Reflectance Pulse Oximetry Signal Processing for Wearable Accuracy

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Solution Overview

Problem

Conventional pulse oximetry methods, particularly in reflectance mode, face inaccuracies due to interference from non-pulsatile tissue components, leading to unstable and erroneous oxygen saturation (SpO2) estimates, especially in wearable devices used for chronic outpatient monitoring.

Innovation Solution

A wearable device that employs a specific light reflectance model to account for non-pulsatile tissue contributions, using a calibration process to determine correction factors for accurate SpO2 readings, and incorporates band-pass filtering and autoregressive models to remove noise from photoplethysmogram signals.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If conventional transmission model signal processing is employed for reflectance oximetry, then device simplicity is maintained, but measurement precision deteriorates due to unstable and erroneous SpO2 estimates

Engineering Contradiction:

Improvesignal processing modelVSAvoidSpO2 estimation accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent changes the fundamental parameters of the signal processing model by transitioning from a transmission model to a reflectance-specific model. This involves modifying the mathematical relationships and physical assumptions to account for light reflection characteristics, thereby improving measurement precision without excessively increasing device complexity

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent substitutes the conventional transmission model (analogous to a mechanical framework) with a reflectance model better suited to the optical physics of reflected light. This substitution replaces an inappropriate theoretical framework with one that accurately represents the physical phenomenon being measured

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Ease of operation

If reflectance mode is used for wearable monitoring, then ease of operation and patient comfort are improved, but measurement precision deteriorates due to non-pulsatile tissue interference

Engineering Contradiction:

Improvewearable device usabilityVSAvoidSpO2 measurement accuracy

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent segments the optical signal into pulsatile and non-pulsatile components, allowing separate analysis and processing. By isolating the pulsatile component that contains the oxygen saturation information, the system can eliminate interference from non-pulsatile tissue while maintaining the ease of wearable operation

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent extracts the pulsatile signal component from the total optical signal, removing the interfering non-pulsatile tissue contributions. This extraction process isolates the relevant physiological information while discarding the noise, thereby improving measurement precision without sacrificing wearable convenience

Inventive Principle:

Principle #2Taking out (Extraction)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution enables reliable and accurate SpO2 measurements in reflectance mode, reducing errors caused by non-pulsatile tissue interference and environmental noise, thus enhancing the usability and accuracy of wearable pulse oximetry devices for chronic disease monitoring.

Implementation Method 1

A wearable device that employs a specific light reflectance model to account for non-pulsatile tissue contributions

Methodology Applied

Scientific EffectLight reflectance: Reflection

Implementation Method 2

SpO2 can be estimated using the Beer-Lambert Law, which relates to light absorbance due to the concentration of a substance in media

Methodology Applied

Scientific EffectBeer-Lambert Law: Absorption (EM radiation)

Implementation Method 3

Each LED can be activated separately, and accompanied by a 'dark' period where neither is on (to obtain ambient light levels). The sensor records light transmitted or reflected for each LED

Methodology Applied

Scientific EffectPhotodetection: Photoelectric Effect

Implementation Method 4

incorporates band-pass filtering and autoregressive models to remove noise from photoplethysmogram signals

Methodology Applied

Scientific EffectBand-pass filtering: Filter (electronic)

Implementation Method 5

incorporates band-pass filtering and autoregressive models to remove noise from photoplethysmogram signals

Methodology Applied

Scientific EffectAutoregressive modeling:

Data Source

PatentEP3307162B1Pulse oximetry

Publication Date: 2021.11.17 CHRONISENSE MEDICAL LTD

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Figure 3A~3B

AI summary

Provided are a method and systems for performing pulse oximetry. A light signal is emitted for a period of time and a modulated light signal is detected. The modulated light signal includes a red signal and an infrared signal. The modulated light signal is originated by an interaction of the light signal with a pulsatile tissue and a non-pulsatile tissue. The modulated light signal is processed to estimate an oxygen saturation in the pulsatile tissue during the period of time. The processing includes removing a non-pulsatile component resulting from the interaction of the light signal and the non-pulsatile tissue. The non-pulsatile component can be removed by removing a first parameter from an intensity of the infrared signal and a second parameter from an intensity of the red signal. The parameters are pre-determined using a calibration process to reproduce a true value for a ratio used to determine the oxygen saturation.