GB2382406A

GB2382406A - Analytical apparatus

Info

Publication number: GB2382406A
Application number: GB0118966A
Authority: GB
Inventors: Geoffrey Richard Mathews; Veronica Mary Mathews
Original assignee: Electrode Co Ltd
Current assignee: Electrode Co Ltd
Priority date: 2001-08-02
Filing date: 2001-08-02
Publication date: 2003-05-28
Anticipated expiration: 2021-08-02
Also published as: WO2003011127A1; GB2382406B; GB0118966D0

Abstract

Apparatus for use in pulse oximetry comprises a pair of light emitting diodes 2, 4 (LEDs) capable of transmitting light through a test medium at two different frequencies, such as one in the each of the red and infrared parts of the spectrum, and a photodetector 8. The photodetector provides as an output the attenuation of light at each of the frequencies and a processing means derives a ratio of the outputs. The processing means contains a memory of experimentally and/or theoretically obtained data corresponding to all possible frequencies within a range. When the test medium is a human finger 6, data relating to attenuation ratios of the two frequencies in question gives an indication of the level of oxygen in the blood. A spectrometer is used to identify the correct frequencies.

Description

ANALYTICAL APPARATUS

Pulse oximetry is a widely used technique for determining the level of oxygen saturation of a subjects blood. The technique involves measuring, at two or more pre 5 determined frequencies, the level of attenuation of light transmitted through or reflected from a human or animal body part (the transmission technique typically being applied to an ear lobe or finger) and comparing those measurements with pre-

stored, experimentally and/or theoretically derived reference 10 data to provide an estimate of the level of oxygen saturation (SaO2) of the subject's blood.

A typical pulse oximetry apparatus comprises a monitor and a sensor, the sensor comprising a pair of light emitters, such as a pair of light emitting diodes (LEDs), for 15 transmitting light, at red and infra-red frequencies respectively, through a body part.

However, a serious problem arises where the frequency of light emitted from one or other of the light emitters deviates from its expected value, so that the attenuation 20 measurements no longer correspond with the experimentally and/or theoretically derived reference data. This problem becomes particularly acute at the low values of blood oxygen saturation, where the accuracy of estimates are most critical.

A deviation of the frequency of light emitted by a 25 light emitter from its expected value might, for example, result from an incorrect light emitter having been selected during the assembly of the sensor, from manufacturing tolerances in the formation of the light emitter, from the complete failure of the light emitter, from a gradual 30 degradation in the performance of the light emitter over a period of time, or from a change in the ambient conditions (such as the temperature) under which the sensor is operated.

It has been proposed to overcome this limitation of existing apparatus by providing means for adjusting the drive

current supplied to each light emitter (and thus the frequency of its emitted light) to achieve a desired emission frequency for that emitter.

However, such an arrangement is only of limited use, as 5 firstly, only a small degree of correction can be achieved by varying the drive current, before other performance characteristics of a light emitter are effected. Secondly, the degree of correction achieved might not be that predicted theoretically. Thirdly, the correction is usually applied 10 during the manufacture of an apparatus, which is therefore still vulnerable to subsequent changes in the emission frequency of a light emitter thereof.

It is a first object of the present invention to provide an arrangement which obviates the requirement for 15 varying the drive current supplied to a light emitter of an analytical apparatus, to regulate the frequency of its emitted light. It is a second object of the present invention to provide an analytical apparatus wherein a single light emitter 20 may be used to obtain light attenuation measurements over a range of frequencies.

In accordance with the present invention, there is provided an analytical apparatus comprising: a light source for transmitting light through a test 25 medium at at least two different frequencies; means for calculating a ratio of the respective levels of attenuation of the light transmitted through the test medium at each of said two frequencies; a memory storing, for different combinations of 30 transmission frequencies, experimentally and/or theoretically derived reference data corresponding to different attenuation ratios at those frequencies; and photosensitive means, preferably a spectrometer, for measuring at least one parameter of the light emitted by the 35 light source to obtain appropriate data from the memory.

In a first preferred embodiment of the present invention, the photosensitive means are used to identify said two frequencies from respective peaks in the spectrum of the light emitted by the light source, to obtain, from the memory, 5 stored data corresponding to the calculated attenuation ratios at the two frequencies identified.

Thus, the apparatus does not require means for varying the drive current applied to the light source to adjust the frequency of the light emitted therefrom.

10 In this case, the level of attenuation of the light emitted at each of said two frequencies may be measured either by the photosensitive means or by some further photosensitive means, such as one or more photodiodes. In the former case, the photosensitive means may be arranged to receive light from 15 the light source, either before or after that light has been transmitted through the test medium.

The light source preferably comprises a pair of light emitters, such as a pair of light emitting diodes, arranged to emit light at the first and the second of said two frequencies 20 respectively.

In a second preferred embodiment of the present invention, the light source is arranged to emit a range of frequencies of light, and the photosensitive means are used to measure the level of attenuation of the light emitted at each 25 of two chosen frequencies within that range, to obtain, from the memory, stored data corresponding to the calculated attenuation ratios at each of the chosen frequencies.

In this case, the light source may comprise a plurality of light emitters, such as a pair of light emitting diodes, 30 arranged to emit light at the first and the second of said two frequencies respectively.

Alternatively, a single light emitter, e.g. a white light emitter, may be used to obtain attenuation measurements within a range of frequencies.

35 Preferably the test medium comprises a human or animal

body part and the apparatus is arranged such that the attenuation ratio is calculated from attenuation measurements taken over a period of time (to take into account the pulsatile nature of the subject's oxygenated arterial blood 5 flow), the stored data preferably comprising a set of experimentally and/or theoretically derived values of the level of oxygenation of the subject's blood.

Conventional pulse oximetry apparatus assume a substantially uniform rate of venous blood flow when deriving 10 an estimate of blood oxygenation. However, it can be shown that, in certain circumstances, venous blood flow can have a pulsatile component which can effect the accuracy of the blood oxygenation estimate.

In the apparatus according to the present invention, 15 the undesirable influence of noise factors, such as pulsatile or irregular venous blood flow, on the analytical accuracy of the apparatus is preferably reduced by measuring the level of light attenuation at at least one reference frequency, in addition to said two frequencies.

20 It will be appreciated that attenuation measurements may also or otherwise be taken at more than said two frequencies, to simultaneously identify and/or quantify a plurality of constituents of the test medium which, with a subject's blood as the test medium, might include 25 carboxyhaemoglobin, bilirubin, methaemoglobin or sickle-cell haemoglobin. Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which: 30 Figure l is a schematic view of a first embodiment of analytical apparatus in accordance with the present invention; Figure 2 is a graph showing the relationship between the level of oxygen saturation of a subject's blood and a ratio of the levels of attenuation of light transmitted 35 through a body part from red and infra-red light emitters

respectively. Figure 3 is a schematic view of a second embodiment of analytical apparatus in accordance with the present invention; Figure 4 is a schematic view of a third embodiment of 5 analytical apparatus in accordance with the present invention; and Figure 5 is a schematic view of a fourth embodiment of analytical apparatus in accordance with the present invention.

Referring to Figure 1, an analytical apparatus is shown 10 comprising a pair of light emitting diodes (LEDs) 2,4 arranged to transmit red and infra-red light respectively, through a subject's finger 6, to a photodetector (PD) '3.

The PD 8 provides, as output, respective measurements corresponding to the levels of attenuation of the red and 15 infra-red light through the finger 6.

Processing means (not shown) derive a ratio of the two attenuation measurements and obtain from a memory a pre-

stored, experimentally and/or theoretically derived estimate of the level of oxygen saturation (SaO2) of the subject's blood 20 for that particular attenuation ratio.

In conventional apparatus, reference data is stored for only a single pair of emission frequencies. For example, Figure 2 shows a set of standard reference data, in the form of a so-called it-curve, relating blood oxygen content to the 25 attenuation of red and infra-red light at frequencies of 665nm and 900nm respectively.

It will be appreciated that in a conventional apparatus using reference data for only a single pair of emission frequencies, any deviation in the frequency of light emitted 30 by either the red or the infra-red LED 2,4 will result in an incorrect estimation of the subject's blood oxygenation level.

The apparatus of Figure 1 overcomes this problem by storing respective itcurves for a large variety of combinations of red and infra-red frequencies and by providing 35 a miniature spectrometer 10 for analyzing the spectrum of the

light transmitted by the two LEDs 2,4, to select an appropriate it-curve for those LEDs.

In the apparatus of Figure 1, a portion of the light emitted by the LEDs 2,4 is collected prior to being 5 transmitted through the subject's finger 6 and channeled to the spectrometer 10 by a fibre-optic light guide 12.

The apparatus of Figure 3 operates in substantially the same manner as that of Figure 1, except that the fibre-optic light guide 12 is instead arranged to collect a portion of the 10 light which has already passed through the subject's finger 6, thereby taking into account any scattering of the light which might occur as the light travels through the finger.

In the apparatus of Figure 4, the spectrometer 10 serves both to select an appropriate it-curve (according to the 15 respective emission frequencies of the two LEDs 2,4) and to provide measurements of the respective levels of attenuation at each of those frequencies, for obtaining an appropriate estimate of blood oxygenation from the selected curve.

In the apparatus of Figure 5, the red and infra-red 20 LEDs 2,4 are replaced by a single, large-bandwidth light-

emitter 14 and the spectrometer 10 serves to provide attenuation measurements at any two chosen frequencies within that bandwidth, which measurements may then be used to obtain a blood oxygenation estimate from an it-curve stored in memory 25 for the chosen frequencies.

The analytical apparatus thus described do not require any complicated feedback means for controlling the frequencies of light emitted by their light emitters and can each be used to obtain light attenuation measurements over a range of 30 frequencies using one or more light emitter(s).

Claims

Claims

1) An analytical apparatus comprising: a light source for transmitting light through a test medium at at least two different frequencies; 5 means for calculating a ratio of the respective levels of attenuation of the light transmitted through the test medium at each of said two frequencies; a memory storing, for different combinations of transmission frequencies, experimentally and/or theoretically 10 derived reference data corresponding to different attenuation ratios at those frequencies; and photosensitive means, for measuring at least one parameter of the light emitted by the light source to obtain appropriate data from the memory.

15 2) An analytical apparatus as claimed in Claim 1, wherein the photosensitive means are used to identify said two frequencies from respective peaks in the spectrum of the light emitted by the light source, to obtain, from the memory, stored data corresponding to the calculated attenuation ratios at the 20 two frequencies identified.

3) An analytical apparatus as claimed in Claim 2, wherein the level of attenuation of the light emitted at each of said two frequencies is measured by the photosensitive means.

4) An analytical apparatus as claimed in Claim 3, wherein 25 the photosensitive means are arranged to receive light from the light source before that light has been transmitted through the test medium.

5) An analytical apparatus as claimed in Claim 3, wherein the photosensitive means are arranged to receive light from the 30 light source after that light has been transmitted through the

test medium.

6) An analytical apparatus as claimed in Claim 2, wherein the level of attenuation of the light emitted at each of said two frequencies is measured by further photosensitive means.

5 7) An analytical apparatus as claimed in Claim 6, wherein said further photosensitive means comprise one or more photodiodes 8) An analytical apparatus as claimed in any of Claims 2 to 7, wherein the light source comprises a pair of light 10 emitters, arranged to emit light at the first and the second of said two frequencies respectively.

9) An analytical apparatus as claimed in Claim 8, wherein said light emitters comprise a pair of light emitting diodes.

10) An analytical apparatus as claimed in Claim 1, wherein 15 the light source is arranged to emit a range of frequencies of light, and the photosensitive means are used to measure the level of attenuation of the light emitted at each of two chosen frequencies within that range, to obtain, from the memory, stored data corresponding to the calculated attenuation ratios 20 at each of the chosen frequencies.

11) An analytical apparatus as claimed in Claim 10, wherein the light source comprises a plurality of light emitters, arranged to emit light at the first and the second of said two frequencies respectively.

25 12) An analytical apparatus as claimed in Claim 11, wherein said light emitters comprise a pair of light emitting diodes.

13) An analytical apparatus as claimed in Claim 10, wherein

a single light emitter is used to obtain attenuation measurements within a range of frequencies.

14) An analytical apparatus as claimed in Claim 13, wherein said single light emitter comprises a white light emitter.

5 15) An analytical apparatus as claimed in any preceding claim, wherein the test medium comprises a human or animal body part and the apparatus is arranged such that the attenuation ratio is calculated from attenuation measurements taken over a period of time (to take into account the pulsatile nature of 10 the subject's oxygenated arterial blood flow), the stored data preferably comprising a set of experimentally and/or theoretically derived values of the level of oxygenation of the subject's blood.

15 16) An analytical apparatus as claimed in any preceding claim! wherein the undesirable influence of noise factors on the analytical accuracy of the apparatus is reduced, by measuring the level of light attenuation at at least one reference frequency, in addition to said two frequencies.

20 17) An analytical apparatus as claimed in any preceding claim, wherein attenuation measurements are taken at more than said two frequencies, to simultaneously identify and/or quantify a plurality of constituents of the test medium.

18) An analytical apparatus as claimed in any preceding 25 claim, wherein said photosensitive means comprise a spectrometer. 19) An analytical apparatus substantially as herein described, with reference to the accompanying drawings.

,