GB2316171A

GB2316171A - Pulse Oximeter

Info

Publication number: GB2316171A
Application number: GB9715525A
Authority: GB
Inventors: Michael William Brampton
Original assignee: THAMES MEDICAL Ltd
Current assignee: THAMES MEDICAL Ltd
Priority date: 1996-07-29
Filing date: 1997-07-24
Publication date: 1998-02-18
Anticipated expiration: 2017-07-24
Also published as: GB9715525D0; GB9615895D0; GB2316171B; GB2316171A8

Abstract

An oximeter probe comprises an elongate rod of resilient material longitudinally divided in its central region by a slot or slit into first and second central portions joined at their respective ends, the first central portion having embedded therein a light source 5 and a light detector 6 facing the second central portion, and the second central portion having a reflective surface 7 facing the first central portion, the arrangement being such that light is directed from the light source to the reflective surface and is thence reflected into the light detector, passing twice through the tissue of a part of the body such as the tongue T inserted into the slot or slit. The source may comprise two LED's 5a and 5b emitting red and/or infrared light.

Description

PULSE OXIMETER The present invention relates to oximeters, and particularly concerns medical or veterinary oximeters which can measure blood oxygen content and pulse rate without causing trauma or discomfort to a patient.

Conventional pulse oximeter probes are constructed for medical use by providing a spring clamp which engages the patient's finger end, a source of light and a light detector being pressed against opposite sides of the finger by the spring. Light emitted by the source passes through the patient's flesh, and the pulsation of blood in the patient's blood vessels causes the amount of light transmitted to vary in synchronism with the pulse. By detecting the amount of light transmitted and measuring the variation with time, the patient's pulse rate can be calculated.

By further processing the received signal, using light of a predetermined wavelength, the amount of light absorbed during its passage through the patient's tissue can be detected and processed to calculate the amount of oxygen in the blood.

Conventionally constructed probes, as well as being difficult to clean due to their multi-component construction, have the disadvantage that they are susceptible to interference from external light sources, and for example in outdoor conditions of bright sunlight the detector is unable to discriminate between the light from the light source and ambient light, thus preventing any calculation of pulse rate or blood oxygenation.

The present invention seeks to provide an oximeter probe which can be used to measure blood oxygen levels and is usable in all ambient light conditions, and is simple to use and easy to sterilise between uses.

According to a first aspect of the present invention, an oximeter probe comprises an elongate rod of resilient material, longitudinally divided in its central region into first and second central portions joined at their respective ends, the first central portion having embedded therein a light source and a light detector facing the second central portion, and the second central portion having a reflective surface facing the first central portion, the arrangement being such that light is directed from the light source to the reflective surface and is thence reflected into the light detector.

In an advantageous embodiment the central portion is divided by a longitudinally extending slit, preferably formed at its ends with enlarged stress-relieving cutouts.

Most preferably the rod is divided by a central slot with smoothly rounded internal contours.

According to a second aspect of the invention, an oximeter probe comprises a light detector, a first light source emitting light of a first wavelength, and a second light source emitting light of a second wavelength, a probe body being arranged to support the light sources and detector so that light from both sources can traverse a patient's tissue, strike a reflector in the probe, and pass again through the patient's body before entering the detector.

According to a third aspect of the invention, there is provided a blood monitoring system comprising a probe having a light detector, a first light source emitting light of a first wavelength, and a second light source emitting light of a second wavelength, control circuitry to cause light to be sequentially emitted by the light sources into the detector, and processing circuitry to calculate, from the magnitude of the signals received, the blood oxygen level.

Preferably the probe of the blood monitoring system has a first light source emitting light with a spectrum including a first wavelength susceptible to absorption by oxygenated blood, and a second light source emitting light with a spectrum including a second wavelength susceptible to absorption by deoxygenated blood.

Embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a lingual pulse oximeter sensor; Figure 2a shows a longitudinal section of the lingual pulse oximeter of Figure 1 in the unstressed state; and Figure 2b shows a longitudinal sectional view of the lingual pulse oximeter of Figure 1 in the operating state.

Referring now to Figures 1 and 2, the probe 1 comprises a rod of resilient material, with sensor components mounted inside the elongate body of the probe 1.

The body 1 is made of a resilient, flexible, waterproof material, such as natural or synthetic rubber, and has a central longitudinally extending slot 2 with stress-reducing enlargements 3 at both ends. The material of the probe is preferably opaque, a preferred colour being white, but the material may be transparent or translucent.

The slot 2 divides the central part of the probe body into first and second central portions la and lb, which in the unstressed state are parallel to each other.

Figure 2a shows a section along the main body 1, with a light source 5 and the detector or receiver 6 mounted in an optically clear window 4. The window 4 is made of an optically clear material, preferably silicone rubber, the material being equally inert and having a flexibility similar to the body of the probe 1. The electrical components 5 and 6 are thus completely encapsulated. Electrical connections are made by wires passing through the body of the probe and exiting the probe at one of its ends.

The light source 5 preferably comprises two light emitting diodes, each having a known emission wavelength.

The light emitted is preferably red and/or infra-red with a first LED having an emission wavelength between about 600nm and 700nm, preferably about 660nm. This light is strongly absorbed by oxygenated blood, but is absorbed to a much lesser degree by deoxygenated blood. The second LED has an emission wavelength between about 850nm and 950nm, preferably about 920nm. This light is strongly absorbed by deoxygenated blood, but is absorbed to a much lesser degree by oxygenated blood.

The preferred method of producing the probe is to place the light source 5, detector 6, windows 4 and leads in a mould cavity and injection mould the body 1 to embed the other components therein.

Figure 2a shows the light source 5 and receiver 6 spaced longitudinally in the first central portion la.

Alternatively, the light source 5 and receiver 6 may be positioned side by side. The light source 5 and receiver 6 are in direct opposition to a reflective surface 7 on the second central portion lb. The reflective surface may be a silvered mirror either on the surface of the second central portion lb or mounted beneath a transparent window 4, or may be a gloss white surface finish on the second central portion lb.

In a preferred embodiment, not illustrated, a probe having a clear window surrounding the light detector is provided with opaque masking regions or plates which prevent light from entering the detector except from the light emitter or emitters. It is envisaged that the masking region may be constituted by the body of the probe being made opaque by the incorporation of opaque fillers in the elastomeric body material, or alternatively opaque plates may be incorporated into the probe during the moulding stage of production. The accuracy and sensitivity of the device are enhanced by the reduction or elimination of ambient light entering the detector.

Figure 2b shows the probe in use as a lingual probe, with the tongue of the patient (human or animal) inserted in the slot 2. The first and second central portions la and lb are deformed outwards into arcuate shapes by the presence of the tongue T between them.

Arrows A indicate the passage of the light from the light source 5 across the slot 2 and back again. If the distance between the light source 5 and receiver 6 is around 10mum then the light passage through the tissue to be measured (in this case the tongue) will be that of a semi-circular shape and not a straight line.

The tongue is placed in the slot in such a position that light is transmitted through the lingual arteries.

The spacing of the light sources from the receiver may be arranged to suit the dimensions of the tongue of a particular species, by moving the light source and receiver in relation to the mid-line to facilitate the light's passage through the lingual arteries.

Figures 2a and 2b show the light sources 5 being positioned equal distances either side of the receiver 6, but asymmetric arrangements may be preferred in some situations. For example, both light sources may be to one side of the detector.

By utilizing a light source Sa having a spectrum including a first wavelength responsive to oxygenated blood, and a second light source Sb having a spectrum including a second wavelength responsive to deoxygenated blood, the monitor will be able to measure the blood oxygen level.

It is envisaged that the light sources of the probe will be controlled by control circuitry to emit light in pulses, for example by emitting a pulse from light source Sa followed by a period of darkness, then a pulse from light source Sb followed by a second period of darkness, and finally simultaneous pulses from both light sources Sa and 5b followed by a third period of darkness.

This cycle could then be repeated, with the greater magnitude of signal from the combined pulse serving to mark the receiver output, enabling the individual pulses to be identified as being either the first or second pulse following the combined pulse. Preferably the frequency of the pulses will greatly exceed any pulse rates likely to be encountered by the probe, and will be in the kiloHerz range.

Detection circuitry in association with the receiver 6 will then analyse the signals from the receiver 6. By measuring the cyclic variation in the overall signal strength, the pulse rate may be calculated. Analysis of the relative absorption of light of the two wavelengths will allow a calculation of the relative amounts of oxygenated and deoxygenated blood to be made, and a determination of the oxygenation level of the blood can then be made.

Claims

1. An oximeter probe comprising; a light detector; a first light source emitting light of a first wavelength; a second light source emitting light of a second wavelength; and a reflective surface; the probe having a body engageable with a patient so as to support the light sources and detector so that light from both sources can traverse a patient's tissue, strike the reflective suface, pass again through the patient's body, and enter the detector.

2. An oximeter probe comprising an elongate rod of resilient material, longitudinally divided in its central region into first and second central portions joined at their respective ends, the first central portion having embedded therein a first light source a second light source and a light detector all facing the second central portion, and the second central portion having a reflective surface facing the first central portion, the arrangement being such that light is directed from the light sources to the reflective surface and is thence reflected into the light detector.

3. A probe according to claim 2, wherein the central portion is divided by a longitudinally extending slit.

4. A probe according to claim 3, wherein the slit is formed at its ends with enlarged stress-relieving cutouts.

5. A probe according to claim 2, wherein the rod is divided by a central slot.

6. A probe according to claim 5, wherein the slot is formed at its ends with enlarged stress-relieving cutouts.

7. A probe according to claim 4 or claim 6, wherein the enlarged stress-relieving cutouts are formed with smoothly rounded internal contours.

8. A probe according to any preceding claim, wherein the reflective surface is flexible, and is curved into a concave configuration to focus light into the detector when positioned for use on a patient.

9. A probe according to any preceding claim, wherein the reflective surface is a silvered mirror on the surface of the second central portion.

10. A probe according to any of claims 1 to 8, wherein the reflective surface is a silvered mirror mounted beneath a transparent window.

11. A probe according to any of claims 1 to 8, wherein the reflective surface is a gloss surface finish on the second central portion.

12. A probe according to any preceding claim, wherein the first light source emits light of a wavelength of between 600nm and 700nm, and the second light source emits light of a wavelength of between 850nm and 950nm.

13. A blood monitoring system comprising; a probe having a light detector, a first light source emitting light of a first wavelength, a second light source emitting light of a second wavelength, and a reflective surface; the probe having a body engageable with a patient so as to support the light sources and detector so that light from both sources can traverse a patient's tissue, strike the reflective surface, pass again through the patient's body, and enter the detector; control circuitry to cause light to be sequentially emitted by the first and second light sources into the detector; processing circuitry to calculate, from the magnitude of the signals received at the detector, the blood oxygen level of the patient; and display means to display an indication of the blood oxygen level.

14. A blood monitoring system according to claim 13, wherein the first light source emits light of a wavelength of between 600nm and 700nm, and the second light source emits light of a wavelength of between 850nm and 950nm.

15. An oximeter probe substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

16. A blood monitoring system comprising an oximeter probe substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.