PREVENTING PRESSURE ULCERS in users of seat cushions. Pressure Mapping in Seating: A Frequency Analysis Approach. Ingrid Eitzen, PT, MSc

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1 1136 Pressure Mapping in Seating: A Frequency Analysis Approach Ingrid Eitzen, PT, MSc ABSTRACT. Eitzen I. Pressure mapping in seating: a frequency analysis approach. Arch Phys Med Rehabil 2004; 85: Objectives: To discuss the methodologic challenges related to pressure mapping in seating and to present a new approach to the analysis and interpretation of results: the frequency analysis approach. Design: Pressure mapping was performed on 3 prototypes of a newly developed foam and gel seat cushion. Setting: Data collection was done in a private laboratory. Participants: Eight nondisabled men. Interventions: Not applicable. Main Outcome Measures: Average pressure, peak values, and the size of the contact area were measured continuously for 74 minutes on each cushion prototype. A supplementary frequency analysis provided information on the number of times each value occurred during the measurement period. Results: Average pressure and peak values showed only very small, nonsignificant changes over the measurement period for all variants. The frequency analysis, however, showed significant differences that enabled the manufacturer to select the prototype best suited for further development. Conclusions: Verifying significant differences in pressurerelieving properties between products has to date been difficult. Findings from this study indicate that a frequency analysis approach may enable more adequate and precise ways to perform such studies. Key Words: Methods; Pressure; Rehabilitation; Skin ulcer by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation PREVENTING PRESSURE ULCERS in users of seat cushions is important because of the severe social and economic consequences, both to the patient and to society at large. In clinical settings, individual adjustment of seat cushions for each patient is essential to find the product best suited to prevent pressure ulcer development. However, finding the ideal cushion requires knowledge and documentation of the mechanisms behind the development of pressure ulcers and of the characteristics of the different products. In 2001, Haukvik and Skøien 1 did a literature review to outline the existing work and established methods of pressure measurement in seating. Their findings confirmed the hypothesis that existing work in this area was done without common methodologic guidelines and procedures, resulting in a growing body of experimental data that separately may be useful but as a whole is very difficult to compare. From SINTEF Unimed, Health and Rehabilitation, Oslo, Norway. Supported by the SINTEF Unimed Research Foundation and Vital Base AS. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the author(s) or on any organization with which the author(s) is/are associated. Reprint requests Ingrid Eitzen, PT, MSc, SINTEF Unimed, Health and Rehabilitation, Pb 124 Blindern, 0373 Oslo, Norway, Ingrid.Eitzen@sintef.no /04/ $30.00/0 doi: /j.apmr After their study, a project was conducted for a manufacturer who had developed 3 prototype seating cushions made from a new type of foam and gel. The prototypes differed in both materials and construction; the manufacturer needed pressure measurements for each to identify the ideal combination of materials. This article discusses the methodologic challenges related to pressure measurements and presents a new approach to the analysis and interpretation of pressure mapping in seating. Theoretical Framework Pressure ulcers are a dominant health problem for people who, for various reasons, must spend most of their time in a seated position. Many causes have been identified as contributing to the development of pressure ulcers. 2-4 Among the external causes, pressure related to time is believed to be the most significant. When the interface pressure exceeds the mean pressure in the capillary veins, the veins tend to collapse. If such a situation is maintained over a period of time, ischemic necrosis will evolve in the affected area. 5,6 In addition to this main cause, other external factors believed to contribute to tissue damage are shear stress, increased skin temperature, and humidity. 3,7-11 Frequently cited internal causes are age, physical condition, degree of mobility, sitting posture, quality of skin and tissue, reduced sensibility, incontinence, and nutrition. 3,12 Treatment of pressure ulcers is not only painful but also time consuming and costly. Therefore, preventing pressure ulcers should be a priority. 2 The use of pressure-relieving seat cushions is important in this regard. 13 An efficient seat cushion should contribute to a functional and balanced posture. In addition, it should reduce the level of pressure in 2 ways: by distributing pressure away from critical areas (like the sacrum and ischial tuberosity) to more tolerant areas and by distributing pressure over a larger contact area. 14 Persons engaged in the field agree that seat cushions should be selected according to individual needs. However, there are no existing common guidelines for selecting seat cushions Subsequently, cushions are still chosen on the basis of affordability, availability, and experience-based judgments of therapists rather than on scientific data. 6 Pressure-mapping systems provide knowledge about interface pressure. Pressure values and pressure distribution are measured by a thin sensor mat that is placed between the seat surface and the user s buttocks. 15 However, different producers and distributors, as well as researchers, approach the issue with different types of measurement equipment. As a result, there are no generally accepted standard methodologic guidelines for pressure measurement. Existing studies are therefore difficult to compare and are of limited external validity, although, taken separately, they may be valid and reliable. 3,5,18,19 METHODS Participants Eight men participated in the study, none of whom were wheelchair users. Their ages, heights, and weights are presented in table 1.

2 SEAT PRESSURE MAPPING, Eitzen 1137 Table 1: Anthropometric Measures in Subjects Subject Age (y)* Height (cm) Weight (kg) A B C D E F G H Average *Rounded to closest year. Apparatus Data were collected with 2 identical pressure-mapping mats, a with measuring cells sized 1cm 2. The pressure mats were placed on 2 identical wheelchairs (Netti III Comfort b ) equipped with a standard back support cushion (Smart b ), with a back height of 48cm and a seat width of 45cm. The position for measurement was standardized (fig 1) and the data were collected simultaneously on 2 subjects, seated in 2 separate parallel chairs. Average pressure, highest peak value, and the size of the subjects contact area on the cushion were measured continuously over a period of 74 minutes and stored in the software that was developed for the system. b To show pressure development over time, average values were collected for each minute during the first 5 minutes (to note possible adjustments in the material of the cushions from change in load and temperature) of data collection and thereafter every 3 minutes. Furthermore, the frequency distribution of all observed pressure values was calculated for each subject. All data were exported and further analyzed. c All pressure mappings were monitored online while data were collected, to ensure proper quality of the data and to help isolate eventual disturbances or extreme values caused by equipment errors or sudden movements. Protocol All subjects were dressed in identical clothing consisting of a pair of wide pants and a 100% cotton sweater. Subjects wore their own underwear; more precisely, boxer shorts. The fabric of the underwear was not known. The subjects were seated next to each other, watching a television screen placed in front of them. The person responsible for the data collection was positioned to the left and slightly in front of the subjects so that she could observe them. The subjects could not see the monitor showing the pressure mappings during measurement. To ensure similar positioning of the subjects, the 2 wheelchairs used were individually adjusted to each subject before data collection. A person experienced in adjusting wheelchairs made all the adjustments. After adjusting the chairs, 1 pressure mat was placed on each chair. The front edge of each mat was placed on top of the underlying cushion, aligned with its front edge. The subjects then entered the chair by lifting and supporting their body weight on the armrests, lowering their buttocks onto the chair, and then resting their feet on the foot supports. Test assistants ensured that the position of the pressure mats did not change during the placing maneuver. The absence of eventual disturbances (in the form of peak values or data error) stemming from protuberances in clothes or mats was ensured via an online monitoring check. Once seated, subjects where requested to put their arms on the armrests and look straight ahead at the television screen. They were further instructed to sit as still as possible during the entire test. They were explicitly told not to lift their arms from the armrests, not to lift their buttocks from the seat of the chair, and not to move their legs or feet. Before measurements were taken, subjects were asked if they felt comfortable. If they did not, the procedure for positioning in the chair was repeated and potential protuberances in the clothes or pressure mat were checked once more. After a comfortable position was achieved, data collection began. RESULTS The aim of this study was to perform pressure measurements on 3 different seat cushion prototypes to identify the ideal combination of materials in the cushions. The remainder of this article discusses the study design in light of recognized methodologic challenges and presents the approach used in analyzing the material; this may represent a new way to interpret pressure measurement data. Equipment Pressure-mapping equipment may, in itself, cause several methodologic weaknesses. Size of the sensor mat, the number of sensors, and the sensitivity of the system will influence the resolution, accuracy, reliability, and replicability of the measured pressure values. 19 Introducing an object between the person and the seat surface will affect the pressure distribution and shear stress to some degree. 8 In this study, the subjects were dressed in identical clothing except for underwear. Although all subjects wore boxer shorts, possible differences in model and fabric may have caused some degree of inaccuracy in the measurements. Thus, to eliminate this possible shortcoming, future studies should also require subjects to wear identical underwear. The accuracy of the measurements may also be affected if recording is done on any surface that is not firm and even, 20 which is a definite possibility with human subjects. This was the case in the present study, although the pressure mat a was thin and flexible. A problem actually resulting from this flexibility is that the mat will follow the shape of the body when it is placed on top of a pliable cushion. The sensors will thus register contact from the sides of both thighs if the lower level of pressure is set to zero. When interpreting data, one must be aware that this may give the impression that the pressure-bearing area is larger than it really is. For example, if a person is seated on a cushion that is very soft, he/she might sit through the cushion, which is negative for pressure relief. The contact area, however, will seem large because of the Fig 1. Standardized position for measurements.

3 1138 SEAT PRESSURE MAPPING, Eitzen Fig 2. Possible misinterpretation of contact surface versus real weight-bearing area. contact of the mat on the sides of the thighs. In reality, no body weight is carried on the sides of the thighs, even though the load may be sufficient for the cell to register this. This phenomenon is illustrated in figure 2. If accurate pressure measurements are achieved, but the effects of external forces and deformation of the seat surfaces are not incorporated, then the results may lead to invalid generalizations of real-life situations. 21 Using actual subjects requires attention to the aforementioned methodologic challenges when interpreting results. Participants The primary goal of the investigation was to test the properties of the materials in different types of cushions. Hence, it was appropriate to create a homogeneous test group. Therefore, nondisabled subjects who were matched for age, sex, height, and weight were chosen. However, the majority of studies have used wheelchair users as subjects. Ferguson-Pell and Cardi 9 emphasized the importance of this, because the pressure distribution generated by a person with atrophied tissue and bony prominences will differ from that generated by nondisabled subjects. However, the variance will be larger within a group of wheelchair users, because the level of atrophy and possible physical deformities will be highly individualized. 22,23 Categorizing users into homogenous groups will consequently be a problem. Studies with a small number of participants will have questionable reliability. To build up data material based on wheelchair users as subjects will require parallel data recordings from several different user groups, because one cannot in any way define the typical wheelchair user. Such categorizations should not be based on diagnosis alone, but rather on relevant individual features, such as age, sex, weight, height, body shape and build, and the type of disability. Categorization should also be in accord with descriptions of level of functioning in the International Classification of Functioning, Disability and Health. 24 Hence, accumulating reliable data using wheelchair users as subjects is an extremely demanding challenge. Research using more homogeneous subjects may be preferable in some cases, especially if the aim is to compare the properties of different cushions and/or materials. One should, however, always remember that using nondisabled subjects can compromise the study s validity if the aim is to establish an acceptance level for pressure-relieving effects. Positioning of Subjects and Time of Measurement Positioning and time are parameters of great importance in the eventual development of pressure ulcers. A pressure value recorded in a single position over a short period will at best provide some baseline knowledge of pressure level. Haukvik and Skøien s literature review 1 showed that, in most studies, pressure mapping was conducted with subjects in 1 static position. The duration of measurement varied from less than 1 minute to more than 30 minutes. General conclusions indicating guidelines for real users seated hours a day in various positions could not be drawn from such data. Measurements should continue for more than just a couple of minutes. After test measurements, the subjects in this study were instructed to remain seated in the same position for 74 minutes. This is a longer measurement period than has been reported in any other comparable study of pressure measurements. As described earlier, the subjects were given explicit instructions to sit as still as possible during the entire test period. They were told not to turn their heads, not to lift their arms from the armrests, not to lift their buttocks from the seat of the chair, and not to move their legs or feet. Eventual movement or muscle flexion was not prevented by other means (ie, belts, stabilizing side parts). However, during the measurement period, the pressure map of each subject was constantly monitored. Hence, possible movements caused by sneezing, coughing, or other potential disturbances could be noted in the protocol and considered in the analysis. The reasoning behind this project design was to create a worst-case scenario, to test the cushions under constant, static load for a longer period of time. Materials like gel and foam change because of variations induced by body temperature. In addition, weight load will affect the carrying capacity and resilience of the materials in the cushion. The measurements taken during the first 5 minutes were analyzed in more detail than were those taken in the remaining time, just to see what happened while the subjects sank into the cushion. However, sitting still for 74 minutes is an artificial situation. As mentioned, this could be comparable to the loads a cushion would receive from passive users, who are most at risk for pressure ulcers. Thus, it is important to create a test situation that reflects relevant conditions. In addition, it was important to create a test situation in which subjects sat as still as possible yet in a natural and relaxed position. To limit possible movement further would have most likely resulted in subjects aborting the study, thus compromising the length of the measurement period. This methodologic choice led, in all likelihood, to movement during the test period, resulting in some differences in measurements from person to person and from test to test. However, these differences were deemed negligible for 2 reasons. First, all recordings were monitored, and all movements affecting the data were noted in the protocol and considered during analysis. Second, the extensive measurement time provided large amounts of data and numerous observations over time. Hence, isolated observations of irregular values would not have an appreciable effect on the total result. Static position measurements may be applicable to passive users, but they will not be sufficient to describe the demands on cushions during the different daily activities of more active wheelchair users. Ideally, data should be recorded over much longer periods of time, allowing for subject movement and preferably including all hours a user is seated during the day. DISCUSSION Because of the aforementioned methodologic problem with size of contact area versus real body weight load and to the electronic drift that always poses a challenge to the interpreta-

4 SEAT PRESSURE MAPPING, Eitzen 1139 Fig 3. Average pressure development over time for all subjects on one of the cushion variants. tion of pressure-mapping results, analysis based solely on average and peak pressures would not be satisfactory. Thus, a frequency analysis was performed, registering the number of times each value occurred. Such an approach is not restricted to showing peak values or average values; rather, it can show how often all the observed values occur on a defined interval scale. In this way, one can easily determine which cushion has the fewest observed values at the high end of the scale. Thus, the frequency analysis approach will, by reflecting both average pressure and peak, illustrate how many of the total observations are in the higher end of the scale that is, the observations that may be characterized as risk values. The problem of the size of contact area is thus minimized, and the only consequence of this possible error is that one will get a somewhat higher number of observations that are near zero than what is actually the case. By using data, one can use a frequency analysis approach. Figure 3 illustrates the average pressure development over time for all subjects on 1 of the cushion variants. Figure 4 shows the same data after a frequency analysis; the distribution of values, and thus the main findings, are immediately evident. Furthermore, this approach provides more detailed information, because all observations not just the calculated average for each time period are captured. Figure 4 shows that the pressure values range more widely than what can be interpreted by an averaged time series. All Fig 4. Pressure data for all subjects on one of the cushion variants after a frequency analysis. averaged values in figure 3 correspond to a single categorized range in figure 4. Thus, to show both the lowest and highest values which are of great interest one must go beyond measuring just the average of observed pressure values. It is important, however, to also show how pressure evolves over time. A time series based on average pressure values may, as shown in the previous examples, at best give a fairly accurate description of pressure development over time, because a considerable number of real observed values in the low and high end of the scale are lost. However, it is hardly an option to present a time series based on all observed values over the measurement period. Such a curve would be hard to interpret. A frequency analysis done in conjunction with the traditional average and peak value time series solves this problem. The frequency analysis diagram captures information that may be said to represent both peak and most common (average) values. Hence, this approach provides far more detailed and accurate results. If the purpose is to compare different products, the chances of finding significant differences based solely on average values could be limited. This may be partly the reason why earlier studies rarely have been able to establish significant differences in pressure-relieving properties among different seat cushions. CONCLUSIONS The amount of experimental research in the area of pressure measurements in the seated user is growing, but a lack of guidelines makes evaluation and comparison of the results of new studies with existing knowledge difficult. The experimentally based knowledge in the field as a whole is fragmented and inadequate for developing reference benchmarks. In this article, I have described a study in which a frequency analysis approach was used to give more accurate information on observed pressure values during a 74-minute measurement period. This approach seems promising for providing more adequate and accurate ways to compare pressure-relieving properties on seat cushions and mattresses in future studies. Finally, it is important to remember that there are more factors contributing to the development of pressure ulcers than pressure alone. Shear, temperature, and humidity, among oth-

5 1140 SEAT PRESSURE MAPPING, Eitzen ers, are acknowledged factors, but they are rarely experimentally investigated. References 1. Haukvik IV, Skøien RM. Determining the effect of seat cushions in preventing pressure ulcers. In: Marincek C, Bühler C, Knops H, Andrich R, editors. Assistive technology: added value to the quality of life. Amsterdam: IOS Pr; p Byrne DW, Salzberg CA. Major risk factors for pressure ulcers in the spinal cord disabled. Spinal Cord 1996;5: Harstall C. Interface pressure measurement systems for management of pressure sores. Edmonton (AB): Alberta Heritage Foundation for Medical Research; Collins F. The contribution made by an armchair with integral pressure-reducing cushion in the prevention of pressure sore incidence in the elderly, acutely ill patient. J Tissue Viability 1999; 4: Allen V, Ryan DW, Lomax N, Murray A. Accuracy of interface pressure measurement systems. J Biomed Eng 1993;15: Willis J. Pressure-relief seating. Prof Nurse 1995;11: Collins F. Sitting pretty. Nurs Times 1998;38: Goossens RH, Snijders CJ, Holscher TG, Heerens WC, Holman AE. Shear stress measured on beds and wheelchairs. Scand J Rehabil Med 1997;3: Ferguson-Pell M, Cardi MD. Prototype development and comparative evaluation of wheelchair pressure mapping system. Assist Technol 1993;5: Williams C. The Vicair Academy and Liberty range of pressurereducing seating. Br J Nurs 1997;6: Eckrich KM, Patterson PE. Dynamic interface pressure between seated users and their wheelchairs. Int J Ind Ergon 1991;8: Cooper P. Cushions and specialist chairs for pressure sore management. Br J Nurs 1998;15: Takechi H, Tokuhiro A. Evaluation of wheelchair cushions by means of pressure distribution mapping. Acta Med Okayama 1998;5: Stockton L. Pressure relief seating: are your patients comfortable? Community Nurse 1998;1: Hastings JD. Seating assessment and planning. Phys Med Rehabil Clin N Am 2000;1: Sumiya T, Kawamura K, Tokuhiro A, Takechi H, Ogata H. A survey of wheelchair use by paraplegic individuals in Japan. Part 1: Characteristics of wheelchair cushions. Spinal Cord 1997;9: Sumiya T, Kawamura K, Tokuhiro A, Takechi H, Ogata H. A survey of wheelchair use by paraplegic individuals in Japan. Part 2: Prevalence of pressure sores. Spinal Cord 1997;9: Gray D. Pressure ulcer prevention and treatment: the Transair range. Br J Nurs 1999;8: Shelton F, Barnett R, Meyer E. Full-body interface pressure testing as a method for performance evaluation of clinical support surfaces. Appl Ergon 1998;29: Buckle P, Fernandes A. Mattress evaluation assessment of contact pressure, comfort and discomfort. Appl Ergon 1998;1: Jürgens HW. Seat pressure distribution. Coll Antropol 1997;2: Ferrarin M, Andreoni G, Pedotti A. Comparative biomechanical evaluation of different wheelchair seat cushions. J Rehabil Res Dev 2000;3: Brienza DM, Karg PE. Seat cushion optimization: a comparison of interface pressure and tissue stiffness characteristics for spinal cord injured and elderly patients. Arch Phys Med Rehabil 1998; 4: World Health Organization. International classification of functioning, disability and health: ICF. Geneva: WHO; Suppliers a. XSENSOR Technology Corp, Ste 111, 319-2nd Ave SW, Calgary, AB T2P 0C5, Canada. b. Alu Rehab AS, Industriveien 15, 4313 Sandnes, Norway. c. Microsoft Corp, One Microsoft Way, Redmond, WA