Navigating the Pressure Gradient

Mapping Technology Helps Clinicians Assess and Teach

Most of us are familiar with the nuances of a weather map. Meteorologists rely on colors to indicate weather changes from precipitation to temperature — think red for temperatures that soar above 100 and blue for those that drop below freezing. But a mapping system also can help clinicians and suppliers see the same color-coded gradients for pressure — red for high and blue for low. Pressure mapping is a useful tool for comparing the efficacy of cushions and teaching clients how to better manage pressure.

Geometry of Pressure

If pressure were a mathematical equation — and by the way, it is — it would look something like this:

Force/Area = Pressure

"Although we say pressure mapping systems, actually the systems, what they do is they measure force and then they calculate pressure," says Dr. Norman Murphy, medical product manager and director of product and market research and development, Medical Division, Tekscan, Boston. "It's the pressure gradient that's important when we're dealing with mobility."

And that pressure gradient — the color-coded map of the area on which pressure is exerted — shows clinicians the areas that are absorbing the most pressure.

The inside of a pressure mapping mat is "like a series of miniature bathroom scales in kind of a large sheet of paper," explains Murphy, who's been involved with pressure mapping for about 15 years. "And these little bathroom scales (called sensing elements) measure the amount of force, just like standing on a bathroom scale. But instead of having one bathroom scale, you've got several thousands of them. By knowing the area of these little bathroom scales, pressure can then be calculated."


It's a great tool; it's just that it's not as absolute as say a thermometer. —Tom Whelan, global vice president, seating business unit, Sunrise Medical

Tom Whelan, global vice president, seating business unit, Sunrise Medical, Longmont, Colo., explains that the pressure mapping mats actually contain sensing points down a row column scanner. The system "shoots a voltage into A, reads it at point 1. And inside of that mat there's a contact point, where as you apply more pressure to it, the voltage changes," he says. "You have a voltage going in and a voltage coming out and … there's a relationship between the force on that point and the amount of voltage that comes through."

Dr. Stephen Sprigle, director of the Center for Assistive Technology and Environmental Access, and associate professor, Departments of Industrial Design and Applied Physiology, Georgia Institute of Technology, Atlanta, runs the Rehabilitation Engineering Research Center on Wheeled Mobility and Seating, which includes interface pressure measurement. He explains, "Each of (the mats) differs in terms of the size of that sensing area as well as the space between them. So, the spatial resolution of the sensors can vary. However, most of them have a high spatial resolution, certainly much better than we've ever had in the history of pressure mapping. So, they work in a very similar way even though they use three basic technologies. You press on them and the voltage changes relative to how hard you're pressing. That's what the software measures is that voltage change and relates that to a force for pressure."

If a Surface Could Talk

OK, so pressure mapping isn't as simple as looking for the high temperature or a belt of rain on a weather map, but the system is easy to use and the data provided is useful.

"To set up a system, put it under a patient and get it to generate data is pretty easy," says Whelan, who helped co-develop a new pressure mapping system with Tekscan. "Most of the systems now are USB; they're plug-and-play. So, you have software, you plug in the sensor in the electronics through USB, they recognize each other and they're ready to work.

"The problem comes (with) now I've got data, what does it mean?" Whelan says. "That's where it starts to fall apart because there is no standardized method for analyzing the data output of a pressure mapping system. There's no clinical validation of any particular way to look at it to make a clinical decision."

In fact, for nearly three years, ISO tried to develop a standard for testing cushions for pressure distribution using pressure mapping and failed, says Whelan. "They couldn't make it work because the variability between the systems and the unreliability of duplicating measurements was so poor that you really couldn't bring it forward as a standard. It's a great tool; it's just that it's not as absolute as say a thermometer."

Sprigle blames a lack of agreement among labs and mats for the failed standards. "Standards tests tend to use models rather than human beings. So, using a standard buttock model, we attempted to use interface pressure mapping technology to characterize cushion performance," he says. "And we could not do that. The problem (is) that we could not get the mats to agree across labs. So, one lab would get different values than another lab. And of course, the mats did not agree. So we looked at different mats at different labs (and) could not get agreement, therefore we could not incorporate them into a standard."

The challenge with pressure mapping lies with the sheer bulk of information provided. "There's a lot of numbers to interpret," says Sprigle. "Part of that interpretation is knowing or recognizing error vs. true information. That's the same for any measurement, but when you have 1,300 of them, it becomes more difficult. But, to cut to the chase, I think the two biggest points I make to clinicians is one, don't sweat a single value, and two, it's a much better tool to rule out cushions or surfaces than to prescribe them."

Keeping the System True

Interpreting the numbers means paying attention to whether or not the data provided is accurate. A process called calibration sets up a lookup table of voltages. "Every time you pressure map, there's a baseline of what each of those sensors is bringing in: 0 pressure, 10 millimeters, 20 millimeters," says Barbara Levy, PT, ATP, Wheelchair Seating Clinic manager, CarePartners Health Services, Asheville, N.C. "As you increase the pressure, it calibrates the sensors to all try to be equal. And then you save that calibration file and that's what the mat is reading relative to that calibration file every time you take a read of somebody. The sensors all need to be reading the same. What one sensor reads as 50 millimeters of mercury, another sensor might read at 200. Even though it's the same amount of pressure, that sensor has just gotten out of calibration."


I think the two biggest points I make to clinicians is one, don't sweat a single value, and two, it's a much better tool to rule out cushions or surfaces than to prescribe them. —Dr. Stephen Sprigle, CATEA, Georgia Institute of Technology

Sprigle says calibration helps to refine the relationship between pressure and voltage in the system. "It's no different than tweaking your bathroom scale to make sure it always starts at 0," he explains. "Or tweaking it to make sure you never weigh more than 125 lbs. In one case, you're trying to prove the calibration; in the other case, you're purposely messing it up. So, (calibration) is trying to improve the accuracy."

But many people who use pressure mapping fail to calibrate the system often enough, if ever. If the system isn't calibrated frequently enough, values can start to lose meaning.

Sunrise and Tekscan actually did a study on calibration before launching the new system, asking participants how frequently they calibrated pressure mapping systems. "The funny thing was that the mean time to calibration was a little over six months," says Whelan. "That means some people did it weekly, (and) some people had had a pressure mapping system for five years and (we asked) when do you calibrate it? 'Well, what's that?' was the response. It's kind of a mixed bag. But what we found was the best system calibration will hold about 30 days tops. And typically if you're using it routinely in a clinical setting, two weeks is about the max."

Sprigle says, "Every mat needs to be recalibrated periodically. And my rule of thumb is ask the manufacturer how often to calibrate and do it three times as often."

"The fact that a lot of therapists don't even know they need to calibrate the mat is a concern because if one sensor gets out of calibration… you're not going to get an accurate read," says Levy, who uses Vista Medical's FSA system and also Xsensor.

Another problem with interpreting data is that the mats interfere with the pressure reading. "Our data suggests that mat design and cushion design interact, meaning certain mats affect the readings on certain cushions more than others," says Sprigle. "So, in fact, the mat effect is not equivalent across product."

Whelan explains that sometimes the mat can wrinkle and interfere with a reading. "If you can imagine trying to force a piece of paper into a coffee cup, it's got a wrinkle and fold that go into the coffee cup," says Whelan. "What happens is when you sit on these mats and you put this layer of non-conforming material between the user, which is compliant tissue, and a cushion, which is some type of compliant material, the pressure mat tends to interfere with the picture that's being formed."

Lauren Rosen, MPT, ATP, Motion Analysis Center program coordinator, St. Joseph's Children's Hospital, Tampa, Fla., calls the wrinkling effect hammocking. "Some of the mats do not stretch or conform to the client, so there is a hammocking effect that limits the ability to see the client's true interface with the cushion," she explains. "Newer systems have mats that conform better."

Rosen also says some cushions cannot provide an accurate reading. "Some cushions … do not map well with pressure mapping because they provide large areas where there is no contact between the skin and a cushion. So, pressure mapping will not give a good representation of the pressure relief for those cushions."

Clinicians should use their hands to check for hammocking during pressure mapping, says Levy, because hammocking "can lead to peak pressures that aren't truly a problem area."

Another Tool in the Belt

Pressure mapping complements other clinical practices during a seating assessment, like visual skin inspection and using the hands to check the fit of a cushion, says Levy, but it is not the be-all, end-all for seating assessments. "I think some folks in this industry and some patients become very caught up in the data collection and tend to get hooked on pressure mapping," she says. "The data can be manipulated to 'look good.' This is not the only variable that needs to be considered in selecting an appropriate cushion for a wheelchair user. Pressure is not the only cause of pressure ulcers."

Pressure mapping is "only useful in identifying which cushions are not good enough, not which cushion is best," asserts Sprigle. "Because choosing your best cushion, you cannot use pressure mapping as the single variable upon which to judge that question. There are way too many other factors: posture, transfers, function, comfort, temperature, all that stuff has to enter into the stew."

In fact, pressure mapping's main clinical use is comparing cushions. "If I wanted to compare two cushions, (pressure mapping) … one of the outputs it will tell you is how many square inches of the mat are actually loaded," says Whelan. "So, if I compare two cushions, one that has greater surface area to one that has worse or less surface area, since you have a fixed load, the one that has more surface area is generally going to have better stability, better pressure distribution just because you've got more area to work with.

"Without worrying about the exact number, you can look at it and say, 'How is the pressure distributed?' " continues Whelan. "If you sit on the cushion and you have the same amount of surface area on two cushions, but in one cushion you can clearly see that most of the pressure is concentrated right under the ischial tuberosities, where on the other cushion it's more evenly spread out over the cushion, the one which has better distribution or is more evenly spread out is likely going to be a better product for that user."

Murphy agrees. "(From) a clinical perspective, those who treat mobility issues want to know if the seat cushion on the other interface that they're using will increase pressure," he says. "If so, then they realize that it might not be the ideal device to use.

"When a person is sitting on a cushion or lying on a mattress, there is no means to know how much pressure there is or where the pressure is located," explains Murphy. "Pressure mapping provides that information. It allows us to see in between the buttocks and the cushion."

The one-of-a-kind view can mean the difference between a pressure sore and healthy skin. In fact, Murphy says pressure mapping is a must for those at high risk for pressure ulcers and for those who already have pressure sores.

"I do not use it for all clients," says Levy, who has worked with pressure mapping for 13 years, "(but mostly for) those who are insensate, have a history of skin breakdown or who cannot communicate. For instance, a user receives a custom molded seat or back cushion and tends to fidget within the system, rotate or pull away from the cushion. If they can't tell me what feels bad due to a cognitive or language disability, I can pressure map them to see if there is an area that is causing a 'hot spot' of pressure, modify the cushion (and) recheck the fit with pressure mapping until the problem is resolved."

In addition to clinician use, pressure mapping has been a valuable tool for research and development.

"You could use it to measure bottoming out; you could match a contour to see how well it performed under a group of actual users," says Whelan. "You could sit there and say, 'I want to load the thighs and off-load the buttocks,' and you could measure through pressure mapping how effectively you did that, distributed pressure against the user set. There's quite a broad number of applications in research and development."

Power of the Pressure Profile

Though its primary use is helping to fit a client with the appropriate seating system, pressure mapping can also serve as an educational tool. Allowing clients to see their pressure profiles helps teach them how to off-load pressure.

Murphy says clinicians can use the systems to take live readings of a client sitting on the cushion and project the pressure profile on a computer screen. "(Clients) see the distribution of the pressures underneath their seat," he says. "And then, if you ask the patient to lean forward or lean sideways, then they can see a reduction of pressure where the body part is being lifted and an increase in pressure where they mold more of the body part."

Whelan says, "As a client feedback tool, it's invaluable. You can use (pressure mapping) under a client, for example, if you're trying to teach a client to do a pressure relief. Well, how much of a lift do they actually have to do before they unload their ischial tuberosities? Because they can see the effect on their interface while they're attempting the activity, it's great.


The fact that a lot of therapists don't even know they need to calibrate the mat is a concern because if one sensor gets out of calibration… you're not going to get an accurate read. —Barbara Levy, PT, ATP, CarePartners Health Services

"You can take a client who has issues with sliding out of the chair over time during active use during the day," says Whelan. Clients can look at the pressure map and then clinicians can instruct them to get them back into the right position against the seating system. "It's a fabulous tool for that because you can sit there and say, 'No, your ischials are still in the wrong place, you're still not loading right, you need to go back farther or you need to do this or you're sitting on one side or the other'… But I would argue that it's not used as much that way as it should."

Sprigle says, "You can also look at how forces on the buttocks change during a tilt-in-space maneuver or a forward lean to pressure relieve or how hard you have to press to do a depression pressure relief. So, they're quite visual and quite useful for many types of feedback and training in the clinic."

Rosen has used pressure mapping to teach a client about pressure relief on many occasions. "I use it with manual chair users who are doing their own pressure relief and with power chair users so they can understand how far they need to tilt their wheelchairs to get proper pressure relief," she says. "It really helps to visually see the pressure relief, especially if the patient has decreased sensation."

Rosen also says the graphs produced help justify specialized cushions to some payor sources. "I have used (pressure mapping) to show a client why a particular cushion is not appropriate for them, and I have used it to send to insurance companies to justify equipment. It has been a wonderful resource to have when I have needed it."

Though pressure mapping is typically considered a clinical tool, some suppliers use pressure mapping for equipment setups.

"I have dealers that use (pressure mapping) when they do setup and adjustment of a wheelchair," says Whelan. "Even simple things like measuring (and) adjusting the footrests for length, where they can put a pressure mapping system down and they can actually see what the ideal dimension for the footrest should be lengthened to so that there's a right amount of pressure under the thigh. Or is one back angle better than another back angle? Or what happens if I tilt the chair a little bit? So, for a good practicing dealer, the idea that they take this out and use it when they set up the chairs to optimize them can be very invaluable, in that it prevents them from having to go back."

In addition, Whelan says pressure mapping has been used on molding frames for custom seating. "When you do the molding, you can't really see what's going on underneath where you can't get to it. So, you can have a client that orthopedically looks great. They're all held up in the right position, but they may have such highs and lows at certain points in the mold that you can predict that they're not going to be comfortable and have much tolerance to the system." Pressure mapping "allows you to see that and make those adjustments before you commit to a custom molded seating system," he says.

Setting a Course Toward Methodology

The utility of pressure mapping speaks for itself as more clinicians and providers are using pressure mapping as standard practice. Developing methodology to streamline the technology is the next goal for manufacturers of pressure mapping systems and is critical for the seating industry.

"It's our job to develop the methodology behind the technology," says Whelan. "What works better? What's the right clinical practice? How often should you calibrate?"

Establishing a best practice "will help us produce data to convince the government that seating intervention is valuable," explains Whelan. "Since every clinic uses a different pressure mapping system a different way, you can't really collect data from multiple centers in a meaningful way because there's no standard protocol for the utilization. So, therefore, you can't compare the data. This could be really valuable data to helping us reduce the $1.3 billion spent on seating-related pressure sores."

Sprigle concurs. "I personally believe it's (pressure mapping) a useful tool. I think the interpretation of the map is the biggest challenge."

How Does Reimbursement Effect Pressure Mapping?

How Does Reimbursement Effect Pressure Mapping?

Within days of press time, the Centers for Medicare & Medicaid Services (CMS) issued a new power mobility device fee schedule. The slash in reimbursement amounted to 30-40 percent across the board. The significant cuts weigh on the minds of industry experts like Barbara Levy, PT, ATP, Wheelchair Seating Clinic manager, CarePartners Health Services, Asheville, N.C., who questioned how someone might be able to afford a pressure mapping system in the future. "I would say that more clinicians are able to get the system, but funding is a problem right now," she said "Reimbursement is not what it used to be. And I'm not sure if people have the money to buy the (pressure mapping) system."

This article originally appeared in the November 2006 issue of Mobility Management.

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