This invention relates to an optical monitoring or measuring device with motion artefact suppression.

Medical instruments are being developed which use non- invasive optical techniques. As is well known in the art, these instruments suffer interference due to patient movement, motion artefact.

For example it is known, in order to measure blood oxygen saturation, to provide a device which passes light through the human finger and monitors the output signal of a photodetector of this device continuously. Movement of the subject leads to changes in the light path and hence to variations in the intensity of light received by the photodetector. This renders the device incapable of distinguishing between changes in received light intensity caused by variations in light absorption by the component being monitored (e.g. oxygen in the blood) , and changes in received light intensity caused by variations in the light path due to movement of the subject.

Motion artefact is a significant problem in pulse oximeters, and can render these devices inoperative for up to

40% of the monitoring period in certain clinical applications. The problem is common to all optical monitoring devices and is particularly severe in critical health care applications, where continuous monitoring is essential. We have now devised an optical measuring or monitoring device which is able to suppress the effects of motion artefact.

In accordance with this invention, there is provided an optical measuring or monitoring device which comprises light source means for emitting one or more light beams which includes light of a plurality of different wavelengths, photodetector means for receiving said light beam or beams after passing through or being reflected within living tissue and arranged to provide signals corresponding to the intensities of the respective wavelengths received by the photodetector means, the arrangement being such that the quiescent (or "DC") signal levels corresponding to the

different wavelengths bear a predetermined relationship with each other, and signal processing means for processing the output signals from the photodetector means to cancel out variations due to motion artefact and to provide an output representing a parameter to be measured or monitored and substantially unaffected by motion artefact.

For example, in a pulse oximetry device, light of two or more wavelengths is passed through or reflected within living tissue. At each wavelength, there is a fixed "DC" intensity of light received by the photodetector, with a pulsatile "AC" component (caused by arterial pulsation) superimposed thereon.

Theoretically, and as confirmed by experiment, variations in signal output due to motion artefact (changes in source-to-tissue and photodetector-to-tissue coupling) are proportional to the respective "DC" signal level. Thus, if the DC intensity of light at the photodetector were doubled, for instance by doubling the output of the light source, a given degree of mechanical disturbance to the device would produce double the motion artefact variation in the detector output signal. Thus if the DC signals are equal at the different wavelengths, the amplitude of the motion artefact signal components will be the same for all those wavelengths.

Normally a pulse oximeter uses two wavelengths and the signal processing unit processes the signals from the photodetector, corresponding to these two wavelengths, to provide a measure of pulse rate or blood oxygen saturation. In accordance with this invention, such a pulse oximeter uses three wavelengths: the signal processing unit processes the three signals from the photodetector firstly to cancel out the motion artefact and then to determine the pulse rate and/or blood oxygen saturation.

In general, where a device normally uses n different wavelengths of light and processes the n corresponding output signals from the photodetector to determine the value of the parameter being measured or monitored, in accordance with this invention the device will use an additional wavelength (n + 1) wavelengths altogether, so that the motion artefact can be cancelled out.

Preferably high accuracy readings are made by the controller of each of the three sources via a single sensor and a single hardware conversion block. From these readings, three difference signals are produced. Because these (AC) difference signals have reduced motion artefact signal and directly reflect the different absorption levels in the tissue, they can be used in combination as indicators of the oxygen saturation level.

Preferably additional real time digital filtering of the signals is used to reduce 50/60Hz components and to correlate input signals to improve noise rejection.

The input channels may be continuously monitored by the processor to ensure that the effects of component aging or temperature drifting are eliminated. Preferably most of the signal processing is performed in software so the costs of the unit is low and there is no requirement for very accurate hardware matching.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, the single figure of which is a diagrammatic block diagram of a device in accordance with this invention.

Referring to the drawing, there is shown a pulse oximetry and blood oxygen saturation monitoring device. The device comprises three LED's 1,2,3 emitting different wavelengths of light for transmission through or reflection from within living tissue, indicated schematically at T. The device further comprises photodetectors 11,12,13 for receiving each of the transmitted or reflected light beams. The device includes a control means 20, arranged to adjust the power applied to the LED's, to give a "DC" signal level, in the outputs from the photodetectors 11,12,13, which is equal for all three wavelengths: this can be achieved very accurately. Then:

the OAC's are the observed AC signals from the photodetector, the AC'S are the true AC signals, and MA is the motion artefact.

The device further includes a signal processor 30, in which:

OAC λl is subtracted from OAC λ2, giving ACλl - ACλ2; and

OAC λ2 is subtracted from OAC λ3, giving ACλ2 - ACλ3

In this manner, the motion artefact signals are subtracted out. There is a known relationship between the values of ACλl and ACλ2 and ACλ3 of the general form :

ACλl + Kl = K4. ACλ2 + K2 Equation 1

ACλ2 + K2 ACλ3 + K3

in which the K's are known constants for the three wavelengths used. The signal processing unit then extracts the original AC components in accordance with the following:

Let K5 = OACλl - 0ACλ2 = ACλl - ACλ2 Equation 2

Let K6 = OACλ2 - 0ACλ3 = ACλ2 - ACλ3 Equation 3

Let Al, A2 and A3 equal ACλl, ACλ2 and ACλ3 ^~

From equations 2 and 3 : Al = K5 + A2 Equation 4 and A3 = A2 - K6 Equation 5

Replacing Al and A3 in equation 1 using Equations 4 and 5 :

K5 + A2 + Kl = K4. A2 + K2 Equation 6

A2 + K2 A2 - K6 + K3

Cross-multiplying Equation 6 :

(K5 + A2 + Kl) . (A2 - K6 + K3) = K4 (A2 + K2) ² Equation 7

Multiplying out the first pair of brackets in Equation 7 :

A ² + (K5 + Kl - K6 + K3) A2 + [ (K5 + Kl) (K3 - K6) ] = K4 A2 ² + 2K4. K2. A2 + K4. K2 ² Equation 8

Rearranging Equation 8 to quadratic form :

(K4 -1). A2 ² + (2K4. K2. + K6 - K3 - K5 - Kl) A2 +

K4. K2 ² - [(K5 + Kl) (K3 - K6) ] = 0 Equation 9

Let the first term in Equation 9 = a Let the second term in Equation 9 = b Let the third term in Equation 9 = c

A2 = ACλ2 = -b ±Vfb ² - 4ac)

2a and from Equations 4 and 5 : ACλl = K5 + ACλ2 and ACλ3 = ACλ2 - K6

Since Kl, K2, K3 and K4 are fixed and known for the three wavelengths in use and K5 and K6 are measured, the processing unit is able to recover the three original "AC" components. It may readily be shown that, where F is a simple function :

ACλl

%SA02 = F. DCλl

ACλ2 DCλ2 for a two wavelength oximetry system.

For a three wavelength system, this modifies by direct analogy to :

ACλl ACλ2 %Sa02 = F DCλl +K4. DCλ2

ACλ2 ACλ3

DCλ2 DCλ3 wherein K4 has the same value as previously.

This effectively takes the mean of two wavelength pairs to obtain the blood oxygen saturation % Sa 02.

Since the DC levels are the same in this case, the formula simplifies to :

ACλl ACλ2

%Sa02 = F. ACλ2 + K4. ACλ3

With the "DC" level equalisation in the three light channels and the motion artefact signals therefore identical, it is theoretically possible to null out the motion artefact entirely. In practice, under many circumstances, the motion artefact signals differ from each other by 5 to 20% implying a degree of rejection of between 20 and 5 times. How close this figure is to 0% depends on the severity of the physical disturbance causing the artefact, and probably on the sensor design. The light source at all wavelengths are point sourced, that is they originate as far as their path through the medium is concerned at a single point.

The configuration of the light sources is constrained by the physical dimensions of the sources, i.e. they are as close as is physically possible. Alternatively, a light guide method may be used to make all light sources originate from a single point.