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Pressure Mapping used in SCI

Pressure mapping technology has been available for many years but remains controversial in its use and interpretation from both clinical and research perspectives (Jan 2006). Pressure mapping systems measure interface pressure. Pressure is defined as force over area (Gutierrez et al. 2007). Interface pressure is defined as the pressure that occurs at the interface between the body and the support surface (Barnett & Shelton 1997).

A pressure mapping device is an array of sensors contained in a flexible mat that measure interface pressure between the user and the support surface. The pressure values and surface contact area measured by the sensors is displayed in a colour-coded image on a computer screen, which includes a numerical value at each sensor location on the image. The clinician must determine the location of bony prominences on the image through manual palpation (Jan 2006).

There are many factors that confound the use and interpretation of pressure mapping data. Interface pressure is only one of many contributing factors to the development of pressure ulcers. Some authors caution that the relationship between interface pressure and pressure ulcer incidence has not been studied well enough (Brienza et al. 2001), and that other contributing factors (extrinsic: skin moisture, friction, shear; and intrinsic: nutrition, age, arterial pressure) should also be taken into consideration (Rondorf-Klym & Langemo 1993; Barnett & Shelton 1997; Shelton et al. 1998). Subject variability, such as body weight, muscle tone, body fat content and skeletal frame size also influences interface pressure (Barnett & Shelton 1997; Shelton et al. 1998; Hamanami et al. 2004). The subject themselves influence interface pressure in terms of how they get onto the support surface as well as how they position themselves on that support surface (Hanson et al. 2006; Shelton et al. 1998).

Time is also a confounding factor. Pressure applied between the surface and the subject changes over time (Hanson et al. 2006). Interface pressure is impacted by both magnitude and duration (Sonenblum & Sprigle 2011). Studies are suggesting that high pressure over a short period of time and less intense pressures over a longer period of time have similar risk in terms of potential skin damage (Kernozek & Lewin 1998; Stinson et al. 2003; Barnett & Shelton 1997). This time factor has also caused considerable controversy in how long a client should sit on the pressure mat to obtain a reliable reading of pressure (Kernozek & Lewin 1998; Stinson & Porter-Armstrong 2007; Eitzen 2004).

Pressure mapping systems are highly dependent on material properties of the pressure transducer, soft tissue and the support surface which may cause variability in data output. “Pressure mapping equipment may, in itself, cause several methodological weaknesses. Size of sensor mat, the number of sensors, and the sensitivity of the system will influence the resolution, accuracy, reliability and replicatability of the measured pressure values.” (Eitzen 2004, p. 1137).

All of these confounding factors contribute to the difficulty in interpreting the results of pressure mapping data collection. Since there is much variability between data collected for each client, an absolute threshold of pressure values has not been identified (Jan 2006; Brienza et al. 2001). “Research has not identified a general interface pressure threshold below which pressure ulcers will not develop…There is no proven relationship between 32 mmHg threshold and pressure ulcer susceptibility” (Jan, 2006, p. 33-34). Several articles point out the need to use caution when interpreting quantitative measurements from different pressure mapping systems, as there are no industry standards in terms of data output for these systems and there are no standard methodological guidelines for pressure measurement (Ferguson-Pell & Cardi 1993; Hanson et al. 2006; Barnett & Shelton 1997; Eitzen 2004). “The gage pressure values generated by the system should be used with caution. Valid comparisons can be made between one surface and another for a single user. It is suggested that interface pressure measurement is better for identifying inappropriate support surfaces than for determining appropriate ones.” (Jan 2006, p. 33)

The critical parameters for an interface pressure measurement system include: overall mat size (smaller pads may not capture distribution of tissue loading), flexibility of the mat so it can conform to the deep contours of a cushion as the client settles into it, resolution (number of sensors per square inch – more sensors improves reliability), accuracy and repeatability(Barnett & Shelton 1997; Eitzen 2004).

The type of data used is also important. While clinicians tend to use the colour-coded image, researchers depend more on the numerical data. The most commonly used numerical data is maximum pressure, defined as the highest individual sensor recorded as a single value, usually seen at a bony prominence. Use of this single measurement value is limiting, as there is no indication of number of peak pressures, the size of the peak pressures or the average pressure for the entire surface. The other commonly used data measure is average pressure, which is defined as the mean, or average, of all sensor values (Shelton et al. 1998). Many researchers believe that “interface pressure data should only be used for relative judgments between surfaces tested under the same conditions” (Shelton et al. 1998, p. 33).Stinson et al. (2003) identified that controversy exists over stability of the average pressure values versus the maximum pressure values as a measure for research.

The inter rater reliability of interpreting pressure mapping data output was evaluated by Stinson et al. (2002). The study used Occupational Therapy students with little/no experience in pressure mapping system use and Occupational Therapists (OT) with experience in use. Both groups ranked pressure map images from different cushions from best to worse. Significant agreement was noted for students (p<0.001) and experienced OTs (p<0.001), however it was noted that students experienced greater difficulty ranking the group of pressure maps done on mid-high pressure reduction cushions. Inter-rater reliability indicated perfect agreement of modal ranking for OT students and experienced OTs. In comparing pressure map ranking with numerical ranking agreement was found to be perfect for maximum pressure data (w=1.000) and near perfect for average pressure data (w=0.9).

Stinson et al. (2002) conclusions  were:

  • Pressure mapping systems users can reliably use pressure maps to guide intervention.
  • Visual Interpretation of pressure maps is as reliable as the use of numerical data from pressure map systems.

In 2007, Stinson & Porter-Armstrong completed another study which evaluated whether using just colour coding is an appropriate method of assessment compared to the use of the numerical output of average and maximum pressure values using 27 subjects with Multiple Sclerosis (15 wheelchair users and 12 non-wheelchair users). Visual ranking of colour coded images was correlated with average pressure and with maximum pressure for each pressure mapping image.

The author suggests using numerical valueswith the visual image for interpretation. The use of visual interpretation alone may be helpful only in eliminating inappropriate cushions with extremes of pressure. Pressure mapping can be a helpful adjunct to clinical judgment as there are other contributing factors besides pressure in wound development that need to be considered in the provision of appropriate seating surfaces (Stinson & Porter-Armstrong 2007).

Stinson and Porter-Armstrong (2007) results were as follows:

  • Low to little correlation between visual ranking and average pressure on all 6 cushions for wheelchair users; no statistical significance was found.
  • Statistical significance found for visual ranking and maximum pressure for wheelchair users on foam (p<0.005) and polyester fiber (p< 0.01) but no significance found on any other cushions.
  • Areas of maximum pressure can easily be identified on the colour code pressure image and therefore are used as bench marks when visually comparing surfaces for pressure distribution
  • Sole reliance on visual interpretation of pressure maps may lead to inappropriate cushion provision.

Many studies throughout the remaining sections have used pressure mapping to assist in identifying the levels of in interface pressure related to posture and positions as well as changes in postures in positions. The reader is asked to keep the above considerations in mind when reviewing the following studies that use pressure mapping.

Table: Comparison of Interface Pressure to Other Pressure Ulcer Risk Factors

Discussion

Taule et al. (2013) identified significant relationships between unsatisfactory sitting pressure and the type of wheelchair and having a history of pressure ulcers. However, in the article there were no indications how the conclusion of satisfactory or unsatisfactory sitting pressure were made as the comparator to these types of factors. The results of the study are based on the conclusion drawn at the start of the study related to satisfaction or dissatisfaction with seating position/pressure however the methods used to make the decision was not clear. Items were dichotomized however, the methods by which factors such as cushion and wheelchair type or co-morbidities were considered and/or weighted in that determination of satisfactory or unsatisfactory sitting pressure was not indicated.

Conclusion

There is level 5 evidence (from one observational study; Taule, et al. 2013) to suggest that pressure mapping can be used to augment clinical decision-making related to pressure management. 

  • Pressure mapping can be used to augment clinical decision-making related to pressure management.

Summary

The use of pressure mapping systems for evaluating interface pressure in the SCI population is not well documented. This may be due to limited standardized methods and procedures for use with these systems and the difficulty in comparing results across systems. It may also be due to the multiple factors that influence the variability in interpretation of results with any confidence for not only the SCI population but for the wheelchair user population as a whole. The effectiveness of pressure mapping systems for education of clients in terms of visual feedback for proper pressure relief techniques, impact of postural changes and proper cushion set up, especially of the air floatation cushion, has proven valuable (Henderson et al. 1998; Jan 2006). While these are worthwhile uses of the pressure mapping systems, more research is required to allow the wealth of information output from these systems to provide more meaningful and clinically relevant data in relation to interface pressure.