Abstract
In this study, we proposed a novel method for extracting the instantaneous respiratory rate (IRR) from the pulse oximeter photoplethysmogram (PPG). The method was performed in three main steps: (1) a time-frequency transform called synchrosqueezing transform (SST) was used to extract the respiratory-induced intensity, amplitude and frequency variation signals from PPG, (2) the second SST was applied to each extracted respiratory-induced variation signal to estimate the corresponding IRR, and (3) the proposed peak-conditioned fusion method then combined the IRR estimates to calculate the final IRR. We validated the implemented method with capnography and nasal/oral airflow as the reference RR using the limits of agreement (LOA) approach. Compared to simple fusion and single respiratory-induced variation estimations, peak-conditioned fusion shows better performance. It provided a bias of 0.28 bpm with the 95% LOAs ranging from −3.62 to 4.17, validated against capnography and a bias of 0.04 bpm with the 95% LOAs ranging from −5.74 to 5.82, validated against nasal/oral airflow. This algorithm would expand the functionality of a conventional pulse oximetry beyond the measurement of heart rate and oxygen saturation to measure the respiratory rate continuously and instantly.
Highlights
Respiratory rate (RR), along with other vital signs like heart rate (HR) and blood pressure, is monitored for primary or continuous assessment of patient wellness
We have presented an algorithm for robust estimation of instantaneous respiratory rate (IRR) from PPG with the aim of developing a portable solution based on pulse oximetry, suitable for both continuous monitoring and spot-check applications
We have proposed a novel method for extracting the instantaneous respiratory rate (IRR) from PPG
Summary
Respiratory rate (RR), along with other vital signs like heart rate (HR) and blood pressure, is monitored for primary or continuous assessment of patient wellness. Fieselmann et al analyzed the measurements of vital signs during the 72 h prior to cardiac arrest and showed that a high respiratory rate (>27 breaths/min) was a significant predictor of cardiac arrest in hospitals (Fieselmann et al, 1993). Subbe et al showed that relative changes in respiratory rate are much more significant than changes in HR or systolic blood pressure in unstable patients and the respiratory rate is more likely to be a better predictor for identifying the patient at risk (Subbe et al, 2003). A pulse oximeter measures the blood volume changes or PPG, based on the light absorption characteristics of the blood at the measuring site on the body (e.g., finger, forehead, and earlobe). The PPG signal generated in the photo-detector, has a pulsatile waveform (AC) whose peaks and troughs reflect light transmitted through the tissue when blood volume is minimal and maximal, respectively. Respiration may induce variation in PPG in three different ways (Meredith et al, 2012) (Figure 1):
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