Pressure Mapping – Important Considerations
To derive any benefit from a session with a pressure mapping system in a seating evaluation there are some basic elements that must be considered.
The pressure mapping system must be calibrated. If it is out of calibration the results are worthless because they can be off by as much as 50%. It is sometimes very evident when a pressure mapping system is not calibrated because the screen will show an unusual grouping of high or low readings that cannot possibly be right.
In the example below there is a large rectangular section on the left side of what appears to be maximum pressure. This cannot be accurate because the posterior does not follow this shape.
Clearly this pressure mapping system is in need of calibration.
Calibration takes time and it is bothersome especially for therapists that typically do not have the time to calibrate. The X sensor pressure mapping system is even less convenient because has to be sent to the factory for calibration.
The following statement is taken from an article written by Kendra B. PT. where she describes one of the potential problems with an adjustable scale feature pressure mapping system when comparing wheelchair cushions.
A key feature of this technology is the ability to adjust the sensitivity of the pressure reading. Values can be set between 10 to 200 mmHg; a higher level yields a higher pressure reading and shows more red on the digital image. “This is so important if you’re comparing one pressure map to another,”. To make comparison valid, they both must have the same sensitivity setting. “Lowering (the sensitivity setting) makes it look better (more blue, less red). You can’t compare one map that’s calibrated to 125 mmHg and one that’s calibrated to 200 and try to draw inferences about which one is better for the individual. You need to compare apples to apples.”
Another factor to consider is manipulating the full scale of the pressure mapping system. This is the bar graph to the right of the image above. The full scale is simply the range the pressure mapping sensors are displaying. As you can see above, the full scale range is 200 mmHg. This means red represents the corresponding maximum pressure of 200 mmHg while light blue represents 20 mmHg and green, yellow and orange are between the two extremes. In this standard set up green represents 100 mmHg.
Now imagine of the full scale were changed from 200 to 100 mmHg and how this would affect the pressure mapping result. The former 100 mmHg represented by green would now be shown as red. The results are off by 50% and an incorrect decision may be made based on these inaccurate pressure map readings.
The PS 256 pressure mapping system has a fully adjustable peak value slider bar which allows the user to change the colors of the scheme. The operator can manipulate the colors to make and pressure mapped cushion look horribly bad or fantastically good. They suggest that this is so the operator can control where the red graphic display will start. For example, if the slider bar is set to 125 then any reading over 125 mmHg of pressure will present in red.
Another pressure mapping system that allows complete manipulation of the results is the x sensor by roho which lets the operator change the full scale (red display set point) at 50 mmHg or higher. Instead of the full scale being 200 mmHg it can be set as low as 50. This degree of manipulation is like comparing apples to oranges and the results are useless.
The bottom line is that pressure mapping can be a somewhat useful tool if used properly and fairly. The operator must keep the settings the same when comparing cushions. The equipment must be properly calibrated too or the results are useless. Be aware and ask questions when having pressure mapping done. In comparing the efficacy of a cushion, the best indicator is the condition of the skin and tissue after a reasonable amount of sitting time on the cushion.