Multi-Wavelength PPG Depth-Specific Signal Extraction
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Solution Overview
Problem
Current photoplethysmography (PPG) methods using single-wavelength light fail to accurately distinguish blood volume changes in specific tissue layers, leading to inadequate measurement of cardiovascular and respiratory parameters due to the inability to separate arterial and capillary blood volume changes, which degrades the precision of Pulse Transit Time (PTT) and blood pressure tracking.
Innovation Solution
Generating depth-specific PPG signals from multi-wavelength PPG signals by selecting appropriate wavelength combinations, calibrating a multi-layer light-tissue interaction model, and using physiological signals like ECG to extract depth-specific signals that reflect arterial blood volume pulsations, thereby improving the accuracy of cardiovascular and respiratory parameter measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If single-wavelength PPG is used, then the measurement is simple and low-cost, but the measurement precision is insufficient due to inability to distinguish depth-specific blood volume changes
Solution Approach 1:
The patent segments the tissue into multiple layers (superficial capillary layer and deep arterial layer) and uses different wavelengths to probe different depths. By dividing the measurement into depth-specific components, the system can separately analyze arterial and capillary blood volume changes, thereby improving measurement precision for specific tissue layers.
Solution Approach 2:
Different wavelengths are assigned to probe different tissue depths locally. Shorter wavelengths (e.g., green light) are absorbed more by hemoglobin and probe superficial layers, while longer wavelengths (e.g., infrared) penetrate deeper to reach arterial layers. This local differentiation of measurement quality by depth enables precise depth-specific monitoring.
2Measurement precision
If single-wavelength PPG is used, then the device structure is simple, but the ability to separate arterial and capillary blood volume changes is lost
Solution Approach 1:
The patent adds the wavelength dimension to the traditional single-wavelength PPG measurement. By incorporating multiple wavelengths with different tissue penetration depths, the system transforms a one-dimensional measurement into a multi-dimensional measurement space, enabling separation of blood volume changes from different tissue layers based on their depth-specific optical absorption characteristics.
3Measurement precision
If IR PPG measures sum total of all blood vessels, then the measurement covers all vascular beds, but the pure arterial blood volume changes cannot be separated
Solution Approach 1:
The patent extracts the depth-specific arterial blood volume information from the mixed PPG signal by using multi-wavelength measurements. Through the multi-layer light-tissue interaction model, the system mathematically separates the contribution of different tissue layers, extracting pure arterial blood volume changes from the superimposed signal that contains both arterial and capillary components.
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 method provides more accurate measurements of blood pressure and Pulse Transit Time along arteries compared to conventional single-wavelength PPG methods, enhancing the performance of cardiovascular and respiratory monitoring applications.
Implementation Method 1
Different wavelengths of light are absorbed by hemoglobin in a depth-specific manner
Implementation Method 2
A PPG sensor system typically includes a light source and a light detector
Data Source
AI summary
Accurate and effective methods for measuring cardiovascular and respiratory parameters are provided. The method for deriving a depth-specific photoplethysmography (PPG) signal from multi-wavelength PPG signals includes choosing light wavelength combinations, calibrating a multi-layer light-tissue interaction model referring to a physiological signal, and generating the depth-specific PPG signal from the multi-wavelength PPG signals based on the calibrated light-tissue interaction model. The disclosed method for cuff-less blood pressure measurement includes recording a physiological signal and multi-wavelength PPG signals of a predetermined body part, deriving the depth-specific PPG signal reflecting the arterial blood volume with the physiological signal as a reference, calculating the pulse transit time (PTT) from the physiological signal and the derived arterial blood PPG signal, and calculating the blood pressure from the calibrated PTT and blood pressure relationship.