GB2269012A

GB2269012A - Colour sensor; Fetal blood oximeter

Info

Publication number: GB2269012A
Application number: GB9315039A
Authority: GB
Inventors: Martin Richard Holman
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-07-22
Filing date: 1993-07-20
Publication date: 1994-01-26
Also published as: GB9315039D0; GB9215584D0

Abstract

A sensor for detecting colour changes in a substrate, as in a fetal blood oximeter, comprises a first optical fibre 5 which detects light emitted by a second optical fibre 4 which is back-scattered by the substrate (skin), the relative positions and acceptance angles of the optical fibres being such that only light reflected at distances greater than a predetermined distance D from the sensor is detected. The sensor may be clamped to the substrate by a vacuum clamping device which comprises a clamping pad 10 having a plurality of compartments 11-20, and means for applying a vacuum intermittently to each of the compartments, the arrangement being such that the vacuum is applied to each compartment in a sequential fashion such that the clamping force is maintained, while minimising blood vessel compression. <IMAGE>

Description

MONITORING DEVICE This invention relates to monitoring devices, and more particularly to a monitoring device comprising a novel sensor for detecting colour changes in a substrate, and a novel vacuum clamping device. Other novel aspects of the invention will be described in detail hereinafter.

Many medical and surgical operations and procedures require monitoring of the subject or patient, often on a more or less continuous basis. This is the case for example during intensive care, operations carried out under anaesthetics, and similar surgical practices; and particularly during childbirth.

An important parameter which it is often considered desirable to measure is the amount of oxygen in the blood of a subject or patient. For example, if a baby becomes deprived of oxygen during birth it can develop a condition known as acidosis, which can lead to brain damage. Therefore, it would be very useful for the doctor or midwife to be able to monitor, both accurately and reliably, the level of oxygen in the baby's blood, whilst its mother is in labour. If the baby's bloodoxygen level falls below a certain value the doctor is then able to intervene and deliver the baby, for example by Caesarian section, thus avoiding the possibility of a spastic or brain-damaged child being born.

It has been proposed, to use an optical technique for fetal monitoring during labour. In this known technique a sensor is placed on the baby's head whilst it is in the birth canal. It is known that haemoglobin in the blood changes colour, depending upon the amount of oxygen therein, and that a person that is suffocating due to lack of oxygen appears "blue", whereas a healthy person appears "pink". Thus by measuring this colour change, it is possible to estimate the blood oxygen level. In addition, since the blood pulsates through the body, this measurement is also pulsatile and is accordingly known as "pulse oximetry".

Oximetry utilizes the fact that the light absorbtion spectra of oxygenated and reduced haemoglobins differ, a characteristic of both fetal and adult haemoglobins. A pulse oximeter uses two reference wave lengths, one in the visible red and one in the near-infra-red part of the spectrum (660nm and 940nm). At both wave lengths, absorption occurs in skin, subcutaneous fat and connective tissue (the non-pulsatile component) as well as in the pulsatile vascular bed. The pulse oximeter is mainly concerned with the pulsatile alternating component of the signal; the ratio of non-absorbed pulsatile/non-pulsatile components is calculated for both wave lengths and the non-pulsatile values are allowed to cancel each other out.The ratio of the two pulsatile components (red /infra-red) is then used through an algorithm to calculate the percentage of haemoglobin that is saturated; if this ratio is calculated at the peaks of the pulsatile signal, the pulse oximeter can be said to indicate oxygen saturation levels of arterial blood.

A number of practical problems have arisen in applying pulse oximetry to the measurement of fetal blood oxygen levels, in particular, the size of the probe required, and devising an effective technique for attachment of the probe to the baby's head. At the present time, these problems have not adequately been solved, and there is accordingly no suitable oxygen monitoring system routinely available for use on babies.

The present invention overcomes the problem of the size of the probe by using a fibre-optic system in which optical fibres carry the light from a remote source to the baby's head and convey the back scattered light to a detector. It can also use white light without the need for any special light source.

It is known to attach the probe to the baby's head by means of hooks, or by suction devices. However these can become detached and lead to false readings, and in addition suction devices can affect the blood flow to such an extent that no meaningful blood measurements can be made.

The present invention substantially overcomes these problems by providing a clamping device having a plurality of compartments to which a vacuum is intermittently and sequentially applied.

In one aspect the invention provides a sensor for detecting colour changes in a substrate, which comprises a first optical fibre through which light is emitted, and a second optical fibre through which light is detected, the arrangement being such that the second optical fibre detects light emitted by the first optical fibre which is backscattered by the substrate, the relative positions and acceptance angles of the optical fibres being such that only light reflected at distances greater than a pre-determined distance from the sensor is detected by the second optical fibre.

In a second aspect the invention provides a vacuum clamping device comprising a clamping pad having a plurality of compartments, and means for applying a vacuum intermittently to each of the compartments, the arrangement being such that the vacuum is applied to each compartment in a sequential fashion such that the clamping force is maintained.

In a further aspect the invention provides an oximetry sensor which comprises a white light source, a sensor probe adapted to be positioned adjacent to the subject or patient and comprising a light emitting means coupled to the white light source and a light detecting means, an opto-electrical receiver means for receiving the light reflected from the subject or patient and detected by the light detecting means, and optical fibres connecting the light source and opto-electrical receiver means respectively to the light emitting means and the light detecting means.

In yet another aspect the invention provides a device for bending an optical signal through a 900 angle, which comprises a pair of optical fibres each having a mitred end, adjacent mitred surfaces of the fibres being optically connected, and the remote mitred surfaces of the optical fibres being silvered, in contact with a silvered reflecting surface, or totally internally reflecting.

Although the invention will be hereinafter more particularly described with reference to a fetal blood monitoring device, it is to be understood that it is not limited thereto, and for example the monitoring device could be used for detecting colour changes in child or adult patients in other medical and surgical procedures, or for detecting colour changes in other substrates for example caused by temperature changes and the like, and for detecting colour changes in chemical reactions. The vacuum clamping device could be used in other monitoring or sensing medical devices, or indeed in applications outside the medical field, and the bending devices could find other applications in optical fibre technology.

A particular advantage of the monitoring device of the present invention is that it can be arranged only to accept back-scattered light which has been reflected from surfaces at a distance greater than a pre-determined distance from the sensor head or probe. One disadvantage of prior art optical systems is that if there is a gap between the sensor face and the skin, emitted light is able to reach the detector without passing through the skin and tissues. By accepting only light that is reflected from points at a distance greater than a predetermined minimum distance from the sensor this source of inaccuracy is substantially obviated. Preferably the minimum distance is at least 0.5 mm, most preferably from 0.5 mm to 2.0 mm from the face of the monitoring device.

Preferably each of the compartments of the vacuum clamping pad comprises a thin flexible membrane which covers the compartment and forms the face of the pad. The membrane is preferably impermeable to body fluids and thus prevents any such fluids from being sucked up into the compartments by the applied vacuum. It is found in practice that the presence of such a thin membrane has little or no effect upon the clamping force exerted by the vacuum pad.

The invention will now be more particularly described with reference to, and as illustrated in, the accompanying drawings in which: Figure 1 shows a sensor head or probe for a fetal monitoring device in plan view and in side elevation; Figure 2 shows the bottom half of a vacuum clamping pad for a fetal monitoring device according to the invention in plan view, side elevation, and cross-section; Figure 3 shows the top half of the vacuum clamping pad of figure 2 in plan view, showing the position of the sensor head and optical fibres; Figure 4 shows the arrangement of part of the optical system for the fetal monitoring device; Figure 5 shows diagrammatically the ends of the optical fibres and their acceptance angles; Figure 6 shows diagrammatically the electronic and optical system for the fetal monitoring device of the invention; and Figure 7 illustrates graphically the vacuum cycle as applied to the compartments of the clamping pad.

Referring now to figure 1 the central head or probe comprises a moulded housing 1, within which there are firmly retained optical fibres 2 and 3. The optical fibres may for example be of the acrylic type having an external diameter of imam. Each of the optical fibres is mitred at 450 in two directions so that each fibre has an end with two flat surfaces at an angle of 900. Two short lengths of similar optical fibre 4 and 5, are cut so as to have one flat end and one mitred end.

Adjacent mitred surfaces of optical fibres 2 and 4, and optical fibres 3 and 5, are placed adjacent to each other within the moulded housing so that the fibres 2 and 4, and 3 and 5, are at right-angles to each other. The arrangement can be seen in figure 1 in connection with fibres 3 and 5, the adjacent mitred surfaces being designated 6 and 7. The remote mitred surfaces of fibres 2, 4, 3, and 5 are silvered as shown at 8 and 9 in figure 1. The effect of the arrangement is that light from or entering the two parallel fibres 2 and 3 is turned through an angle of 900 within the housing 1 and is emitted or detected through the ends of fibres 4 and 5 at the face 52 of the sensor head.

The sensor head or probe is carried by a vacuum clamping pad as shown in figures 2 and 3. The device comprises a vacuum pad 10, of generally cylindrical construction which is cast or moulded from a soft rubber material, for example a silicone rubber. It is to be understood, however, that the vacuum pad may have any suitable shape or dimensions for example it may be lozenge-shaped, oval, or elliptical in shape. The vacuum pad is in two parts, and as shown in figure 2, the bottom half has a number of compartments 11 to 20 communicating with a centrally disposed recess 21. Within the recess 21 there is fitted the top half of the vacuum pad 22#, as shown in figure 3. Communicating with the vacuum pad is an elongate flexible soft rubber conduit 23, which has four axially directed holes therein. The conduit can be of any suitable cross-section, for example, round, square or rectangular.Of these, holes 24 and 25 are for receiving the optical fibres 2 and 3, and 26 and 27 are vacuum tubes connected to a vacuum pump. The housing 1 of the sensor head or probe is situated in a hole 28 in the bottom half of the vacuum clamping device and is held in place by the top half, 22.

Ducts 29, 30, and 31 moulded into the bottom half of the vacuum clamping device connect respectively compartments 11 and 19 (duct 29), 12, 18 and 14 (duct 30), and 15 and 17 (duct 31).

Tubular passageways 32 and 33, moulded into the top half of the vacuum clamping device connect respectively compartments 11, 13 and 15 (passageway 32), and 16, 18, and 20 (passageway 33) via holes 32a and 33a. It can be seen, therefore, that when the top half and the bottom half of the vacuum clamping device are assembled together there is produced two sets of interconnecting compartments, 11, 13, 15, 17 and 19, and 12, 14, 16, 18 and 20. Each of compartments, 11 to 20 is covered by a thin flexible membrane 34, formed from silicone rubber.

Referring now to figure 4, the sensor head or probe, 35, comprising the opaque plastic housing 1, with the optical fibres 2 to 5 embedded therein, is connected via the optical fibres 2 and 3 to respectively a source of pulsed white light 36 and a detection system 37. Referring now to figure 5, there is show in detail the ends of the emitter fibre 4 and the detector fibre 5. the fibres have flat polished end surfaces 41 and 42 and are situated within the housing about lmm apart.

As can be seen from the drawings, the emitter fibre will emit light in the form of a cone whose angle is the acceptance angle of the fibre. Similarly the detector fibre 5, will receive light falling within a cone determined by its acceptance angle.

Only light that falls within the overlap of the two cones will be detected, and thus no reflected light which is reflected from points less than distance D from the surface of the sensor probe can be detected by the detector fibre. Preferably the fibres are arranged such that the distance D is at least 0.5 mm, and preferably from 0.5 mm to 2.0 mm.

The electronic and optical system of the fetal monitoring device is shown diagrammatically in figure 6. The light source 36 comprises a white light source 43, such as a tungsten lamp positioned behind a slotted wheel 44 which is driven by a step motor 45. Light passing through the slots in the wheel as it is rotated is effectively pulsed, and is collected in one or more optical fibres 46. The pulsed white light is then conveyed to the sensor head 35 wherein light from the emitted fibre 4 is reflected from the baby's head and collected by the detector fibre 5. The detected light signal containing red scattered light is conveyed by optical fibre 47 to a grating spectrometer 48, together with white light from a reference fibre 49 which has not passed through the sensor head.By comparing the detected light signal with the reference signal spectral variation and attenuation in the fibres can be eliminated.

In the spectrometer the light is split up into its component parts by a diffraction grating and the defracted light allowed to fall onto a photodetector array. The number of detections in the array can be varied but, for example, about 10 is a preferred number. The signals from the photodectors are then passed via an amplifier and analogue to digital converter 50 to a microprocessor 51, where the signals are compared with a set of calibration data stored in the microprocessor memory. The set of data that best fits the observed readings is then taken as the best estimate of the oxygen level in the blood and recorded on the digital display 52. The accuracy of the fit will give a measure of the accuracy of the oxygen reading. If desired, the sensor probe can also carry an ECG electrode 51 which is connected to the signal analyzer so that pulse rate can also be displayed.

The sensor head or probe is held on the baby's head by means of the vacuum clamping pad, whose operation may be described with reference to figure 7. A vacuum is first applied to tube 26 which communicates with compartment 11 in the vacuum clamping device 10. As previously explained, compartment 11 is connected via ducts 29, 31, and passage way 32 to compartments 13, 15, 17 and 19 and thus these compartments will all be under vacuum at substantially the same time. The vacuum in tube 26 is then returned to atmospheric pressure or slightly above, but before so doing, the vacuum is applied to tube 27, and thence to compartments 12, 14, 16, 18 and 20 via passageway 33 and duct 30. Similarly, before the vacuum in tube 27 is released to atmosphere the vacuum is applied once more to tube 26. By applying the vacuum alternately to compartments 11, 13, 15, 17, 19 and 12, 14, 16, 18, and 20, it is ensured that the blood vessels underneath the vacuum pad are not compressed for too long, which might otherwise lead to unreliable readings of blood oxygen level.

Preferably the cycle time is from 1 to 10 seconds, most preferably about 5 seconds. In a preferred arrangement, the vacuum is applied for more than 50% and preferably around 70% of the time to each of the tubes 26 and 27 and released for around 30% of the time. By arranging for an overlap of the vacuum cycles in the tubes 26 and 27 it is ensured that the sensor does not become disengaged from the babies head inadvertently.

By the use of the present invention practical and reliable methods of obtaining optical measurements on a baby's head may be achieved. The light source is positioned remote from the sensor and the use of optical fibres still enables sufficient light to be detected to ensure accurate readings. s use of the vacuum pad avoids the necessity to use stainless steel hooks with their consequent risk of infection, and also improves the mechanical fastening of the sensor to the babies head. By applying the vacuum intermittently the risk of damage to blood vessels underlying the vacuum pad, which may restrict blood flow and lead to bruising, is substantially obviated.

In a typical arrangement, the vacuum pump may be operated by a pair of solenoids, operating diaphragms or pistons connected to the vacuum lines 26 and 27. The sensor head, vacuum pad, conduit and optical fibres constituting the sensor assembly may be supplied as a disposable item, thus further reducing the risk of infection.

The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps or any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features

Claims

CLAINS 1. A sensor for detecting colour changes in a substrate, which comprises a first optical fibre through which light is emitted and a second optical fibre through which light is detected, the arrangement being such that the second optical fibre detects light emitted by the first optical fibre which is back-scattered by the substrate, the relative positions and acceptance angles of the optical fibres being such that only light reflected at distances greater than a pre-determined distance from the sensor is detected by the second optical fibre.
2. A sensor according to claim 1 that is a blood monitoring device.
3. A sensor according to claim 1 or 2 in which the pre determined distance is from 0.5mm to 2.0mm from the face of the sensor.
4. A sensor according to any one of the preceding claims substantially as described and illustrated in the accompanying Drawings.
5. A sensor according to any one of the preceding claims substantially as hereinbefore described.
6. A vacuum clamping device comprising a clamping pad having a plurality of compartments, and means for applying a vacuum intermittently to each of the compartments, the arrangement being such that the vacuum is applied to each compartment in a sequential fashion such that the clamping force is maintained.
7. A vacuum clamping device according to claim 6 that is combined with a sensor so as to form a blood monitoring device.
8. A vacuum clamping device according to claim 6 or 7 in combination with a sensor according to any of claims 1 to 5.
9. A vacuum clamping device according to any of claims 6 to 8 in which each of the compartments comprises a thin flexible membrane which covers the compartment and forms a part of a face of the pad.
10. A vacuum clamping device according to any of claims 6 to 9 in which the vacuum is applied to each of the compartments for more than 50% of the cycle time.
11. A vacuum clamping device according to claim 10 in which the cycle time is from 1 to 10 seconds.
12. A vacuum clamping device according to claim 10 or 11 in which the vacuum is applied continuously for around 5 to 15 seconds whilst sensor readings are taken.
13. A vacuum clamping device substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings.
14. A vacuum clamping device substantially as hereinbefore described.
15. An oximetry sensor which comprises a light source, a sensor probe adapted to be positioned adjacent to the subject or patient and comprising a light emitting means coupled to the light source and a light detecting means, an opto-electrical receiver means for receiving the light reflected from the subject or patient and detected by the light detecting means, and optical fibres connecting the light source and opto-electrical receiving means respectively to the light emitting means and the light detecting means.
16. An oximetry sensor according to claim 15 in which the light source is pulsed.
17. An oximetry sensor according to claim 15 or 16 in which the light source is a white light source.
18. An oximetry sensor according to any of claims 15 to 17 in which the opto-electrical receiver means comprises a spectrometer.
19. An oximetry sensor according to claim 18 in which the light from the spectrometer is passed via a photo detector to an amplifier, an analogue to digital convertor, and a microprocessor, and the resultant signal displayed on a digital display.
20. An oximetry sensor according to any of claims 15 to 19 substantially as described and illustrated in the accompanying Drawings.
21. An oximetry sensor substantially as hereinbefore described.
22. A device for bending an optical signal through a 900 angle, which comprises a pair of optical fibres each having a mitred end, adjacent mitred surfaces of the fibres being optically connected, and the remote mitred surfaces of the optical fibres being silvered, in contact with a silvered reflecting surface, or totally internally reflecting.
23. A device according to claim 22 substantially as described and illustrated in the accompanying Drawings.
24. A device according to claim 22 or 23 substantially as hereinbefore described.