Feeling the Heat
- By Laurie Watanabe
- May 01, 2013
While longer days and more sunshine are being welcomed by many after a cold winter, for seating & mobility clients and their rehab teams, those rising temperatures offer a whole new challenge.
The very seating systems that can work wonders for clients — redistributing pressure more evenly, providing stability, supporting functionality, increasing comfort, lengthening time spent in the chair — can also hold onto heat and unintentionally contribute to a client’s discomfort. As temperatures become warmer in the spring and summer, clients may find themselves at increasing risk for skin breakdown and other heat-related illness.
So what role does heat play in a seating system’s ultimate success? And when should heat risk be assessed?
Several seating & mobility experts contributed to this story:
- Lois Brown, MPT, ATP, rehab clinical education specialist, Invacare Corp.
- Evan Call, MS, CSM (NRM), lab director, EC-Service Inc., and adjunct faculty at Weber State University
- Kim Chaney, BSc, MSc, occupational therapist, technical product manager for seating & positioning, Ottobock
- Ivan Samila, product designer, Motion Concepts
- Brad Stern, global sales support, Supracor
- Stephanie Tanguay, OTR, ATP, clinical education specialist, Motion Concepts
Identifying the Issues
When we discuss heat and seating, there are several factors to consider.
First, Kim Chaney notes, is the client himself.
“People are homeotherms,” Chaney says, “which means that their bodies respond to environmental factors, such as air temperature and humidity, to try to preserve a constant core/internal temperature of 37°C [98.6° F]. As the human body converts nutrients and oxygen into energy, heat is produced.”
To maintain a steady temperature, the body has a couple of strategies, Chaney adds: “When the body gets too hot, our blood vessels expand (vasodilation), increasing blood flow to the skin and allowing greater heat loss to the air. When this is not effective, the body then induces sweating to benefit from the cooling effect of evaporation. “However, materials held against the body, such as clothing and seating, oft en provide thermal insulation, creating a microclimate between the human body and the clothing/seating surface.”
That’s a second issue in the heat equation: The seating materials meant to support, stabilize and relieve pressure
also end up impacting the client in another significant way.
“Materials such as foam, rubber and gel trap and absorb heat from the body and from outside temperature,” Brad Stern says. “This also occurs when the user is stationary, which results in heat that can’t be circulated.”
Lois Brown says of maintaining a comfortable body temperature, “Staying cool and dry is a function of airflow. The more surface contact between seating surface and client, the hotter the client will be, and the more moisture entrapment.”
While those factors are true for all seating& mobility users, some consumers can be particularly impacted by heat.
“Certain users such as those with multiple sclerosis or whom are considered bariatric are more prone to heat and sweating,” Stern notes. “This makes the seating surface material that they sit or sleep on very important to their skin.”
In addition to medical conditions, lifestyle factors can also contribute to how much a consumer is impacted by heat.
“Retention of heat and moisture does pose the same amount of risk for all seated consumers,” Stephanie Tanguay says. “There are so many factors and variables that come into play for each consumer: intrinsic variables such as nutrition and circulation. Does the person smoke? Do they have diabetes? Can they perform any type of pressure relief? Do they have sensation? Exposure to heat & moisture can lead to skin maceration, but some people are at greater risk than others.”
How Cushions & Seating Systems Affect the Situation
So let’s take a closer look at seating system components and how and why they can ratchet up the heat that consumers and their rehab team have to deal with.
First, says Evan Call, different cushion media vary in their ability to hold heat.
“All materials that cushions are made out of have what’s called a thermal mass, which is a measure of how much heat can they hold,” he explains.
“For example, if you see a pitcher, you know it holds two quarts of water. If you look at a foam cushion, you can say that cushion can hold x joules or x BTUs (see sidebar), whichever units you want to use to measure how much heat they hold. And their thermal masses are all different, so for example if I have a cushion that is 4 inches thick and 16 inches square, the thermal mass of a piece of urethane foam in that shape is moderately low. It doesn’t hold too much heat because it’s largely air.”
For other foams, it’s a different story, Call says: “If I have a viscoelastic foam that’s 4x16x16 inches, it will hold two to two and a half times the amount of heat as a regular piece of foam. If I have a gel cushion — nobody makes a gel slab that’s 4 inches thick, but the visual image here is if I had a gel cushion that’s 4 x16x16 inches, it would hold about 30 to 35 times as much heat, all in the same space.
“So the material plays a very, very important role in what goes on, heat wise.”
Fortunately, the system’s media are just the starting point, thanks to current design and engineering options. Add in more options for cushion covers, and rehab team members have many different ways to try to keep heat under control.
For example, Ivan Samila says, “A material that has a smooth surface, such as vinyl or plastic, reduces air flow and can retain more heat and moisture than other materials. So what is the best seating surface? A porous material that allows air to pass through as the moisture is wicked away by the free air flow. Spacer fabrics, woven or knit textiles are the best.”
Indeed, Call says a seat cushion’s thermal mass is just the beginning of the heat equation.
“That’s just one feature,” he notes. “These products all also have breathability. Breathability is based on where can air move, or can air move at all through the product? Is there a designed path or channel?
Certain products actually have a specific feature that allows the air to move in relation to the body’s interaction with the surface. So for example, if a product has grooves cut in the foam so there are blocks in the top of the foam, those channels become a place for the air to move.”
When those design features are paired with action on the consumer’s part, the results can be significant, Brown says.
“There are two ways to promote airflow and reduce heat and moisture buildup,” she says. “One, and the most effective, is motion. The movement between the user and the seating surface pumps air over the seating surface and client, lowering the temperature and drying the area.”
Call explains how that motion could occur: “If the person does a forward lean for pressure relief, then air is able to rush into those channels as the cushion uncompresses, as their weight comes off and the cushion swells back up. And that draws fresh air into that cushion.
“Then when they sit back up, that air is expelled again. That breathability we like to describe as an air wash. That breathability combines with the thermal mass to determine how much heat is displaced each time a person moves or does a pressure relief lift , or how much heat is trapped and how much of it can actually be given up.”
The Impact on the Human Body
Studying and understanding heat, breathability and all these factors are important, of course, because of the serious effects that heat can have on consumers.
Chaney explains, “If the body cannot counter this increase in skin temperature, the body will start to sweat. If the materials next to the skin are unable to wick away moisture and do not have sufficient ventilation to allow evaporation, then the sweat will remain next to the skin.”
While that moisture can be uncomfortable or embarrassing for the consumer, Chaney points out there can be even more serious results if that moisture is unable to evaporate.
“Pressure ulcer development is known to be influenced by multiple factors, including pressure, shear, moisture, temperature, mobility and the tissue’s ability to tolerate mechanical forces as influenced by the individual’s health conditions,” she says.
“It is known that a 1° C increase in skin temperature will cause an increase in metabolic demand by 10 to 13 percent,” Chaney adds. “However, in sitting the tissue is compressed, restricting the blood flow and meaning that this extra demand for nutrients and oxygen needed to maintain healthy skin cannot be met. This means that an increase in skin temperature as small as 1° C can significantly increase the potential risk of pressure ulcer development. There is also research that suggests that cooling the skin can reduce the likelihood of pressure ulcer development.”
When that doesn’t happen and moisture is held next to the skin for extended periods, the result can be maceration, a softening of the skin, Chaney says.
“Macerated skin tends to look whiter and swollen and feel damp to the touch,” Chaney says. “Moist macerated skin is more vulnerable to fungal and bacterial infections, which can cause discomfort and result in a foul smell. In addition, as the skin becomes softer it becomes much more vulnerable to breakdown from pressure, shear or friction.”
What Media Are Best?
It would seem that the best way to combat heat and moisture would be to choose the media “best” able to keep both to a minimum. But as with so many seating & mobility considerations for highly involved clients, choosing a medium to work with is complicated.
Samila says, “It is a bit of a quandary. What material is best as the best materials for heat and moisture control are not hygienic or easy to clean. The easiest-to-clean-and-maintain materials, just by their nature, are smooth, shiny surfaces that are very poor heat- and moisturecontrol materials.”
Call also says all these materials have a conduction value: how much they conduct heat.
“For example, when you fill that pitcher of water at your kitchen sink, if you have low water pressure or you have an aerator in your sink, then that water flows slower than if you have high pressure or no aerator in your sink or in your faucet,” he says. “That rate of heat loss in a cushion — it can be either conduction or radiation by which the cushion gives up the heat — also determines how fast that resets the temperature to a physiologically desirable temperature.”
In addition to conduction value, providers and clinicians need to consider how the materials are used, such as what forms they take.
“People typically think of a rubber material that’s filled with air as being a hot surface to sit on,” Call points out. “They feel that way based on how well the air moves through the product.
“However, if that product has fingers and air channels, then when a person does a forward lean or they do a pressure-relief lift , the thermal mass of the rubber is actually quite low because it’s just filled with air. So the air that rushes in during that pressure-relief lift and then washes back out is able to carry a large portion of the heat the cushion had trapped with it. An airfilled, rubber kind of cushion actually can be fairly cool if the user is trained properly and is a fairly active sitter.”
Incorporating “breather holes” into a cushion design can also help keep temperatures down, Call says: “You look at the breather holes and think, ‘That’s not a very big hole; what can it really do?’ But in testing, we’ve seen that if you put a breather hole in the surface and then you cover the cushion with a spacer fabric — the vertical fibers with a knit surface on both sides — if you use the right thickness of spacer fabric over a pattern of holes put into the product, it actually creates a slight chimney effect, where air is able to enter as the body begins to heat up the air in the spacer fabric. The air naturally begins to rise, drawing air through these breather holes as it vents out of the spacer fabric. So design is critical to make this work right, but you actually can get a couple of degrees’ temperature drop for a patient who is confined and not able to perform pressure-relief lift s or be mobile, just by how you structure the fabric and the breather holes that would go with it.”
The Effects of Climate Change
So far, we’ve discussed “internal” factors related to heat and moisture, from the body heat that humans generate, to the materials that hold heat and get rid of it, and the designs that can help heat to dissipate more quickly.
But there are also factors to consider that go beyond the client or seating system, including the climates your client will be living in, visiting and spending time in.
How much does the weather impact what your clients feel and endure? Call cites a paper he and Japanese researcher Hideyuki Hirose wrote that described wheelchair users as sweating on par with extremely active athletes.
“Patients in Japan were sitting in wheelchairs in the sun on a typical day — not an overly hot day,” Call says. “Hirose used Attends — essentially, diapers — weighed them, put the patients in them and set the patients in this activity area in the sun. They weren’t active; they were reading and sitting.
“In that hour because of the temperature and the humidity, they sweated as much as an aggressive athlete performing their sport does. We’ve documented wrestlers that lose 12 to 18 lbs. of water in a three hour wrestling practice. That’s intense. Eighteen pounds of water is just over three gallons of water that the person loses. These patients were sweating about the equivalent of 12 lbs. of moisture loss. And they were just sitting, just at rest.”
Additionally, relieving heat can be much more difficult in warmer climates.
As Call points out, “If the ambient temperature around you is 100° F and you do a pressure-relief lift , what have you just done? You didn’t gain anything because your body is 98.6° F. The same thing’s true of humidity. The ideal is someone who lives in an environment where it’s 70° F all the time and the relative humidity is about 15 percent, because then you have this large difference between the environment and the body-cushion interface, and a pressure-relief lift makes it work perfectly.”
Unfortunately, those helpful conditions aren’t always possible, and Call adds, “Our population are the ones who don’t always have the money for nice air-conditioned homes, central-air kind of interventions that keep humidity low and temperatures low. Everything I’ve described so far relies on an environment that has lower temperatures and lower humidity than the body interface. Otherwise, these things don’t have the potential of helping.”
Assessing Who Is Most at Risk
All of your complex rehab mobility clients use seating systems and/or seat cushions. Should they all be assessed for heat and moisture issues? Do some clients need more consideration in this area than others?
“There are many factors/criteria that a clinician or provider should consider when determining whether heat retention could be a concern for a particular client,” Stern says. “If the client has a history of skin breakdown, then it should be determined if heat retention played a role in that skin breakdown. But there are other factors, such as environment, client diagnoses, cushion material, etc. I think that temperature should be a concern for most clients whether they’ve had a history of skin breakdown or not.”
“The seating team needs to consider the presentation of the individual consumer,” Tanguay says. “Does the consumer appear to be damp at the cushion interface? Especially at risk are consumers without sensation who may not be aware of moisture retention at their seat surfaces. We should be thinking about heat and perspiration as well as incontinence.”
She adds that even consumers with sensation could be at risk if they’re “without a means of movement to promote airflow at the seat surface interface…That movement could be as simple as weight shift during wheelchair propulsion and transfers throughout the course of the day. Another ‘higher risk’ group would be sedentary consumers who may have decreased cognition and ‘fragile’ skin with decreased muscle tone. Think: frail, elderly consumers with some dementia.”
Chaney mentions consumers could be at higher risk if they have “an impaired ability to regulate their own body temperature as a result of their health condition. Conditions that affect thermoregulation can include fever, overactive thyroid, seizures as well as some neurological conditions such as tetraplegia, where the sweat response may be impaired, and multiple sclerosis, where excessive sweating is common.
“For some wheelchair athletes who do not have intact thermoregulation, there is a danger of heat exhaustion. Therefore such players, e.g., in wheelchair rugby, may wear ice vests before a game, or mist themselves with a spray bottle to try to keep cool.”
Making the Right Choices
The wide range of clients who could be at higher risk for skin breakdown related to heat and moisture points out the enormous challenge of making the “right” call when choosing seating system components.
For instance, an incontinence-resistant cover that’s easily washable might seem a good choice for a senior with symptoms of dementia. But as Call points out, “ You do have to be aware of the reduction in breathability that comes with even just an incontinence cover.”
Similarly, seat cushions that specialize in skin protection and positioning for clients with complex needs may not be made of materials that are the most able to quickly release heat.
“These issues are always things that you have to deal with on a patient-by-patient basis,” Call acknowledges. “So when you’re looking at a patient who perhaps has cerebral palsy and is incontinent, you’ve got a tough one. You either have to have incontinence aids on the patient all the time, or you have to have a product that is incredibly easy to rinse off or wipe off to deal with incontinence. If you look at those in concert, you have to put it together in a way that allows you to create something that is effective for the given case.”
An example of a possible solution, Call says, “would be using one of the combination products. You want some good pressure-relieving characteristics, but you still want to contain the patient, so hybrids that have both the air and the foam” might work well.
With such cushions, Call says, “What you do with those products is you essentially take that low thermal mass of the air product and you combine it with something that would typically be thought of as insulating. The foam would be a hot surface as well. But if the person is active enough to do the pressure-relief lifts, you are able to move the heat out of the air bladder area and maintain the desirable cushioning. Of course, air bladders aren’t good for stability, so if you can’t gain enough stability from the surrounding foam structure, then you have to go to a different technology and provide it in a different way.”
Even custom-molded seating — which can be especially adept at holding heat because it’s foam “that’s been carved very specifically, shaped to hold this person and very intimate in the contact with the body,” as Call says — can be made “cooler” by enhancing its breathability by adding, as Call’s colleague did in one case, a layer of honeycomb atop what Call describes as typically “a very robust surface: a rubber, a PVC [polyvinyl chloride] or some other coating that provides durability.”
Custom seating for clients with high involvement can be among the most challenging, heat and moisture wise.
“People who we find in a custom seat, particularly if it’s a seat and back combination with high involvement with wings to help position their arms or to keep them upright or to provide them some base for what function they can obtain — then that breathability goes away, and you start insulating the body and trapping the heat. Oftentimes you’ll see people with those conditions with heat rash, sweat accumulation and potentially moisture-based maceration or abrasions on the skin based on that. So the best thing you can do in those cases is to provide a path for breathability.”
Call suggests that teaching clients to monitor their bodies, if they’re capable of doing so, can empower them to act quickly when noticing changes in temperature and moisture.
“That’s where training and experience come in again,” he says, “because most of these patients, even though they’re insensate, will actually become able to understand that they’re feeling some heat stress, and they need to cool off .
“Perhaps they’re very active, outdoor sports kind of people, and they do develop a sense of stress. Even though they’re not sensate, they can tell that their bodies are working hard to cool off . So they need to intervene, do a lift , get out of their chair. That training becomes important, and it’s a little bit hard because these are subtle differences: Is my heart rate going up? Why is it going up? Well, your body’s working harder to cool off . Or, If I’m not comfortable, is it time to put a warm or a cool cloth or a damp cloth on the back of my neck to stimulate a response to the heat environment?”
Seating & mobility professionals on the front lines with clients — and particularly those who work with newly injured or diagnosed clients — are in an especially good position to teach consumers to pay attention to the clues their bodies give when heat becomes a problem. “They’re also going to have to educate them in these subtle changes and issues to watch for and the interventions, then making them effective based on habit,” Call says.
And all seating & mobility providers, clinicians, consumers and caregivers might also do well to rethink what they believe about skin breakdown.
“We talk about ‘Is it a pressure sore, or is it a shear sore?’” Call says.
“Well, ‘Is it a pressure sore, a shear sore or a temperature sore? Because all of the interventions actually impact all three.”
Glossary: Heat Defined
BTU: Abbreviation for British Thermal Unit, it’s the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit.
Celsius to Fahrenheit conversion: Start with Fahrenheit temperature; subtract 32; multiply by 5/9.
Fahrenheit to Celsius conversion: Multiply Celsius temperature by 9/5, then add 32 to the product.
joule: Named for James Prescott Joule, it’s a metric unit of work and energy equal to the work done when a one ampere current is passed through a resistance of one ohm for one second.
Sources: Old Farmers Almanac; U.S. Energy Information Administration
This article originally appeared in the May 2013 issue of Mobility Management.