Photoplethysmogram system and method

A photoplethysmogram system, comprising:

• at least three light sources of different wavelengths configured to emit light into a subject's skin, the light sources including at least a red light source, an infrared light source, and a green light source;
• a PPG sensor configured to detect the light from the multiple light sources after it has passed through the subject's skin and to generate, for each light source, a respective PPG signal;
• and
• a processor communicatively coupled to the PPG sensor;
• wherein the processor is configured to:
• (a) for each PPG signal, estimate a first respiratory-effort signal from the PPG signal;
• (b) for each time sample, determine a signal-quality value for each of the PPG signals based on a consistency of the PPG signals across the different wavelengths;
• (c) maintain a cumulative poor-quality indicator, and
• when all of the signal-quality values, or a majority of the signal-quality values, are below a poor-quality threshold, or when any one of the signal-quality values is below the poor-quality threshold and the cumulative poor-quality indicator exceeds a predetermined percentage threshold, amputate the current time sample from the PPG signals and increase the cumulative poor-quality indicator;
• and
• otherwise, when the signal-quality value of a particular PPG signal is below the poor-quality threshold, impute a value of the particular PPG signal at the current time sample based on a regression-based lookup table that uses one or more of the other PPG signals as input;
• (d) after the amputation or imputation, re-estimate, for each PPG signal, a second respiratory-effort signal;
• (e) for each PPG signal, identify segments of the second respiratory-effort signal that satisfy a quality criterion and generate a respiratory-effort event list for the PPG signal;
• and
• (f) apply a fusion algorithm to the second respiratory-effort signals from the different wavelengths to construct a final respiratory waveform, the fusion algorithm assigning different weights to the second respiratory-effort signals according to wavelength-dependent penetration depth in tissue, and determine final respiratory-effort events from the respiratory-effort event lists.