The pressure is on - everyone, everywhere, everyday. Emma Parry, NZROT, ATP & Tania Strickett, NZROT. Introduction. There have been numerous publications that discuss pressure sore aetiology (Bergstrom et al, 1992; Clark, 1998), pressure risk assessment scales (Schoonhoven et al, 2002; Dealey, 1995; The National Pressure Ulcer Advisory Panel, 1989; Crowe and Clark, 1990), different support surface technologies (www.spinlife.com; Ferguson-Pell & Parry, 2000) and pressure mapping (www.seatingdynamics.com.au; Ferguson-Pell & Bain, 1999; Ferguson-Pell & Cardi, 1992; Ferguson-Pell & Cardi, 1993; Ferguson-Pell & Minkel, 1995) The aim of this paper is to unite the information from these sources, combined with the clinical expertise of the authors to create a useful resource for clinicians. What are pressure sores? Pressure sores are variously described as pressure ulcers, decubitus ulcers, bedsores, pressure necrosis, and ischaemic ulcers. Essentially, they can all be characterised as "any lesion caused by unrelieved pressure resulting in damage of underlying tissue" (Bergstrom et al, 1992). Pressure sores have been recognised for hundreds of years. Their presence has also been identified on Egyptian mummified bodies (Clark, 1998). Incidence & prevalence of pressure sores. Pressure sores have long been identified as being a source of great pain and inconvenience to the client, significant cost to the health system, and clinical challenge and frustration to clinicians. The incidence & prevalence of pressure sores is difficult to determine. A Medline database search for articles published and catalogued between January 1 1990 & December 31 2000 yielded over 300 studies on pressure ulcer incidence and prevalence over the past decade. In Australia in 1998, pressure ulcers were cited as the cause of death for 49 people and to have been a contributory cause in the deaths of a further 227 individuals (Newall et al, 2000). In the USA the reported incidence & prevalence of pressure sores in hospital patients has varied widely from 2.7% to 29.5% (Gerson, 1975, Clark & Kadhom, 1988). In 1999, 8 - 10% of hospital patients in the Netherlands had pressure ulcers of grade 2 or worse (Schoonhoven et al, 2002). Cost of pressure sores. In terms of human suffering the cost of pressure sores is considerable and at times catastrophic. Financial costs have been estimated at £200 million in 1995 in the United Kingdom (Bar & Pathy, 1998). The costs associated with pressure sores in the USA in 1992 exceeded $6.4 billion (Marwick, 1992). Schoonhoven et al (2002) reported that in the Netherlands, pressure ulcers are the third costliest condition to the health service after cancer and cardiovascular disorders. Estimates of the annual cost of pressure sores to the NHS vary from £60 million (DoH, 1991) to more than £400 million (Touche Ross, 1993), while the human costs include pain, loss of independence, prolonged hospital stay and, at worst, death from septicaemia (Land, 1995). Factors relating to pressure sore development. There are numerous factors that contribute to the development of pressure sores. For ease of discussion, the literature divides these into those that are considered to be extrinsic (those external to the person), and those that are considered to be intrinsic (those internal to the person) (see diagrams 1 & 4). Extrinsic factors * Pressure * Shear * Friction * Moisture * Quality of care * Positioning * Wheelchair / support surface * Extremes of temperature * Transfer technique Severe or prolonged ischaemia produces changes in the normal cell metabolism that leads to death of the cell. Kosiak (1961) indicated that cellular ischaemia caused by external pressures higher than capillary blood flow pressure is one of the primary factors in the production of pressure sores. Landis (1930) established the average capillary blood pressure to be 32 mmHg. His work led to the belief that internal or external force applied to bony prominences of greater than 32 mmHg can contribute to capillary closure and cause tissue hypoxia resulting in pressure ulcers. This measure of 32 mmHg has been widely accepted as the pressure threshold for ischaemic skin changes, but a different measuring technique has reported capillary pressure to average 47 mmHg. Following an initial brief capillary occlusion phase, auto regulation often ensures skin capillary blood flow at pressures up to 60 mmHg. Therefore if local tissue pressure exceeds capillary closing pressure for a sufficient period of time, ischaemic changes will occur (Bar, 1998). There is widely varying opinion regarding the science behind the measurement of mmHg, with some literature suggesting that occlusion of blood flow may be more accurately measured using laser doppler flowmetry or transcutaneous oxygen tension (Brienza et al, 2001). * Pressure: Forces may be generated by gravity acting on the body, or dynamically when the body moves, such as during wheelchair propulsion or transfers. Normal forces are perpendicular to the surface of the body and shear forces are tangential to it. Pressure is equal to force divided by area (P = F/A). Pressure is a reality on earth; escape from it is only possible in a gravity free environment. We live in an atmosphere where the pressure around us (ambient) is 760 mm Hg or 14.7 psi at sea level (Glover, 1996). * Shear /Friction: Shear is force acting tangentially on a body that changes its shape. Friction is a force that opposes a shear force. It's maximum value before slipping is influenced by the size of the normal force; the frictional properties of the surface in contact, and the prevailing shear force. Shear and friction are often used synonymously in practise, however in reality they are different forces. * Moisture: Skin integrity is diminished in the presence of extremes of moisture levels on the skin. If the skin is too dry, then skin can be vulnerable to cracking. If the skin is too moist, then maceration can occur (Joanna Briggs Institute, 2004). Prolonged exposure to moisture may make the skin vulnerable in situations of turning, altering clothing or bedding where the friction forces may damage the skin (Bar, 1998). * Quality of care: The level of nursing/caregiver support as well as policy and procedures within a health care environment can have an impact on pressure sore development. * Positioning: A person lying semi-recumbent, sitting in one position for extended periods of time, and the timeframe for turning in a health care environment will all have an impact on the development of pressure sores. * Wheelchair / support surface: Assessment for the appropriate mobility option and support surface (for example, mattress or cushion) plays a significant role in pressure management. * Extremes of temperature: Thermoregulatory response in spinal cord injured persons is impaired, especially when associated with sensory loss (Bar 1998). Excessive heat can cause perspiration, which again creates the issue of excess moisture on the skin. * Transfer technique: Appropriate manual handling techniques play a vital role in ensuring skin integrity. Use of hoist transfers and slings play an important role in managing back care but without appropriate training in placement and removal, slings can cause damage to the superficial layers of the skin. Intrinsic factors: * Age * Sensory deficit * Incontinence * Impaired communication * History of insult to skin integrity * Medical diagnoses and medication * Orthopaedic deformities, muscle contractures and altered tone * Mental status * Nutritional status * Body size, type and weight * Vascular insufficiency * Debilitating illness * Infection * Immobility * Age: The majority of pressure sores occur in persons over the age of 70 years (Guralnick et al 1988; Young & Dobrunski 1992). Several changes occur in aging skin, including slower epidermal turnover and decreased vascularity, subcutaneous adiposity, and collagen and elastin content (Dharmarajan & Ugalino 2002). * Sensory deficit: Decreased sensation to pain and discomfort create a situation where there is no stimulus for the need to relieve pressure. * Incontinence: Skin that is exposed to moisture can lead to compromised skin integrity (see moisture under extrinsic factors). Incontinence may well be linked with reduced mobility and more severe health issues. * Impaired Communication: Both expressive and receptive communication issues can cause problems in communicating pain and management techniques respectively. * History of insult to skin integrity: Scar tissue from previous pressure ulcers is generally weaker and therefore more prone to further damage. * Medical diagnoses and medication: Some conditions and medications may cause increased immobility. Medications that cause drowsiness (and therefore decrease active movements) or reduce cardiac function may increase a patient's risk of pressure sore development. * Orthopaedic deformities and muscle contractures: Restrictions in joint movement and fixed or flexible deformity patterns affect a persons ability to maintain symmetrical postures and therefore may in turn create undue risk on individual parts of that person's anatomy. * Muscular paralysis, atrophy, altered tonal patterns: If normal movements are impaired or restricted then ability to relieve discomfort is curtailed. * Mental Status: May cause depression, loss of appetite, apathy, which in turn increases risk of pressure sores. * Nutritional status: Nutrition plays an important role in the maintenance of skin integrity. Studies on nutritional intervention have established a correlation between nutritional status, body weight and rate of wound healing (Himes, 1999). * Body size, type and weight: An overweight person tends to move less and may have issues with increased friction during transfers. An underweight person may have more pronounced bony prominences. * Vascular insufficiency: This may reduce pain sensation (especially in heels and feet of those suffering from peripheral vascular disease). * Debilitating illness: Terminal illness or acute illness generally create a situation of metabolic disturbance in terms of fluid intake, reduced dietary intake, reduction in blood pressure and use of medication creating a situation that may influence the time-pressure threshold for development of pressure sores (Bar 1998). * Infection: Infection has been recorded in 50% of hospitalised patients who develop pressure sores. Sepsis as a risk factor is often associated with pyrexia and metabolic disturbances of acute illness (Bar 1998). * Immobility: An epidemiological study on the distribution of pressure sores in relation to mobility found that 10% of patients were ambulant, 37% were confined to a wheelchair and 53% were bedridden (Gerson, 1975, Clark & Kadhom, 1988). Grading of Pressure sores. In order to measure the pressure sore problem it is necessary to have a way of describing sores by their severity and depth. This is particularly important, as it is the deep sores, which cause the most problems in terms of money and patient suffering. Several grading systems exist which attempt to describe sores by depth and severity. Because for many years pressure sores were not given a high priority people involved in trying to measure and solve the pressure sore problem began to develop their own tools to describe the severity of pressure sores. This has led to several different systems existing with anything from 4 - 6 basic categories and some with further sub categories. The most commonly used grading system in the author's experience is that devised by the European Pressure Ulcer Advisory Panel (EPUAP). Grading of Pressure Sores - European Pressure Ulcer Advisory Panel Guideline Grading System: Stage 1: Non-blanchable erythema of intact skin. Discolouration of skin, warmth, oedema, induration, hardness may also be indicators, particularly inindividuals with darkly pigmented skin. Stage 2: Partial thickness skin loss involving epidermis, dermis or both. Ulcer is superficial and presents clinically as an abrasion or blister. Stage 3: Full thickness skin loss involving damage to or necrosis of subcutaneous tissue that may extend down to but not through underlying fascia. Stage 4: Extensive destruction, tissue necrosis, or damage to muscle, bone, or supporting structures with or without full thickness skin loss. Pressure Sore Risk Assessment Scales Risk assessment can be seen as a form of clinical risk modification, which will lower the potential for healthcare risk (Dealey, 1997). The Clinical Practice Guidelines for the Prevention and Management of Pressure Sores states that the assessment of risk should be part of a comprehensive patient assessment. In other words, all patients in vulnerable areas should be assessed as a matter of routine. Once a patient is identified as being at risk, appropriate preventive measures should be implemented, as failure to do so could be viewed as negligent. Risk assessment is a dynamic process, as people change over time so does their level of risk. One approach to risk assessment is the practice of routine assessment with the recognition of the need to undertake more frequent assessment in the event of changes in an individual patients condition (Dealey, 1995). Pressure sore risk assessment scales represent an objective attempt to determine an individual's risk status by measuring a range of the most commonly recognised risk factors. They are used to attempt to achieve standardisation of assessment and early identification of a patient's level of risk from pressure injury. The idea of using a risk assessment score appears to be attractive to clinicians, as there are over 21 such scales reported in the literature (McDonald, 1995; Clark & Farrar, 1992; Lowthian, 1989). Although so many scales exist there is little evidence to support their sensitivity and specificity, which varies enormously (Flanagan, 1998; McDonald, 1995; Smith & Booth et al, 1995), and there is little evidence to suggest that the incidence of pressure sores has dramatically improved as a direct result of using a risk assessment scale. A recent study of routine use of risk assessment scales for prediction of pressure sores found that the effectiveness of available scales is limited. The same study also found that use of the outcome of risk assessment scales leads to inefficient allocation of preventive measures (Schoonhoven et al, 2002). The National Pressure Ulcer Advisory Panel (1989) advised that an ideal 'at risk' assessment tool should: * Have good predictive value * Have high specificity * Have high sensitivity * Be easy to use Crowe and Clark (1990) suggested the following criteria be used to inform choice: * Accuracy in predicting those at risk * Ease and convenience of use * Reliability between staff and patients It would not be practical to discuss all the available risk assessment scales, so the authors have selected the most popular. Following is a brief overview of each of three scales: Norton, Waterlow and Braden. Norton. The origin of pressure sore risk assessment can be attributed to Norton and associates in the 1960's. According to the Norton Scale, five predisposing factors are rated on a scale of one to four. These five items include mental status, incontinence, mobility, activity, and physical condition. The five scores are summated, and a score of fourteen or below indicates high risk, and preventive measures are prescribed. The Norton Scale. A Physical Condition: 4 good, 3 fair, 2 poor, 1 very bad. B Mental State: 4 alert, 3 apathetic, 2 confused, 1 stuporous. C Activity: 4 ambulant, 3 walk/help, 2 chair bound, 1 bedfast. D Mobility: 4 full, 3 slight, 2 limited, 1 immobile. E Incontinence: 4 none, 3 occasional, 2 urine, 1 double. Although the Norton Scale has been modified by several others in the ensuing years, the main criticism is that it over predicts pressure sores, there being many patients deemed at risk who do not go on to develop pressure sores. Other criticisms of the Norton system are the subjectivity of the terminology in section A, relying on interpretation and the possible duplication of information as a result of the interdependence that exists between physical condition, mental state, activity and mobility. The Waterlow Scale. The Waterlow Risk Assessment Scale was developed in 1984. Waterlow reviewed the intrinsic factors associated with the aetiology and pathogenesis of pressure sores and concluded that many factors relevant to acutely ill people had been omitted from the Norton Scale. The assessment tool, which was subsequently developed, could be used in a variety of clinical specialties and included such factors as nutrition, pain, reduced cardiac output, and the effects of anaesthesia (Flanagan, 1993). The scale comprises 10 categories with ratings of one to five, but more than one category can yield the same score, and more than one rating is selectable from a given category. The categories include: body weight for height, continence, skin type, mobility, age/sex, appetite, and a special risks category split into four groups - tissue malnutrition, neurological deficit, major surgery/trauma and medication. Despite the numerous adaptations that have been made to the Waterlow Scale, the main criticism is that it over predicts, even more so than Norton. The Waterlow Scale weights the sexes unequally (1 for male, 2 for female) when the literature in fact supports the assertion that males are equally likely to develop pressure sores (Gosnell, 1973; Gerson, 1975; Bergstrom et al, 1987). The Braden Scale. Bergstrom & Braden et al presented the Braden Scale (1987) following a prospective study of the aetiology of pressure sores. This scale is comprised of six subscales: the first three (sensory perception, activity and mobility) reflect the primary factors contributing to intense and prolonged pressure while the other three subscales (skin moisture, nutritional status, friction and shear) reflect factors contributing to diminished tissue tolerance for pressure. Each section has 3 - 4 levels with a clear description of qualifying attributes for each level. The Braden Scale has been shown to be reliable when assessment is undertaken by registered nurses (Bergstrom et al, 1986). Provided that nurses are trained to use the Braden Scale it is relatively self explanatory and easy to use (Smith et al, 1995). Validity of the scale is generally good and compares favourably with the Norton and Waterlow scales (Bergstrom et al, 1987). None of these scales really takes into effect the skeletal impact in pressure sore prevention. A lightweight person with tone compared to a heavy weight person with no tone, or simply sitting long term. This is one of the flaws of pressure risk assessment scales (O'Sullivan, 2004, personal communication). Pressure mapping can assist in this area. What is pressure mapping? Pressure mapping can be used as an everyday clinical tool to aid the clinician in the decision making process. It provides objective data for decision making in the clinic setting. It gives an illustration of weight distribution over the mapped surface, and allows monitoring of the position of the weight bearing surfaces. It allows for observation of symmetry or otherwise of body parts, and provides this without the perturbation effects the traditional 'hands in/under/on' method provides. It also provides a means for confirmation of clinical findings from the mat evaluation, and if there is no congruence between the two sources of information, it allows a means for checking and rechecking. It is a powerful biofeedback tool for clinicians and clients (www.seatingdynamics.com.au; Ferguson-Pell & Parry, 2000; Ferguson-Pell & Minkel; 1995; Garber & Krauskop, 1978; Lipka et al, 1997). Pressure mapping can be helpful in the assessment of people when providing suitable support surfaces, for educating clients and clinicians about the effectiveness of pressure relieving strategies, and to help in the provision of seating, mattresses and other support surfaces (Ferguson-Pell & Bain, 1999). Recommendations can be made following a rigorous assessment process regarding appropriate support surfaces for the client to address their identified needs (Shmeler & Buning, 1999). What does pressure mapping measure? In real terms, pressure mapping measures millimetres of mercury (mmHg), which is a measure of pressure. Pressure mapping does not measure shear, stability, maintenance or comfort. Much discussion has been published regarding the validity of the numbers generated by pressure maps, and many manufacturers are now using pressure-mapping data in the marketing of their products (Shapcott & Levy, 1999). It is vitally important for clinicians to be well informed of what this data really means in terms of their clients, and their particular clinical setting. It is also important to remember that pressure mapping is a clinical tool, much like a tape measure or digital camera, and must be used as an integral part of thorough assessment and clinical reasoning processes. How does pressure mapping technology work? There are several different brands of pressure mapper available in the market place. These mappers all work on slightly different technology. These differences are best described in terms of the types of sensor, and number of sensors per mat. For example, there are pneumatic sensors that work on air connected to some type of pressure measuring device (e.g. Talley). There are resistive sensors that work on changes in the resistance of individual sensing elements (e.g. Tekscan, FSA). And there are capacitive sensors that work on electrical capacitance of individual sensing elements (e.g. Novel, Xsensor). Each system has a specific number of sensors per mat. Generally speaking, depending on the type of sensor technology used, the more sensing elements in a mat the higher the resolution. Single sensor technologies. Whilst discussing pressure mapping, it is important to briefly overview single sensors. There are several different types available (for example, Flexiforce from Tekscan, Therapoint Dynamic Pressure Monitor from ROHO, Pressore range from Cleveland Medical Devices, and the Talley single sensor). A single sensor is either one sensor, or a small cluster of sensors that is placed under one bony prominence at a time. The hand held device gives a reading of the interface pressures under each prominence. Each reading is manually recorded, and the summary of information must be analysed and synthesised. Single sensory technology is relatively inexpensive, easy to use and quite simple. However it is often difficult to identify the bony prominences without introducing error into the system (i.e., your hand), and it is very difficult to know if you have positioned the sensor correctly or not. There continues to be debate as to whether to attach the sensor to the skin or not. What do the numbers mean? There has been much debate in the literature regarding what the numbers actually mean clinically. As discussed earlier, 32mmHg has typically been used as the 'gold standard' for optimal pressure - this is not humanly possible on earth. 32mmHg is the capillary pressure at the heart level. Capillary pressure is much greater at the feet, and research by Bar (1999) showed that 60mmHg is in all likelihood a much better number to use. However single sensors and pressure mappers have certain margins of error, so how can a clinician be sure that what reads as 60mmHg on the tool is not 50mmHg, or even 80mmHg? In the author's experience, one cannot be unequivocally certain. For that reason, the authors use the distribution and general patterns, rather than the numbers in and of themselves in the clinical setting. Using pressure mapping in clinical practice Pressure mapping is a clinical tool, and should be integrated with the other skills and tools already used. Pressure mapping can be incorporated in the assessment process, and is used by the authors to verify their hands on assessment and clinical 'hunches' (Lipka et al, 1997). The following is suggested as a way to incorporate the use of this technology into practice: * Take a baseline recording. This shows the pressure distribution of the client on their existing support surface. Do this before you change anything about the support surface. * Take a distribution on a firm surface (if evaluating a client's seating). This allows the clinician to really see what is happening at the interface, in terms of obliquities, rotations and weight bearing surfaces. * Whilst in the process of trialling different support surfaces, map each one. However it is important to remember the characteristics of each support surface in order to adequately map each one. Further information regarding support surfaces is detailed below. * Document clinical decisions and the reasoning behind it. Pressure mapping offers a unique way to present information to funding agencies. And when combined with photographs, pressure mapping helps form a powerful justification. Pressure mapping, like humans, is not infallible. It is possible to see unexpected things on a pressure distribution. It is important to check these out. Firstly, blame the tool - ask whether it is set up correctly, switched on, calibrated, plugged in, Secondly, blame the environment. Check the mat is straight, right way up and not creased. Thirdly, look at the client. Does the client have anything in their pockets? What is the client wearing (rivets on jeans and underwear seams are commonly seen on pressure distributions). Fourth, check whether the distribution is repeatable. Finally, what seems to be odd, may in fact be the client as they really are. However it is vitally important that other possibilities are excluded first (Ferguson-Pell & Parry, 2000). Interpreting pressure mapping data. There are several important points to bear in mind when looking at a pressure map: * Always try and validate physically what you see on the screen. What appears to be peakiness may in fact be car keys or loose change. * Always check which display mode the system is operating and consider whether this is the most appropriate for the situation. Different mapper manufacturers will be able to advise regarding this. Remember to use the software features provided with the system. * Remember that about 80% of the time the changes in distribution validate the expected outcome of the intervention. The other 20% of the time the results will not be as expected, and a potential mistake is avoided. Characteristics of support surfaces Materials used for the manufacture of support surfaces (mattresses & cushions) have developed significantly over the past 10 - 15 years. Cushions and mattresses greatly influence skin integrity and the clients ability to function, for example reaching, transferring, propelling. There is concurrence in the literature that 'no one cushion/mattress is best for all people', however there is much discussion regarding which type of support surface technology is the most appropriate for each client. The determining factor for the choice of support surface is not the pressure map. The pressure map forms one part of the equation (Ferguson-Pell & Parry, 2000). These support surface technologies include the following: * Foam or flexible matrix - a lightweight, flexible cellular material used in support surfaces. * Viscoelastic foam or matrix - a compressible material that has both elastic (spring like) and viscous (time-dependent) properties. Viscoelastic foam is different from ordinary foam by having time dependent behaviours such as creep, stress relaxation and hysteresis. This type of foam is also known as memory foam as it maintains the shape of an indentor (like your hand) before springing back to its original shape. * Non-deforming foam or matrix - a support material that does not compress, deflect or deform under force, often used in cushion bases (e.g., Jay J2 cushion base). * Bonded foam - adhesion of foam material including different foams being glued together. * Viscous fluid - relatively incompressible substance with viscous properties. Viscous fluids do not flow as readily as water and include maple syrup and grease (e.g. Otto Bock floam, Jay Flo). * Air - a cushion or mattress with an impermeable membrane (or multiple membranes) containing air (e.g. ROHO). * Water - a cushion or mattress with an impermeable membrane containing water. * Gel or solid elastomer - solid rubber like, relatively incompressible material. * Segmented - a cushion or mattress whose surface is divided into separate and distinct segments. * Convoluted - cushion or mattress surface is composed of convex protrusions separated by depressions or sulci, often called 'egg crate foam'. * Contoured - shaped to fit or reflect the form or shape of the body. * Cut out - surface having a disruption or removal of material to alter the load bearing characteristics of the surface. (www.spinlife.com) Conclusion. The aim of this paper was to create a useful resource for clinicians that unites some of the discussions in the literature regarding pressure sore aetiology, pressure risk assessment scales, different support surface technologies, and pressure mapping. Pressure sores remain a complex issue, with many contributing factors. Attempts have been made by many scientists, researchers, and clinicians to predict which clients will develop pressure sores, through pressure risk assessment scales. These remain a controversial and inconclusively reliable tool for preventing pressure sores. It is vitally important for clinicians to have an understanding of the aetiology of pressure sores, and the grading of pressure sores. This is so that clinicians are well informed and can assist their clients in the healing process, and in the prevention of further sores. Pressure mapping has been presented as another tool for the clinician's toolbox. What it is, how it works, and what it means for the clinician and client has been discussed. Pressure mapping can be used by clinicians in a clinic setting and in the community in order to provide a quality service to consumers. Pressure mapping provides quality data that enables the clinician in partnership with the client to make well-informed clinical decisions. There is no easy panacea for resolving the problems of pressure sores and it is hoped that this paper will promote clinicians to review their clinical reasoning about their recommendations in the choice of product in order to prevent their clients being affected. Glossary of terms. * Averaging: most mapping systems have a feature that smoothes out large differences between neighbouring sensors. This is achieved by taking the average between adjacent sensors. Manufacturers have different formulae for performing this averaging process. Care must be taken to ensure that real local feedback and pressure are not lost in this process. * Calibration: the method by which the digital output is converted to an actual unit of pressure such as mmHg. Calibration enables comparison of the output of the same sensor in various environments and allows comparison of calibrated outputs of various sensors. * Creep: is the change in sensor (and system) output when a constant pressure is applied over a period of time. A potential source of error. * Curvature effect: some sensors produce an output when they are draped over curved surfaces. Some sensors give different readings when loaded with the same pressure, depending upon whether they are on a flat or curved surface. * Equilibration: the sensitivity of each sensor in the matrix differs somewhat from its neighbours. The software used by the mapping systems accommodates for these differences during calibration, and this process is called 'equilibration', and is usually performed automatically during the calibration process. * Friction: is a force that opposes a shear force. Its maximum value occurs before slipping which is influenced by the size of the normal force, the frictional properties of the surfaces in contact and the prevailing shear force. * Forces: may be generated by gravity acting upon the body, or dynamically when the body moves, such as during propulsion or transfers. Normal forces are perpendicular to the surface of the body, and shear forces are tangential to it. * Hysteresis: is the difference in the sensor output response depending upon whether the applied pressure is increasing or decreasing. A potential source of error. * Incidence: the rate at which new cases occur in a population over a given time period such as the number of new cases per year among the patients at a long term care facility. * Interpolation: all mapping systems use a technique in displaying their data that will make the image smooth rather than the mosaic that would appear if you drew each sensor on the screen and represented its output with a square of colour. This process is called interpolation and is rather like making a picture by joining the dots. * Ischaemia: any condition that interferes with the circulating blood flow. * Noise: after a period of sue, or in the presence of electromagnetic interference, a sensor may indicate 'noise' as low level pressure which usually fluctuates with time. The effect of this can be reduced by setting a noise threshold using the software. A potential source of error. * Pressure: (normal stress) is determined by dividing an applied force by the area perpendicular to it. * Pressure ulcer: an area of localised damage to the skin and underlying tissue caused by pressure, shear, friction and/or a combination of these (EPUAP, 2004). * Prevalence: the number of both new and old cases at any one time in the population such as the proportion of patients in a long term care facility with pressure ulcers on a specified day. A cross sectional view of the problem. * Repeatability: is the ability of a device to respond in the same way to the same pressure applied multiple times. * Saturation: pressure is the point at which the sensor output no longer varies with applied pressure (the level of pressure at which this occurs (or the display) varies with different sensors and with the way the calibration was performed). A potential source of error. * Sensitivity: is a measure of the amount of pressure needed to generate a given unit of change in the output. For example, a very sensitive system would give a full-scale output (red display) at a much lower pressure than a less sensitive system. * Shear: is a force acting tangentially to a body that changes its shape. * Spatial resolution: is a measure of the ability of a p\mapping system to discriminate between two or more pressure features separated by a given distance. * A threshold: is created when the lower limit displayed is increased from '0'. All pressures below this level are disregarded, so as to suppress the display of noise. (Ferguson-Pell & Parry, 2000). References. Bar, C., & Pathy, M.S.J. (1998). Principles & Practice of Geriatric Medicine. (3rd ed). New York: John Wiley & Sons. Bergstrom, N., Allman, R., Carlson, C. (1992). Pressure ulcers in adults: Prediction and prevention. 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