GB2377489A

GB2377489A - Optoelectronic device for characterising tissue perfusion in dermis

Info

Publication number: GB2377489A
Application number: GB0115331A
Authority: GB
Inventors: Martin Ferguson-Pell; Duncan Shirreffs Bain; Graham Philip Nicholson
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-06-22
Filing date: 2001-06-22
Publication date: 2003-01-15
Also published as: GB0115331D0

Abstract

The device consists of an array (columns and rows) of sensing elements (7, figure 4), each comprising a light guide (1) arranged to emit light to illuminate an area of the dermis of a person or animal, and a receiving optical fibre (2), arranged to receive the light transmitted from the illuminated dermal area and a light measurement device. The plurality of such sensing elements can be assembled into an array (, figure 4), or affixed to a flat flexible substrate. A reflective backing plate (3) is provided to augment the delivery of light from the light guide to the skin. The light guide may be an optical fibre(s) or transparent sheet illuminated by single source, which may produce two or more specific colours. . The measuring device may be a spectrometer, a photodiode, or a CCD.

Description

Device for mapping the spatial distribution of blood parameters in the skin Martin Ferguson-Pell, Duncan Shirreffs Bain, Graham Philip Nicholson Background Pressure sores are a serious secondary complication affecting a wide range of people who receive hospital care. Frail elderly inpatients and nursing home residents are particularly vulnerable, especially when seated in restchairs, or when prolonged bed-rest is necessary. Measures to prevent pressure sores include the provision of special support surfaces, such as alternating pressure mattresses and wheelchair cushions that may comprise advanced materials and strategic contouring. Clinicians select support surfaces for their patients based on clinical risk assessment tools such and assessment for posture, tissue status and functional activities. Recently pressure mapping systems have been introduced to provide dynamic contour plots of pressure beneath the seated or recumbent patient. This technique has been used for comparing the ability of competing products to relieve pressure, for cushion selection with individual users, or for training patients how to attain an optimal sitting posture.

Pressure sores, however, are not caused by pressure per se. The tissue necrosis which constitutes a pressure sore is more likely the result of prolonged ischaemia (lack of blood supply) or hypoxia (lack of oxygen delivered by the blood supply). A number of variables intrinsic to the individual affect the patency of blood supply, and so the level of pressure required to bring about ischemia, and the level and duration of ischemia required to produce hypoxia.

Furthermore, the nature of pressure required to cause ischemia is complex : pressure gradients are thought to be of more consequence than absolute pressures, and tangential forces as well as normal forces also appear to contribute to ischemia.

Thus, although pressure mapping yields valuable insight into how body weight is distributed by a cushion or mattress, the physiological response of the skin to a support surface is unknown and likely to be influenced by a wide range of factors specific to the individual. Current methods for the assessment of support surfaces are subject to significant measurement errors, do not include the influence of mechanical factors such as shear, or differences in physiological responses associated with different and varying medical conditions.

This invention relates to a device which maps the distribution of blood content and blood oxygenation over the surface of the skin Laser doppler flowmetry and measurement of transcutaneous partial pressure of oxygen and carbon dioxide (TcP02, TcPC02) have been used to characterise tissue response to mechanical loading. However the dimensions of the sensors used in these systems are thought to produce substantial localised tissue loading which in a matrix configuration would produce significant disruption of the characteristics of the support surface.

Furthermore the unit cost of these systems make multi-sensor applications prohibitively expensive.

Using tissue reflectance spectroscopy the applicants have demonstrated methods for detecting changes in blood content (lHb), oxygenation (lax) and the presence of pulsatile blood flow in the dermis using very thin ( < 1 mm diameter) fibre optic probes. They are connected to a spectrometer operating in the visible region (450-620nm). These parameters can be determined with minimal cross-talk and algorithms have to been developed by the applicants so that the values are independent of the subject's skin pigmentation. Using data generated by this system it is possible to characterise tissue perfusion during and following a period of loading (see figure 1). Initial resting levels, and changes in these parameters while supporting body weight, and the subsequent response of the tissues when they are unloaded (reactive hyperaemia peak response) all give promising measures of tissue response to a period of sitting on a support surface.

Referring to figure 2, the graph shows the absorption spectrum of oxyhaemoglobun (solid line) and deoxyhaemoglobin (dotted line) in the visible spectrum. It can be seen that the 2 curves cross at various isosestic points, and the absorption at these points is therefore independent of oxygenation.

An index of haemoglobin concentration, and so blood content, can therefore be constructed using these ponts eg: Blood Content Index = (2 x Absorption at 545nm)- (Absorption at 522nm + Absorption at 568nm) which is independent of blood content.

Having established IHB, it is possible to determine the oxygenation state of the blood by examining the Absorption at 557nm relative to the absorption at isobestic points 545nm and 568nm:

Eg: Blood Oxygenation Index = ( (2 x Absorption at 557)- (Absorption at 545nm + Absorption at 568nm))/Blood Content Index These indices are merely examples, and numerous variants may be constructed to represent these perfusion parameters.

It was observed by the authors that skin blood content as measured using tissue reflectance spectroscopy showed pulsatile behaviour.

Description A sensing element may be constructed as in figure 3, comprising 2 optical fibres and a backing plate. White light is delivered to the sensing area by transmitting fibre, 1. Light is emitted from the end of the fibre as a cone, the angle of which is determined by the optical properties of the optical fibre.

When the sensing area is placed on the skin, a proportion of this light will diffuse into the skin. Within the skin, light is absorbed according at various wavelengths according to the extinction coefficients of the various constituents of the skin, including blood. Back-scattered light from the skin is therefore a coded spectrum which may be analysed using techniques such as the indices mentioned above. Alternatively, pulse waveforms may be picked up as modulations of the optical signal.

A receiving fibre, 2, receives back-scattered light from the skin, and delivers it to a spectrometer or other such device.

A reflective backing plate, 3, may be affixed to the fibre pair, with the fibres between the backing plate and the skin, to augment the light delivered from fibre 1 to the skin, and to augment the light delivered from the skin to fibre 2.

As an alternative to full spectroscopy, blood information may be derived from a small number of wavelengths of light using similar techniques to those employed in photoplethysmography, or pulse oximetry. This would entail delivering light of 2 or more specific colours (centred on specific wavelengths in the visible or infra-red range) to the transmitting fibre by such means as light emitting diodes. 2 or more light emitting diodes could be made to illuminate the fibre alternately, and respective returned signals could be measured from the receiving fibre using a photodiode, ccd, or similar device.

Sensing elements may be assembled into an array as in the schematic in figure 4. Multiple sensing elements are affixed to a flat, flexible substrate, 6.

Transmitting and receiving fibres may be assembled into rows and columns respectively. Each light source, 4, which may be a light emitting diode, or other light source, illuminates a bundle of transmitting fibres, 5. Each of the fibres within each bundle delivers light to a single sensing element, 7. Light returned from the skin at each sensing element is received via a receiving fibre, which joins fibres from sensing elements in the same column to form a fibre bundle, 12. Light from each column is delivered to a light measurement device, 8, such as a photodiode, spectrometer, or ccd. These measurement devices may be arranged as an array. By alternately illuminating individual light-sources, 5, the signal from each measurement device, 8, relates to a specific sensor. This constitutes a means of multiplexing the information from each sensor on a given column.

An alternative arrangement shown in figure 5 consists of a number of light sources, 9, each of which supplies light to a single fibre. In this embodiment, all transmitting fibres may be gathered into a single bundle, 10, and separate within the array of sensing elements so that each sensing element, 7, is supplied with light by a single specific transmitting fibre. Similarly, each sensing element returns light coded by the skin to a single specific receiving fibre. All receiving fibres are gathered into a single fibre bundle, 11, which delivers the received light to a spectrometer or other measurement device, 8.

In this embodiment, each light source is illuminated separately, so that a single spectrometer or other receiving device may separately receive signal from each sensing element.

An alternative construction for the sensor that may enhance the delivery of light to the skin, and enhance receipt of back-scattered light from the skin is shown in figure 6. A fibre 13, terminates in an oblique face. A hole is drilled in a slab of transparent material, 14, to accommodate the fibre, 13. Light travelling down the fibre is reflected by the oblique face, and so enters the skin at near to a right angle, so minimising losses. The reflection may take place by internal reflection. Alternatively, the oblique face of the fibre may be mirrored. Alternativelt, a mating plug 15, with mating oblique reflective surface, 16, may be inserted into the hole opposing the fibre, 13.

Figure 7 shows such a sensor complete with both transmitting and receiving fibre components. The receiving fibre also has an oblique face, to optimise receipt of backscattered light at right angles from the skin.

Alternatively, light may be made to escape from the transmitting fibre and enter the receiving fibre by provision of a rough surface on the outside of the fibre. Abrasion of a region of the fibre to produce a frosted effect inhibits

internal reflection, and causes leakage of light across the interface between media. Figure 8 shows the rough surface, 20, allowing leakage of light, interrupting the internally reflected light path, 21.

Alternatively, in figure 9, a flexible transparent sheet material, 22, may be employed as the light-guide. Light delivered to the edge of such a material for example by abuttment of an optical fibre, 23, will reflect internally until it meets a discontinuity in the surface. Rough patches that may be frosted finish may be provided, 24, to allow light escape to and from the light-guide. Opaque divisions in the sheet, or scores in the sheet may isolate rows or columns, and prevent cross-talk between rows or columns. Multiple transmission or receiving fibres, 23 may be provided to supply or read from separate rows and colums. Similar multiplexing strategies to those employed in figure 4 may be employed.

An alternative embodiment is shown in figure 10. Light sources may occur directly on the sheet for example by means of application of an electrophosphorescent material to the surface of the sheet. In effect, these take the form of thin-film light-emitting diodes, 26 which may be applied to the surface of the sheet, 22. These are supplied by electric wires, 27, allowing alternate switching of light sources. Light backscattered from the skin may be picked up by the sheet, and a proportion of light returns by internal reflection to one or more optical fibres, 23.

The array of sensing elements may consist of 2 or more elements which are detached from each other. Figure 11 shows an embodiment with 2 sensors, 16 and 17, which may be affixed to the body, 18. Sensor 16 is affixed to the body in an area which does not experience pressure. Sensor 1 7 is placed on an area of the body which is subject to pressure. By comparing the readings from the respective sensors, appraisal may be made of the local effects of pressure.

Claims

Claims 1 A device for characterising tissue perfusion in the dermis of a person or animal comprising : a light guide arranged to emit light to illuminate an area of the dermis of the person or animal, and a sensing element, each sensing element having; an optical fibre acting as a receiving fibre arranged to receive light transmitted from the illuminated dermal area, and a light measurement device arranged to receive light from the receiving fibre
2. A device according to claim 1 wherein the device comprises a plurality of sensing elements assembled into an array
3. A device according to claim 2 wherein each sensing element is affixed to a flat flexible substrate.
4. A device according to claim 2 or claim 3 wherein a reflective backing plate is provided to augment the delivery of light from the light guide to the skin.
5. A device according to claim 2 wherein a light source is provided to transmit light through the light guide.
6. A device according to claim 5 wherein the light guide is an optical fibre.
7. A device according to claim 6 wherein: the sensing elements are arranged in columns and rows,

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each row of sensing elements is illuminated via a single light source which transmits light thereto via a bundle of optical fibres communicating one each between the light source and the sensing element, each column of sensing elements communicates with a single common light measurement device, and means is provided to alternately illuminate each of the light sources whereby a signal can be read out from each sensing element
8. A device according to any one of claims 1 to 5 wherein the light guide is a flexible transparent sheet matenal having patches to encourage light to escape from a surface of the material.
9. A device according to any one of the preceding claims wherein the measurement device is a spectrometer.
10. A device according to any one of claims 1 to 8 wherein the sensing device is a photodiode or CCD and each light source is adapted to emit light of two or more specific colours.
11 A device for characterising tissue perfusion in the dermis of a person or animal comprising : an array of light sources disposed on the surface of a sheet each associated with an optical fibre, each said optical fibre acting as a receiving fibre arranged to receive light transmitted from the illuminated dermal area, and a light measurement device arranged to receive light from the receiving fibre.
12. A device for characterising tissue perfusion in the dermis of a person or animal as herein described with reference to the accompanying Illustrative drawings.