Imaging Tissues for Pressure Ulcer Prevention

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Introduction

Every day, clinicians determine tissue status by visual and physical assessment. These practices, although technologically simple, belie the complex range of physiological responses of the tissues that may be detected by clinical assessment. Observations made by clinicians are used to assess the integrity of tissues and their response to the mechanical, physical and chemical environment. Clinical observation, however, is limited in a number of important ways. Qualitative clinical assessment is difficult to record accurately, particularly when observations require recording of subtly different levels in tissue status or response. Different observers also often record qualitative observations differently. This often results in difficulties when working in teams or shifts, or when information is assessed over time.

Imaging often requires qualitative interpretation of the information re- corded, but the image at least captures information in a way that allows multiple assessors to view the same primary dataset. However, the nature of imaging systems in healthcare settings usually requires them to be oper- ated by clinical and technical specialists working in a dedicated setting.

They are usually used for diagnosis and screening in order to detect early evidence of pathologies, or to assist in the provision of interventions, such as surgery or radiotherapy.

The potential for using imaging systems for the prevention and management of pressure ulcers is an altogether different proposition. With the prevalence of pressure ulcers in district general hospitals reported to be between 15 and 20% [1], use of specialist imaging facilities to assess wound status, or to identify particularly vulnerable patients, would create an overwhelming demand for scarce resources. Practical imaging systems to assist in pressure ulcer management must therefore be simple enough to use at the bedside. It may also be desirable for these systems to make measurements of parameters associated with the interaction of the patient with the bed or seating system, as it is here that problems with tissue integrity first occur.

The cost of managing and preventing pressure ulcers certainly could jus- tify the use of quantitative methods for tissue risk assessment and for monitoring the healing of ulcers. At present, however, confidence in the value of these technologies is modulated by their complexity, their cost and the lack of evidence of their efficacy.

Imaging Tissues for Pressure Ulcer Prevention

Martin Ferguson-Pell

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Fromthe technology developers' perspective most of the technical re- quirements for instruments to provide reliable measurements of physiological or mechanical parameters can be fulfilled. However, a significant bar- rier to their development is the lack of sound physiological and aetiological understanding of pressure ulcer pathology, as highlighted in Chap. 1. With- out a direct link to validated aetiological models, the design of effective clinical tools cannot be accomplished with confidence. For this reason many of the tools in use clinically have their origins as research instruments to measure specific physiological or mechanical parameters. They are most effectively used by clinicians, who use the information provided by these instruments to supplement their clinical skills and intuition.

Much can, however, be learnt fromwork to date, fromtechnologies developed for related application areas and fromthe use of instruments by researchers seeking to develop an improved understanding of the pressure ulcer problem. This chapter reviews many of these technologies and the in- sight they can provide into pressure ulcer aetiology.

Wound Assessment

X-rays have in the past been used to assess the extent of necrotic undermining by injecting contrast media into the wound. The disadvantages of needing to use dedicated imaging services are obvious.

A very simple way to record pressure ulcer dimensions is to use photography. With the advent of digital photography and the introduction of digital patient records it is much more practical for clinicians to record the progress of wound healing and to make simple measurements of changes in wound dimensions. Of course, a major limitation is that simple photography only yields information in two dimensions and therefore measurements of wound depth are not reliable from simple photographs. The Vi- sion Engineering Research Group (VERG; Winnipeg, Canada) has produced a simple-to-use wound measurement system (VeV MD) using digital photography that provides reliable three-dimensional information. To achieve this, the VeV MD software uses target plates placed in the field of the image to determine the camera position and orientation in relation to the wound and corrects for the distortion caused by the curvature of the lens. With these correction techniques it is possible to record the area, length, width, perimeter, depth and volume, along with the hue of different regions of the wound. Easy-to-use software enables the clinician to track changes in all the parameters over time so that the progression of the wound healing process can be monitored. Figure 17.1 shows the target plate in position along with the margins of the wound that are detected automatically by the software. Of course, in cases where the tissues are un- dermined this system would fail to detect the full extent of tissue damage.

Ultrasound imaging offers a more comprehensive means of wound assessment in these cases.

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The use of B-mode ultrasonography to provide a non-invasive record of the extent of wound undermining has been described in detail by Wendelk- en et al. [2]. This approach allows accurate monitoring of wound dimensions by filling the wound with a sterile wound-mapping gel and film dres- sing (Hudson Diagnostic Imaging). The wound is scanned by slowly moving the probe across the wound and a series of images captured digitally.

(Fig. 17.2). Measurements of the wound can then be made using the digital

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Fig. 17.1. The VeV MD system in use, showingthe target plate used to calculated wound dimensions and the margins of the wound detected by the software

Fig. 17.2.Use of B-mode ultrasonography for wound assessment (after Wendelken et al. [2])

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callipers provided with the instrument's software package, as indicated in Fig. 17.3.

Lyder [3] has used higher-frequency ultrasonography (Longport Inc., Swarthmore, PA, USA) to detect more subtle changes in tissues associated with a developing pressure ulcer. Lyder claims that experienced users of the systemcan differentiate phases of development, including pockets of oedema, inflammatory changes and frank breakdown (Fig. 17.4). Assessment of these images is a skilful process and open to differences in interpretation.

Elastography is a non-invasive method for imaging tissues based on differences in the tissue stress±strain modulus associated with different tissue Fig. 17.3. Ultrasound image of stage IV pressure ulcer being measured using digital callipers (after Wendelken et al. [2])

Fig. 17.4. Use of high-frequency ultrasonography to detect early pressure ulcer development (after Lyder [3])

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constituents and structures. Srinivasan et al. [4] have developed a technique using ultrasound that measures tissue strain and a nano-indenter which measures tissue modulus. By comparing the results from these two techniques on tissue phantoms they have demonstrated that there is an in- trinsic relationship between tissue modulus and tissue strain. This offers a promising technique for future imaging of tissues where modulus changes are associated with pathology, for example the oedema associated with early pressure ulcer onset. A further development of elastographic technologies has been demonstrated by Sinkus et al. [5]. In this case, magnetic resonance imaging (MRI) is coupled with mechanical wave propagation.

The tissue modulus can be inferred from the MRI data by taking a se- quence of synchronised measurements at the maximum and minimum in- duced strains in the tissues during the application of the vibration. Further discussion of MRI applications is provided in Chaps. 12 and 18.

Although these more advanced techniques do not at present offer practical everyday tools for patient assessment they are of real value in developing a more detailed understanding of pressure ulcer aetiology. With an improved understanding of the problem, more practical imaging techniques may well be evolved.

Pressure Measurement

Numerous devices have been produced for single-point measurements, of which most successful for routine clinical measurements was the Talley-Sci- medics Pressure Evaluator developed by Reswick and Rogers [6] for assessment of wheelchair cushion pressures. It comprised a thin elliptically shaped air bladder, approximately 100´80 mm, with a copper foil grid laminated to opposing surfaces of the inside of the bladder forming a switch. When the bladder was inflated the grid switches were `open', and when sufficient pressure was applied externally to the bladder it collapsed, causing the switch to

`close'. This device had a number of technical limitations, not least its fragil- ity and tendency to under-read when pressure was concentrated within its sensing area. However, it had the distinct advantages of low cost and ease of use, becoming widely adopted by clinicians needing a tool to measure interface pressures when assessing patients for wheelchair cushions and seating. A smaller sensor was later produced, based on similar principles, for making more localised single-point measurements; this device was thin enough to be considered suitable for measuring pressure beneath pressure gar- ments and bandages. Its size and flexibility made it only practical to place directly on the skin prior to making interface measurements, whereas the Tal- ley-Scimedics sensor could be positioned beneath the fully-clothed patient.

The introduction of these interface pressure measurement systems into the clinical setting raised important issues about interpretation of the data and their validity. These concerns continue to be debated by clinicians, researchers and manufacturers of support surfaces who use pressure measurement to

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promote their products. Ferguson-Pell [7] proposed specifications for interface pressure sensors, pointing out that the sensors can introduce errors by locally perturbing the measurement region in which it is placed. The author proposed minimum specifications for the aspect ratio (ratio of thickness to diameter) of pressure sensors for different applications. Later, Ferguson-Pell and Cardi [8[ demonstrated the limitations of pressure-mapping systems in terms of their potential to hammock across the measurement region, in ad- dition to measurement errors introduced by creep and hysteresis in the sensors. These studies and others have contributed to the widely established view that pressure-measurement systems are of clinical value when making com- parisons between different support surfaces for an individual, where essen- tially qualitative information is required. The imaging of pressure between the body and a support surface is not a new technology. The earliest images were produced by Aronovitz et al. [9] and Reswick et al. [10] using a multi- cellular inflatable mat with 1886 independent reading of pressure made in rapid succession. A similar device was developed by Garber et al. [11], who produced the first commercial pressure-mapping system (TIPE System), which provided a spatial resolution of 144 sensors on a pad measuring 400´400 mm. This system was later modified by Jaros et al. [12] to enable the data to be displayed on a computer. Bader and Hawken [13] introduced the Oxford Pressure Monitor (OPM; Talley, UK), which used a novel pneumatic sensor to monitor 12 sensors placed at approximately 25 mm centres.

In the early 1990s a number of pressure-mapping systems were introduced, filling the void left by the removal of the TIPE from the market due to man-

Table 17.1.Summary of four commercially produced pressure-mappingsystems

System Novel Pliance Tekscan Xsensor FSA

Manufacturer Novel GmbH, Munich, Ger- many

Tekscan Inc.,

Boston, USA Xsensor Tech- nology Corp., Calgary, Canada

Vista Medical, Winnipeg, Canada There are five

models for wheelchairs

Sensor type Capacitive Conductive ink Capacitive Conductive rubber Single sensor

area 600 mm²±

196 mm² 103 mm² 135 mm² 298 mm²

Sensor pitch 25 mm±

14 mm 10 mm 13 mm 25 mm

Number of

sensors 256±1344 1558 1296 225

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ufacturing difficulties. Talley Medical Devices produced a 96-sensor array (Mk III, Talley Pressure Monitor). In Canada the QAPad was introduced, providing an electropneumatic system similar to the TIPE with 256 sensors placed on a 400´400 mm sensing area. Further developments were introduced using capacitive arrays of sensors by Novel (Munich, Germany) and by Xsensor (Calgary, Canada). Simultaneously, Vista Medical introduced the Force Sensing Array and Tekscan the SEAT System using semiconductor materials that decrease in resistance with increasing applied axial stress.

Sample images are shown and the characteristics of these four systems are outlined in Table 17.1.

As can been seen in Fig. 17.5, dramatic differences in pressure distribution are noted when a buttock phantom is loaded on a wide range of wheelchair cushions. The loading conditions are identical in each case, but the pressure distribution is significantly different due to the properties of the cushion.

With the advent of the ability to measure interface pressure distributions came questions about how to interpret them (also discussed in Chap. 5). A number of proposals have been made for ªpeak acceptable pressuresº, many of them linked to physiological studies to determine capillary closure pressure. However Kosiak [14] and later Reswick and Rogers [6], empha- sised the multi-factorial nature of pressure ulcer aetiology, not least that there is a nominal inverse relationship between applied pressure and the duration of its application to initiate a pressure ulcer. This inverse relationship is thought to be substantially modulated by other clinical risk factors and the loading history of the tissues. Bader [15], using tissue partial pres-

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Fig. 17.5. Pressure maps generated by a gel-covered cushion loading indenter for a range of commercial cushions. The pressure map at top left is for a rigid surface for reference purposes

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sure of oxygen measurements, demonstrated how, under repetitive loading conditions, there were changes in tissue response with increasing number of loading cycles.

Of course, pressure mapping images draw attention to differences in the way pressure is distributed spatially and thereby introducing third and fourth dimensions (pressure: amplitude, two spatial dimensions and time) to the measurements obtained from simpler instruments that are relevant to pressure ulcer aetiology. Drummond et al. [16] undertook an important study by measuring the distribution of pressure in wheelchairs beneath children with spina bifida. They defined an index by taking the ratio of the sumof the pressure readings in the sacral±ischial region to the sumof all readings on the cushion. Those children whose index was high, indicat- ing that a greater proportion of the body weight was transferred to the cushions through their sacral±ischial tissues, were found to develop significantly more pressure ulcers. This study, coupled with that of Ferguson-Pell et al. [17], was the first to show in human subjects a link between pressure measurement parameters and actual pressure ulcer incidence. Subsequently Geyer et al. [18] undertook a comprehensive randomised prospective study which showed that peak pressure and average pressure were correlated with increased pressure ulcer incidence in a group of frail elderly people in a nursing home setting.

Discussion of pressure measurement is not complete without addressing reasonable concerns for the biomechanical validity of these measurements.

Interface pressure measurement strictly measures localised axial stresses that occur between two loading surfaces. These stresses are often referred to as contact stresses. Off-axis, or shear, stresses are known to occur and are thought to be of considerable clinical significance. Transducers to measure shear stresses have, however, presented significant technological chal- lenges, although a number have been produced for research purposes. The level of shear stress is influenced by the direction of load transfer between the body and the support surface and the frictional properties of the interface. Interposing sensors will result in substantial changes in local shear unless the shear sensor matches the frictional properties of the interface materials. A satisfactory technological solution is still elusive.

However, recognising that it is the physiological response of tissues to local mechanical stresses that determines their viability, many have suggested that measures of tissue deformation would yield measures more closely linked to the influence of a support surface on pressure ulcer risk.

Cheung [19] and Sprigle et al. [20] employed instrumentation to measure the shape of the buttocks under loaded conditions and related the findings back to interface pressure readings. They found in a group of spinal injured participants that there were distinctive differences in loaded buttock shape linked to level of injury. They suggested that customcontoured seating could be used to accommodate these differences and thereby minimise tissue deformation during sitting. A buttock shape-sensing system was produced (Fig. 17.6) which enabled a record to be taken and input to a nu- merically controlled milling machine. Although the cushions did not match

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the undeformed buttock shape, because the measurements were made with the tissues loaded on spring-loaded displacement sensors, this approach was thought to reduce tissue deformation compared with a planar cushion.

There are concerns, however, that cushions formed to create an intimate fit might restrict opportunities to produce pressure redistribution through changes in posture. Nonetheless buttock shape-capturing technologies are being widely used for patients needing sophisticated postural management and where the risk of pressure ulcer development is relatively low.

Monitoring Tissue Response to Load

Given the many limitations of pressure mapping, interest has been directed towards the use of physiological monitoring where the body's response to a support systemis used to indicate its suitability.

A number of discrete sensing systems have been used for this purpose, including transcutaneous measurements of the partial pressure of oxygen in the tissues [15], tissue reflectance spectroscopy (TRS) [21±23] and laser Doppler flowmetry (LDF) [24]. In all cases these instruments measure physiological changes in tissues associated with either the application of load generating ischaemia, or the reperfusion response (reactive hyperae- mia) when the load is removed.

Figure 17.7 illustrates TRS and LDF responses for a heel placed on an alternating pressure mattress during an inflation and deflation cycle. The cyclic variations in the LDF signal are associated with vasomotion.

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Fig. 17.6.Spring-loaded displacement sensors used to measure loaded buttock contour

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Laser Doppler Flowmetry

Laser Doppler flowmetry imaging uses a stable monochromatic laser light source directed through a fibre optic to illuminate the tissue. Backscattered light is collected using a second fibre optic placed closed to the source (approximately 1.5 mm centres). The backscattered light will be slightly frequency shifted if the scattering medium, namely blood, is moving either towards (blue shift) or away from (red shift) the point of measurement.

The net effect is for the bandwidth of the frequency spectrumof the backscattered light to be broadened by an amount proportional to the average velocity of the scattering medium. The tissue thickness sampled is typically 1 mm, the capillary diameters 10 lmand the velocity spectrummeasure- ment 0.01±10 mm/s.

Most LDF systems make single-point measurements providing an index of the blood flow rate. However, one scanning system(LDF Imager; Moor Instruments, Devon, UK) produces images of the blood flow in tissues by using a mirror to move the laser beam across the region of measurement in a raster pattern. This systemcan measure regions of tissue from 50´50 mm up to 500´500 mm, taking approximately 5 min to complete the scan. The maximumresolution of the systemis 100 lm. A typical image of a healing burn is shown in Fig. 17.8.

Fig. 17.7. Tissue response for a heel of a healthy subject placed on an alternatingpressure mattress. The internal bladder pressure in one of the cells of an alternatingpressure mattress is represented in black. The indices for skin blood content and oxygenation are indicated in blue and red, respectively, and the laser Doppler flux is represented in green

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Transcutaneous Oxygen Measurement

TcPO2 sensors employ a polarographic Clark electrode which comprises a platinumcathode held at 0.7±0.9 V relative to an anode usually made from silver. An oxygen-permeable membrane isolates the electrode from the skin. In order to measure the partial pressure of oxygen in the tissue the electrode is heated to 42±44 8C. This induces maximal vasodilation, so that the oxygen diffusing through the skin is equilibrated closely with arterial oxygen tension. Only discrete measurements are possible at present using TcPO2sensors; however, a four-sensor system is available commercially for multi-point measurements (Perimed, Jårfålla, Sweden). These sensors have been used extensively by Bader [15]. Their repeatability for clinical applications is discussed by Coleman et al. [25].

Tissue Reflectance Spectroscopy

Oxy- and deoxyhaemoglobin have distinctly different absorption spectra, particularly in the green and red regions of the spectrumas well as in the near infra-red (NIR) region of between 700 and 850 nm.

A number of approaches have been adopted to use the absorption spectrumof light in the visible region to characterise the blood content and oxygenation of the superficial vasculature of the skin. Dawson et al. [26]

employed the points on the absorption spectra for oxy- and deoxyhaemoglobin, known as the isobestic points (Fig. 17.9). The level of oxygenation does not influence the absorption of light at these wavelengths. These wavelengths are influenced by the concentration of blood in the sample volume and other factors, such as melanin. By taking ratios of neighbouring isobestic points (absorption = L_xxx at wavelength xxx), it is possible to de- rive indices for the blood concentration (H) in the sample volume, and its level of oxygenation (O_x). The indices proposed by Feather et al. [26] were:

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Fig. 17.8. Laser Doppler flowmetry image of healing burn with corresponding photograph (after Moor Instruments)

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H L544 L527

16:5

L573 L544 29

100 17:1

Ox 100 L573 L558 L558 L544=14:5H 17:2

As the blood concentration tends to zero, then Ox becomes indetermi- nate (one cannot measure haemoglobin oxygenation if there is no blood!) and in practice for low blood concentrations the signal-to-noise ratio becomes very low.

Subsequently Ferguson-Pell and Hagisawa [22] proposed a slightly modified formof these equations for spectrometers with improved resolution.

The isobestic wavelengths are shown in Fig. 17.9.

Scattering is the dominant photon±tissue interaction in skin in the NIR wavelengths, whereas in the visible region light is strongly absorbed and therefore only penetrates the most superficial layers of the skin. Hebden and Delpy [27] and Hebden et al. [28] have demonstrated, using arrays of TRS optodes, that in the NIR it is possible to create images showing regions of differing blood oxygenation of the neonate brain. In Fig. 17.10 this technique is demonstrated for the forearm.

It is also interesting to note that in the NIR an important terminal enzyme in the cellular respiratory chain (cytochrome oxidase, CtOx) produces in its oxidised state a broad peak around 830 nm[29]. This is not present for the enzyme in its reduced state. Thus it may prove possible to monitor and image the degree of oxygenation in tissues at a cellular level.

However, due to the relatively strong absorption at these wavelengths this technique presents practical difficulties when decoupling the simultaneous contributions to the absorption spectrumof haemoglobin and CtOx.

Ferguson-Pell and colleagues made discrete sensors to measure tissue reflectance spectra at interfaces between the body and support surfaces, and Fig. 17.9.The absorption spectrum of blood indicatingthe isobestic wavelengths

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sample data are presented in Fig. 17.11. The dimensions of these sensors (10 mm diameter, 2.5 mm thickness) is approaching the aspect ratio re- quirement for pressure sensors at body±support interfaces [7]. They have also produced an array of sensors that offer the potential to image these parameters, using high-intensity light-emitting diodes with emission wavelengths close to the isobestic points used in the equations above (Fig. 17.9).

Summary

Imaging techniques are available both to measure the interaction of the body with support surfaces and to assess tissue status. Ultrasound provides an effective method for assessing wound status, particularly in determining the extent of undermining of the wound. Photogrammetry techniques permit accurate measurement of wound dimensions using simple digital photography.

Mechanical interaction with a support surface can be visualised using pressure mapping. There are, however, concerns regarding the accuracy of these devices and their influence on the supporting characteristics of some cushions and mattresses.

The physiological response of tissues to ischaemia can be determined with a range of different techniques that provide information about blood

a Summary z 313

Fig. 17.10. NIR imaging of arm (left: optode array around circumference of arm; centre: X-ray image; right: NIR image) (after Elwell and Hebden [29])

Fig. 17.11. Schematic representation of a prototype array of sensors that use high-intensity LED illumination at centre wavelengths consistent with the isobestic points used by tissue reflectance spectroscopy to estimate levels of blood content and oxygenation

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flow, tissue oxygenation, and blood content and oxygenation. Imaging physiological parameters is challenging, especially if information is required while the tissue is under load. However, recent advances in electro- optics are reducing both the size and cost of sensors, suggesting a promising future for using the tissues as a direct indicator of their status, rather than drawing inferences fromsimple observation or pressure measurements alone.

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