 2009 that specifically talk about the positive or downward force on the wound is directly proportional to the setting and the pumps and that pressure can actually cause ischemia.^2,3 And then there's a second or third, sorry, paper in 2020 that actually describes a very similar response.⁴ The reason it hasn't gotten any attention before now is there's really been no alternative to the foam. Traditional black foam really can't perform at lower pressures because it requires typically about 100 millimeters of mercury to 125 to remove that exudate. 
 
And so when we talk about running at lower pressures, you really need to have a totally different wound contact layer, something that's non-absorptive and non-compressible because again, that compression, if the pathways are reduced due to the negative pressure, then you have to get higher pressures to get the exudate out. And so a material that's non-compressible that has large open pathways to allow for easy removal of fluid can then work at a much lower pressure. And so that's part of what we've developed is a polymer based dressing that's transparent, non-absorptive and non-compressible, which actually removes fluid from the wound surface at minus 50 millimeters of mercury, which again is a much lower pressure, but also still clears the fluid and also doesn't cause the positive downward forces that traditional negative pressure does, which actually allows for perfusion. 
 
So it's the exact wounds that nobody can heal. The reason pressure injuries don't heal is because the exact reason for why they're in there in the first place is to sustain positive pressure. And so if the wound occurred due to sustained positive pressure and you're putting a dressing on that wound that puts down 180, 190 millimeters of positive pressure, you're just exacerbating the injury that got them there in the first place. 
 
And so wounds that are either dysvascular or wounds that have bony prominences underneath the wound are the ones that are most susceptible to this positive pressure. And so I like to say time is tissue. That's very common term with heart attacks and strokes. I would argue with dysvascular or bony prominence underneath wounds, times also tissue when it comes to negative pressure. If you're putting a dressing on that's putting 180, 190 millimeters of positive pressure, you're actually continuing to damage those wounds. 
 
It's been really exciting. It's been really exciting to be able to see wounds that actually typically don't heal being able to heal. And I think it's all due to perfusion and it's all due to pressure. And so I think one of the biggest things in the next research that needs to be looked at is being able to determine how you can heal these wounds that traditionally don't heal and understanding the fact that positive or downward force of negative pressure is probably a big contributing factor of how wounds can or can't heal.

References

1. 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. doi:10.1097/00000637-199706000-00001
2. Kairinos N, Solomons M, Hudson DA. Negative-pressure wound therapy I: the paradox of negative-pressure wound therapy. Plast Reconstr Surg. 2009;123(2):589-598. doi:10.1097/PRS.0b013e3181956551
3. Kairinos N, Voogd AM, Botha PH, et al. Negative-pressure wound therapy II: negative-pressure wound therapy and increased perfusion. Just an illusion?. Plast Reconstr Surg. 2009;123(2):601-612. doi:10.1097/PRS.0b013e318196b97b
4. Biermann N, Geissler EK, Brix E, et al. Pressure distribution and flow characteristics during negative pressure wound therapy. J Tissue Viability. 2020;29(1):32-36. doi:10.1016/j.jtv.2019.12.004