Compensation of human variability in pulse oximetry
First Claim
1. A method for calibrating an apparatus intended for non-invasively determining an amount of at least two light absorbing substances in the blood of a subject and being provided with emitter means for emitting radiation at a minimum of three different wavelengths, the method comprising the steps of:
- (a) according to the Lambert-Beer model, determining a theoretical measurement signal for each wavelength used in the apparatus, (b) according to the Lambert-Beer model, determining an invariant which is a quotient of two pseudo-isobestic signals, each pseudo-isobestic signal being a weighted sum of two measurement signals, the weighted sum being theoretically independent of the relative concentrations of said substances in the blood of the subject, (c) calculating a first value for said invariant by means of said theoretical measurement signals, (d) performing an in-vivo measurement on a living tissue with the apparatus, whereby in-vivo measurement signals are obtained for each wavelength, (e) applying a transformation on said in-vivo measurement signals for transforming said signals to the Lambert-Beer model, whereby transformed in-vivo measurement signals are obtained, (f) calculating a second value for said invariant, the second value being calculated similarly to the first value, except for the replacement of said theoretical measurement signals by the transformed in-vivo measurement signals, (g) comparing the second value with the first value of the invariant, and (h) calibrating the apparatus on the basis of a difference between said first value and said second value.
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Abstract
The invention relates to the calibration of a pulse oximeter intended for non-invasively determining the amount of at least two light absorbing substances in the blood of a subject. In order to take human variability into account, the calibration is based on an invariant which is a quotient of two pseudo-isobestic signals. Each pseudo-isobestic signal is a weighted sum of two signals, and the weighted sum is theoretically independent of the relative concentrations of said substances in the blood of the subject. By using theoretical values of the invariant on the one hand, and values based on in-vivo measurements on the other hand, the calibration curve of the pulse oximeter is adapted to the characteristics of each individual patient.
267 Citations
16 Claims
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1. A method for calibrating an apparatus intended for non-invasively determining an amount of at least two light absorbing substances in the blood of a subject and being provided with emitter means for emitting radiation at a minimum of three different wavelengths, the method comprising the steps of:
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(a) according to the Lambert-Beer model, determining a theoretical measurement signal for each wavelength used in the apparatus, (b) according to the Lambert-Beer model, determining an invariant which is a quotient of two pseudo-isobestic signals, each pseudo-isobestic signal being a weighted sum of two measurement signals, the weighted sum being theoretically independent of the relative concentrations of said substances in the blood of the subject, (c) calculating a first value for said invariant by means of said theoretical measurement signals, (d) performing an in-vivo measurement on a living tissue with the apparatus, whereby in-vivo measurement signals are obtained for each wavelength, (e) applying a transformation on said in-vivo measurement signals for transforming said signals to the Lambert-Beer model, whereby transformed in-vivo measurement signals are obtained, (f) calculating a second value for said invariant, the second value being calculated similarly to the first value, except for the replacement of said theoretical measurement signals by the transformed in-vivo measurement signals, (g) comparing the second value with the first value of the invariant, and (h) calibrating the apparatus on the basis of a difference between said first value and said second value. - View Dependent Claims (2, 3, 4, 5, 6, 7)
determining a new transformation which is such that a value calculated for said invariant as the first value, except for the replacement of said theoretical measurement signals by transformed in-vivo signals obtained by applying the new transformation, corresponds to the first value with a sufficient accuracy, and calibrating the apparatus based on the new transformation.
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3. A method according to claim 1, wherein step (h) includes:
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mapping said difference to an error value indicating a divergence from a specified value of a given quantity, and calibrating the apparatus based on the error value.
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4. A method according to claim 2, further comprising the step of storing in the apparatus numeric data representing the transformation.
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5. A method according to claim 4, wherein the new transformation is determined iteratively by means of said numeric data.
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6. A method according to claim 4, further comprising the step of storing the first value of the invariant permanently in the apparatus.
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7. A method according to claim 6, wherein steps (a) to (c) are performed in the manufacturing phase of the apparatus and steps (e) to (h) for each in-vivo measurement according to step (d).
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8. An apparatus for non-invasively determining an amount of at least two light absorbing substances in the blood of a subject, the apparatus comprising:
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emitter means for emitting radiation at a minimum of three different wavelengths, detector means for receiving said radiation at each of said wavelengths and producing at least three electrical output signals, signal processing means for processing said output signals and producing a modulation signal for each wavelength, each modulation signal representing pulsating absorption caused by arterialized blood of the subject, transformation means for applying a transformation on each said modulation signal, whereby transformed modulation signals applicable in the Lambert-Beer model are obtained, calculation means, responsive to said transformation means, for determining a value for an invariant on the basis of the transformed modulation signals, the invariant being in the Lambert-Beer model a quotient of two pseudo-isobestic signals, each pseudo-isobestic signal being a weighted sum of two transformed modulation signals, the weighted sum being independent of the relative concentrations of the substances, and determining means for specifying a transformation whose application on said modulation signals yields a value for the invariant which meets a predetermined criterion. - View Dependent Claims (9, 10)
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11. An apparatus for non-invasively determining an amount of at least two light absorbing substances in the blood of a subject, the apparatus comprising:
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emitter means for emitting radiation at a minimum of three different wavelengths, detector means for receiving said radiation at each of said wavelengths and producing at least three electrical output signals, signal processing means for processing said output signals and producing a modulation signal for each wavelength, each modulation signal representing pulsating absorption caused by arterialized blood of the subject, transformation means for applying a transformation on each said modulation signal, whereby transformed modulation signals applicable in the Lambert-Beer model are obtained, calculation means, responsive to said transformation means, for determining a value for an invariant on the basis of the transformed modulation signals, the invariant being in the Lambert-Beer model a quotient of two pseudo-isobestic signals, each pseudo-isobestic signal being a weighted sum of two transformed modulation signals, the weighted sum being independent of the relative concentrations of the substances, comparing means, responsive to the calculation means, for determining the difference between said value and a specified value of the invariant, and mapping means for determining an error value based on the difference, the error value indicating a divergence from a specified value of a given quantity. - View Dependent Claims (12)
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13. A sensor for collecting measurement data for a pulse oximeter intended for non-invasively determining an amount of at least two light absorbing substances in the blood of a subject, the sensor comprising:
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emitter means for emitting radiation at a minimum of three different wavelengths, detector means for receiving said radiation at each of said wavelengths and producing at least three electrical output signals, storage means including a first set of data allowing an apparatus connected to the sensor to determine a value for an invariant which is a quotient of two pseudo-isobestic signals, each pseudo-isobestic signal being a weighted sum of two theoretical measurement signals, the weighted sum being theoretically independent of the relative concentrations of said substances in the blood of the subject. - View Dependent Claims (14, 15, 16)
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Specification