Filtering-induced time shifts in photoplethysmography pulse features measured at different body sites: the importance of filter definition and standardization

Haipeng Liu, John Allen, Syed Ghufran Khalid, Fei Chen, Dingchang Zheng

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    27 Citations (Scopus)
    1095 Downloads (Pure)

    Abstract

    Objective. The waveform of a photoplethysmography (PPG) signal depends on the measurement site and individual physiological conditions. Filtering can distort the morphology of the original PPG signal waveform and change the timing of pulse feature points on PPG signals.Weaim to quantitatively investigate the effect of PPG signal morphology (related to measurement site) and type of pulse feature on the filtering-induced time shift (TS). Approach. 60 s PPG signals were measured from six body sites (finger, wrist under (volar), wrist upper (dorsal), earlobe, and forehead) of 36 healthy adults. Using infinite impulse response digital filters which are common in PPG signal processing, PPG signals were prefiltered (band-pass, pass and stop bands:>0.5 Hz and<0.2 Hz for high-pass filter,<20 Hz and>30 Hz for low-pass filter) and then filtered (low-pass, pass and stop bands:<3Hz and>5Hz). Four pulse feature points were defined and extracted (peak, valley, maximal first derivative, and maximal second derivative). For each subject, overall TS and intra-subject TS variability in feature points were calculated as the mean and standard deviation of TS between prefiltered and filtered PPG signals in 50 cardiac cycles. Statistical testing was performed to investigate the effect of measurement site and type of pulse feature on overall TS and intra-subject TS variability. Main results. Measurement site, type of pulse feature, and their interaction had significant impacts on the overall TS and intra-subject TS variability (p?<?0.001 for all). Valley and maximal second derivative showed higher overall TS than peak and maximal first derivative. Finger had higher overall TS and lower intra-subject TS variability than other measurement sites. Significance. Measurement site and type of pulse feature can significantly influence the timing of feature points on filtered PPG signals. Filtering parameters should be quoted to support the reproducibility of PPG-related studies.

    Original languageEnglish
    Article number074001
    Number of pages17
    JournalPhysiological Measurement
    Volume42
    Issue number7
    DOIs
    Publication statusPublished - 28 Jul 2021

    Funder

    High-level University Fund G02236002 of the Southern University of Science and Technology

    Keywords

    • multi-site photoplethysmography (PPG)
    • pulse wave
    • feature point
    • filtering
    • time shift

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