Using Pressure Mapping in Seating and Wheeled Mobility Assessment

Using Pressure Mapping in Seating and Wheeled Mobility Assessment

Elizabeth Cole, MSPT, ATP


Pressure mapping has become more and more widespread as a way to select the most appropriate skin protection product for individuals with disabilities.  Most often this product is a wheelchair seat cushion and/or a wheelchair back support. Pressure mapping can be a valuable clinical tool for this purpose when it is used appropriately and its limitations are understood.  However, we must consider that it can be improperly used, mis-used or over-used. Let’s look first at what pressure mapping is and what it can do and then identify its limitations and precautions.


In the simplest terms, a pressure map is a thin pad with sensors, which is connected to a pump.  When the pad is placed on top of a seating/lying surface and underneath the person, it provides a graphical representation of the interface pressure between the person and the surface.  The “picture” of the interface pressure is then depicted on a computer screen.  The surface could be a wheelchair cushion, a therapeutic support surface on a bed,or some other sitting or lying surface/device. For simplicity, we will use the wheelchair cushion when describing the process in the following paragraphs.


Through a display of pressure readings and different colors the pressure map can show:


1.A diagram of overall pressure distribution over the wheelchair cushion.  This shows the area in inches² or cm²of the person’s body (buttocks and upper legs) that is making contact with the cushion surface.


2.The peak pressure index. This indicates how well the pressure is distributed across the person’s buttocks and upper legs and where there are areas of high (peak) pressure.  The areas of pressure are recorded in mmHg and are depicted in different colors on the screen.  The highest pressures present as red,followed by yellow, then green and finally blue, which represents areas of lowest pressures. With most systems these representations can be displayed as 2-dimensional or 3- dimensional images. 


3.       Significant pressure differences indifferent areas, such as between right and left sides or between the coccyx/sacrum compared to the ischial tuberosities.  This could indicate postural asymmetries that create excessive loading in one area over another.


How then can we effectively and appropriately use the information from pressure mapping?  It could be used as one tool to help with the following:


·Compare different wheelchair cushions with regards to how well they distribute pressure and whether or not one cushion results in areas of higher pressures compared to the other(s)


·Identify any changes in pressure distribution and peak pressures with a particular cushion as one follow-up measure (among others) to track continued efficacy of a seating and mobility system.


Identify postural abnormalities that create asymmetrical pressures on the inferior surface of the pelvis and/or sacrum and coccyx when sitting on a particular cushion. The graphics and measurements could also be used to identify the effect of modifications or secondary supports added with this cushion for correction or accommodation of these asymmetries.  


Identify the effectiveness of different dynamic seating systems in redistributing pressure and reducing peak pressures when using this particular cushion. For example, you might compare the pressure distribution on this cushion using tilt-in-space alone compared to recline alone compared to a tilt and recline combination.  Or you might assess the differences in pressure distribution with different degrees of tilt or different degrees of recline.


Demonstrate the effectiveness of an individual’s weight shift method and the potential increased results using other methods (e.g., wheelchair push-ups, leaning forward or side to side).


Provide feedback regarding the effects of different wheelchair configurations and adjustments in redistributing pressure and reducing peak pressures with this particular cushion.  This could include the effects of different footrest lengths or angles, armrest heights, seat-to-back angles and so forth.


Provide education to the individual and/or caregivers regarding the effect on pressure distribution of proper versus improper use of a particular cushion, such as the effects of improper placement and orientation in the wheelchair or improper inflation.


Identify areas of high pressure for individuals who have cognitive and/or communication limitations and are unable to effectively communicate discomfort or pain.This could also include individuals who may feel pressure or discomfort but cannot distinguish the exact location. 


Demonstrate weight distribution and areas of peak pressures that occur on other weight bearing surfaces, such as toilet seats, shower chairs, tub benches, commodes, household seating surfaces and car seats.


Compare different therapeutic  support surfaces for the bed with regards to how well(in comparison to the others) they distribute pressure and whether or not one surface creates areas of higher pressures compared to the other(s).  It could also be used to compare the effects of changing the position of the bed while using a particular support surface(i.e., compare the pressure distribution in supine versus various degrees of elevation of the head of the bed).


As outlined above there are a number of uses of pressure mapping as a clinical tool.  However, to ensure that it is used appropriately and to the benefit of the individual, it is crucial to be aware of its limitations. These include the following:


·Pressure mapping should not be interpreted as an absolute reading of pressure.  The color displays are relative and are dependent on the calibration of the system and how the sensitivity is set. For example, if the sensitivity is set low, the color scale is more sensitive and areas of red will appear sooner, making the cushion look“bad”.  Setting the sensitivity higher with the same individual on the same cushion could decrease the amount of red and increase the amount of green, yellow or even blue, making the cushion look“good”.  However, although the picture changes, the pressures and the distribution remain the same. The picture can also change significantly if the device is not calibrated often and accurately. And the pad must be placed on the cushion in the same orientation and location each time.


§  The interface pressures recorded on the pressure mapping system do NOT correlate to internal pressures in the body (particularly over bony prominences) or to capillary closing pressure and should not be used as a sole or precise predictor of risk for damage from pressure for an individual.  Few of the pressure points are actually true readings of pressure, especially if a person is sitting on a contoured cushion.They should only be used as relative readings.


·Many pressure mapping systems only provide a picture of the pressures at one point in time and do not record what changes may occur if the person transfers or has been transferred differently into the system, if they are repositioned on the cushion, or if they move around in the system (voluntarily or involuntarily).  It is important to allow the person to sit on the pressure map for 15 – 20 minutes before taking a reading and, if possible, to allow the person to complete some sort of typical activity during the mapping session, such as wheelchair propulsion,reaching, manipulating, using a computer or communication device and so forth. 


·Pressure maps interact differently with different cushions depending on the material they are made from, such as foam,air, gel and viscous fluids.


·Pressure mapping does not always identify the specific cause of pressure.  Peak pressures and/or poor pressure distribution might have nothing to do with the cushion, but instead might be due to improper wheelchair adjustments, lack of/improper correction or accommodation of postural abnormalities, ineffective tone management, etc.  In addition, they might not be present at the time of the mapping session but might occur later due to poor or harmful transfer methods, infrequent weight shifts, weight shifts that are too brief, voluntary or involuntary movements, changes in body size and composition and so forth.  Pressure mapping can point out potential problems at the moment but cannot  predict long-term changes in equipment or anatomy and physiology


·Pressure mapping does not assess the effects to skin integrity from friction, shear, moisture or any of a host of intrinsic risks (i.e. aging skin, poor nutrition, dehydration, incontinence, compromised immune system, immobility, cognitive or behavioral issues, etc).


·Individuals who have had surgery on their buttocks(i.e., muscle flap surgery, shaving of an ischial tuberosity) or orthopedic issues such as lower extremity amputations may display unique results with pressure mapping.  These potential anomalies must be considered when interpreting the data.


·When considering a change in wheelchair cushion the individual should first be mapped on his/her existing cushion and wheelchair,then on a firm flat surface and finally on the cushion or cushions (and in the wheelchair) being considered. This provides the clinician with a baseline with which to compare the new cushion. The individual’s posture and comfort should also be assessed in each trial, including palpation of areas that correlate with peak pressures seen on the display, especially bony prominences.


Pressure mapping can be a very helpful item in your toolbox to be used in the provision of products and education for skin protection for an individual. It can help in selection of a new product, assessment of the effectiveness of a current product, and assessment and education regarding weight shifts, posture, wheelchair set-up, bed position and so forth. However,it is only beneficial if used correctly, it only provides relative information and it is only one small piece in the overall puzzle.  No matter what, it should never be used as the only indicator that is considered when assessing whether or not a product is appropriate and effective.  And it should never take the place of a hands-on assessment and visual inspection by the clinician,caregiver or the individual him/herself.