MVTR - No sweat, all you need to know ....
Did you ever wonder what MVTR is, and why it is important in wound care?
This podcast will provide valuable insights into the vital role MVTR plays in the dressings we use, and the science behind it.
Runi and Chris will also discuss why we should consider MVTR in wound management, and how this can provide an optimal environment for healing.
Welcome to Smith & Nephew's Closer to Zero podcast, a bi-monthly podcast with leading experts in wound care, hosted by Smith and Nephew helping health care professionals in reducing the human and the economic costs of wounds.
Hello, I'm Ruth Timmins from Smith & Nephew and welcome to today's podcast MVTR No Sweat: All You Need to Know. We have some special guests with us today from the UK scientists Runi Brownhill and Chris Casey who will talk us through MVTR and its relevance in wound care. So welcome, Runi and Chris to this podcast session. Would you just like to start off and tell us a bit about yourselves?
Thank you Ruth. My name is Runi Brownhill. I'm a Senior Scientist Manager in the Advance Wound Management R&D Business. I'm based in Hull in the UK, being at Smith & Nephew for over 11 years and I'm part of a group called the Research and Innovations Group. My expertise is in the field of wound biology. It's great talking to you again, Ruth, for a second time as I loved our discussion on the PICO mode of action session earlier. Let me now hand over to Chris. We will do a double act. I'll take all the questions around the why, while Chris will do the how questions and the technical detail.
Thanks Runi. My name is Chris Casey and I manage a team of analytical scientists in AWM R&D, based in the Hull site. My area of expertise is in the analytical testing and I have worked with Smith & Nephew for the past nine years.
Well, thank you for joining us again and welcome. Perhaps, Runi, let me ask you, what is MVTR and why is it important?
MVTR stands for moisture vapour transmission rate. It is a mouthful, I realise. Sometimes, people use WVTR, which is water vapour transmission rate. This is a measure of the movement of water vapour through a material in a period of time. For example, we typically measure it over an hour. So why is MVTR of a wound dressing important? Well, creating a desirable healing environment is essential wound healing. This means an ideal moisture content is one of the most important factors we need to consider. The benefits of moist wound healing were first discovered by George Winter in 1962 (Ref 3). I actually have his original PhD thesis on this writing on my desk. I inherited this from my previous boss who had it donated to him many, many years ago. Anyway, one of the fundamental functions and requirements of a wound dressing is to maintain this optimal moisture balance. This is key for cellular function, such as cell migration and cell proliferation. These functions are pivotal to wound healing and hence, dressings with optimum MVTR to provide the correct moisture balance is something us scientists work towards. Excessive MVTR, which means far too much evaporation may lead to wound dehydration and adherence of the dressing to the wound bed. Whereas a low MVTR, which means not enough evaporation, might lead to maceration of healthy and surrounding tissue (Ref 5). There are in vitro cell-based work that show keratinocytes and fibroblasts work effectively when the moisture balance is maintained (Ref 6). In vivo studies show wound re-epithelialisation and granulation, which are key to wound closure, happening more rapidly in a moist environment than in a dry scab (Ref 6). Also, with our IV dressings, it has been shown that the moisture balance achieved prevent increasing the bacterial numbers which can be clinically useful to prevent infections (Ref 4). So, MVTR is a key factor to consider when developing an optimum wound care dressing.
OK, thank you Runi. So now we know the importance of MVTR, how do we measure it? Perhaps, Chris this might be for you.
Well, thanks Runi, that's a really good explanation of what MVTR means and why it's important. And all of this has a bearing on how we measure the MVTR. Now, we're measuring MVTR, we follow ISO 13726, which covers standard test methods for medical dressings. This defines moisture vapour transmission rate or MVTR as the permeability of materials to the passage of water molecules from the skin contact site to the external atmosphere under controlled conditions of humidity and temperature. Now, that's a bit of a mouthful but don't worry about this too much as I'll explain as I describe the test method. So that's what MVTR means, but how do we measure it? There are two ways of doing this, with the dressing in contact with water vapour and with the dressing in contact with liquid. Let's start by looking at the dressing in contact with water vapour. We start by placing some water into a cup and then placing the dressing across the top of the cup, with the wound contact side facing the inside of the cup and showing that there is an air gap between the surface of the water and the dressing material. The whole thing is then weighed and placed in a chamber set to 37 degrees C and relative humidity at less than 20% for 18-24 hours. If you remember the definition which we gave earlier, this is to meet the requirement for controlled temperature and humidity. The cup is weighed again at the end of the test and the time spent in the chamber recorded to the nearest five minutes. The difference between the weights at the start and end of the test indicates how much vapour has escaped through the dressing material. Measurements of MVTR in contact with liquid is done in the same way, with the exception that the cup is inverted and it's only left in the chamber for 12 hours. This brings us onto the subject of switchable films. You might have noticed that the test for the dressing in contact with the water only lasts for 4 hours instead of the 18-24 hours taken for the test in contact with vapour. So why is this? Switchable polymer films are formed in such a way that when they come into contact with liquid water, their structure opens up to form pores which allow the water vapour to pass through the film much more easily. This is the effect of greatly increasing the MVTR. Therefore, we don't need to run the test for as long to see a decrease in the weight of the cup due to the loss of water vapour.
OK, so how does the MVTR concept span all types of dressings right, from IV products through to, you know, flat films, foams and even more advanced concepts, such as PICO delivering negative pressure wound therapy?
The main application of MVTR fairly obviously, is in controlling the amounts of fluid under the dressing. For the simple film, a high MVTR will enable more fluid to escape in the form of vapour and consequently, any build-up of fluid under the dressing will be reduced. Therefore on intact human skin, which has an MVTR of approximately 200 to 400 grams per metre squared per day, a film which has an MVTR above this level will prevent moisture from building up between the dressing and the skin, avoiding potential issues such as maceration. The situation becomes more complex when the dressing contains additional materials such as foams or super-absorbers. Dressings of this type would typically be used on chronic wounds where you're trying to create an environment that promotes wound healing. In this situation, it is important to create the correct balance between having too much moisture in the wound bed and not drying out the wound. Therefore, the foam and super absorber materials might hold onto a certain amount of moisture, preventing the wound bed from drying out, while the top film allows excess fluid to evaporate, preventing the lower layers from becoming saturated. In the case of PICO, it is a high MVTR that allows the dressing to replace the canisters in traditional NPWT, with as much as 80% of the moisture produced by the wound being removed by evaporation through the top film while the super-absorber layer retains the remaining 20% preventing the wound bed from drying out. Therefore, PICO is able to combine the benefits of negative pressure with good control of moisture levels.
I see. So, could you give an example of how MVTR has provided us with superior clinical outcomes? Perhaps this one's for you, Runi.
Ruth, directly linking MVTR to wound outcomes in the clinic can be difficult and actually, it can be impossible. However, let me also take PICO dressing for an example. Chris talked about this before. PICO has been designed to work without a canister, with complex constructions allowing the evaporation of up to 80% of absorbed exudate. The dressing has a silicone interface which contacts the skin side. It has a layer that transmits the negative pressure, the AIRLOCK™ technology. It also has a super absorbent layer to lock the exudate away from the wound and top of all of that, a top film with a high MVTR. This is where all of it happens. It allows this evaporation of up to about 80% of the absorbed exudate and hence, the reason why we can use a dressing instead of a canister. So let me talk about the potential MVTR-related benefits with the PICO dressing. We have done experimental studies where we have shown that the skin surrounding the wound on the PICO dressing remains uncompromised. We have shown that there is no disruption to the skin barrier (Ref 1). We use a measure called trans-epidermal water loss for this and TEWL readings under the PICO dressing was very similar to normal skin. We also show that this skin is not red or it has got any erythema. So for example, the periwound skin under the PICO dressing looks uncompromised (Ref 1). Therefore, PICO has been engineered to maintain an optimal moisture balance and that can help reduce adverse events such as maceration (Ref 2). Let me also give you an example of how the MVTR of our IV film dressings work. We have done experiments to show that our dressings are more permeable to water vapour than ordinary films. Chris also talked about this. This prevents the accumulation of moisture underneath the dressing, thereby reducing the opportunity for bacteria to grow and proliferate, and hence, overall reducing the risk of catheter related infections (Ref 4). So our dressings are designed with optimal MVTR to provide optimal outcomes.
That's really interesting. So perhaps Chris, can you talk a little bit about the MVTR in Smith & Nephew products such as OPSITE, ALLEVYN and PICO?
So, as Runi has explained Smith & Nephew products are designed to maintain the optimum wound healing environment, given their intended application. Therefore, OPSITE Flexifix has an MVTR of around 650 grams per metre squared, per 24 hours. Remembering that skin typically has an MVTR of 200 to 400, this allows excess fluid to evaporate, while not drying out the wound bed. Products such as ALLEVYN Life and PICO are typically used on wounds which produce significant amounts of exudate, and therefore needs to allow more moisture to evaporate in order to maintain an optimum environment. These have a more permeable film with an MVTR of around 2,000 but the top film is also switchable. So this rate will increase when liquid permeates through the lower absorbent layer. So when PICO achieves an 80% evaporation rate, its MVTR can increase to 8,000-9,000 over the course of its wear time. IV3000 which is used for catheter fixation and also in OPSITE Post-Op has an MVTR of around 3,000, hence the name, increasing to around 14,000 in contact with liquid. This inhibits bacterial growth by keeping the catheter site dry. This is sometimes referred to as REACTIC™ technology.
OK, so when you think about chronic wound care where these products are used, sometimes, you know, clinicians use secondary dressings or bandages, as well. So what impact would that have on the MVTR?
So if extra layers of dressing are placed over the wound, then this will reduce the overall MVTR. However, these secondary dressings will usually have a more open structure than the top film of a PICO or ALLEVYN dressing. This would generally allow freer passage of water vapour than the top film and therefore, it would be the top film that limits the MVTR rather than secondary dressing. So any impact on the MVTR would usually be fairly small.
Just to add to that and let me reinforce what Chris is saying, if we use bandages on top of our dressings with an open weave structure, this allows evaporation so there's minimal impact on MVTR. But yes, the more you cover up the top film of a dressing, the more you reduce MVTR. We know that compression bandaging is essential to manage chronic venous insufficiency and reduce oedema, and this is important for leg ulcer healing. Let's take PICO for example again. We have done tests using compression bandaging to show that the PICO top film MVTR is minimally affected in the presence of bandaging (Ref 7) and the product continues to handle the exudate as it should do. So yes, be mindful of how you use dressings but we do design our products to take into consideration some of these facts also.
OK, that's really great information. So what would be the take home message for today's podcast?
So Chris talked about how we measure MVTR and all the efforts we go into new product design. I can summarise by saying that the microenvironment is essential for wound healing and MVTR plays a key role here. So when you think about a dressing selection, take MVTR into consideration. But rest assured, we as scientists take this important factor into account when we design our wound care products and we create these products to make sure we provide an optimal environment for healing and positive outcomes.
Well, that's great. Thank you both for bringing us up to speed on MVTR and its relevance in wound care, and I'm sure it's given a lot of insights to our listeners. So thanks again for joining us and tune in for our podcast next time. Thank you.
Thank you. It's been great to join you today.
Dressings with low MVTR can result in increased costs, wastage and impact patient quality of life. Our extensive range of moisture responsive dressings including our OPSITE Post-Op range, our OPSITE film dressings, IV3000 and PICO single use negative pressure are scientifically proven to provide high MVTR and are backed by our extensive clinical evidence and education. At Smith & Nephew, no matter what care setting we have a solution to assist you in improving patient outcomes. For more information on how a higher MVTR dressing can help you and your patients, contact a Smith & Nephew representative or email us at ProfEd.ANZ@smith-nephew.com.
The information presented in this podcast is for educational purposes only. It is not intended to serve as medical advice. Products listed and outline of care are examples only. Product selection and management should always be based on comprehensive clinical assessment. For detailed product information including indications for use, contraindications, precautions and warnings, please consult the product's applicable instructions for use prior to use.
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1. Brownhill et al., 2020. Pre-Clinical Assessment of Single-Use Negative Pressure Wound Therapy During In Vivo Porcine Wound Healing. Adv Wound Care. Published Online 7 Jul 2020 https://www.liebertpub.com/doi/abs/10.1089/wound.2020.1218
2. Kirsner R et al., 2019. A Prospective, Randomized, Controlled Clinical Trial on the Efficacy of a Single‐use Negative Pressure Wound Therapy System, Compared to Traditional Negative Pressure Wound Therapy in the Treatment of Chronic Ulcers of the Lower Extremities. Wound Repair Regen 2019; 27:519-29. https://pubmed.ncbi.nlm.nih.gov/31087729/
3. Winter G. D. Formation of the scab and the rate of epithelization of superficial wounds in the skin of the young domestic pig. Nature 193, 293–294 (1962).
4. Richardson MC, 1991. In Maki DG (ed): Improving Catheter Site Care: International Congress and Symposium Series Number 197. London Royal Society of Medicine Services Ltd 1991 pp29-31
5. L L Bolton 1, K Monte, L A Pirone. Moisture and healing: beyond the jargon. Review Ostomy Wound Manage, 2000 Jan;46(1A Suppl):51S-62S; quiz 63S-64S.
6. Rui Xu et al., 2016. Controlled water vapor transmission rate promotes wound-healing via wound re-epithelialization and contraction enhancement. Sci Rep. 2016; 6: 24596. Published online 2016 Apr 18.
7. Fauland et al., 2013. Assessment of moisture management performance of multilayer compression bandages. Textile Research Journal. 83(8), 871–880.
Senior Scientist Manager
Smith + Nephew Advanced Wound Management
Research & Development, Hull UK.
Runi Brownhill is a recognised scientific expert in the field of Wound Biology and the interventions needed to improve patient outcomes. She provides strategic direction to the Research and Innovation programme through the application of deep domain knowledge in Human Biology & Disease relevant to the business. She has an extensive knowledge of appropriate model systems to investigate the mechanisms of action of various therapies and wound-targeted strategies. Runi additionally provides a broad scientific expertise in order to identify and source innovative new technologies, lead technology scouting and drive product development through to proof of concept.
Runi has a PhD in Pharmacology with a background in the Bio Pharma Sector. Runi has been at S&N for over 11.5 years working as part of the Wound Management R&D division.
Runi was pivotal in the research programme to identify the mode of action of PICO sNPWT.
Runi is based in the UK but her parents live in New Zealand!
Scientist Manager, Smith + Nephew.
Chris’s background is in analytical chemistry and he has a Masters in Analytical Science from the University of Hull.
He has forty years’ experience in the Analytical Sciences, having previously worked at the University of Hull and with nutritional and pharmaceutical products before moving to Smith and Nephew AWM nine years ago. Chris currently manages a team of analytical scientists working across a range of AWM development projects.