Determinants of photoplethysmography signal quality at the wrist

Fig 1. Examples of PPG signals: Each column shows an original PPG signal (scaled to fit) in the top row, and individual components of this PPG signal in the subsequent rows, each shown on common scales: (second row) the DC component (low-pass filtered below 0.5 Hz); (third row) the AC component (band-pass filtered between 0.5 and 12 Hz); and (fourth row) high-frequency noise (high-pass filtered above 12 Hz). Each column shows a different signal and its components: (a) a low-quality PPG signal with low-amplitude AC components and barely discernible pulse waves; (b) a medium-quality PPG signal where individual pulse waves are visible but their shapes are corrupted by noise; and (c) a high-quality PPG signal where individual pulse waves as well as their detailed shapes are clearly visible. Definition: au - arbitrary units.

Fig 2. The AC and DC components of the PPG signal.

Fig 3. PPG signal quality metrics: The distributions of PPG signal quality metrics in: (upper row, A–C) the Auscultatory Subset; and (lower row, D–F) the Oscillometric Subset.

Fig 4. Examples of PPG signals whose SNR values did not align with their quality: (a) a reasonably high-quality PPG signal (individual pulse waves are clearly visible and details of their shapes are also visible) with a low SNR value due to a noisy frequency spectrum; (b) a high-quality PPG signal with a low SNR value due to the nonstationary nature of the signal produced by an irregular rhythm (e.g., premature ventricular contractions); and (c) a medium-quality PPG signal (individual pulse waves are visible but their shapes are corrupted by noise) and yet a reasonably high SNR value.

Fig 5. The quality of PPG signals measured in different postures: (upper row, A– C) comparison between postures with arm in the natural position for each posture; (lower row, D– F) comparison between postures with sensor at heart height. Each row shows: (left, A and D) the SNR; (middle, B and E) the PI; and (right, C and F) the TMCC. Definition: An asterisk (*) indicates a statistically significant difference.

Fig 6. The quality of PPG signals measured at different sensor heights. Results are shown for: (upper row, A– C) standing; and (lower row, D– F) sitting. Each row shows: (left, A and D) the SNR; (middle, B and E) the PI; and (right, C and F) the TMCC. Definition: An asterisk (*) indicates a statistically significant difference.

Fig 7. Determinants of the PPG SNR: The plots show the modelled relationship between the SNR and its key determinants on: (upper row, A– C) the Auscultatory Subset; and (lower row, D– F) the Oscillometric Subset. Solid lines indicate predicted values when all other parameters were set to normal values. In the upper row dashed lines indicate non-simultaneous 95% confidence intervals.