Fig. 1.
![The placement of the ECG electrodes with PowerLab, a physiological signal acquisition instruments from ADInstruments Inc. [30].](https://sciendo-parsed.s3.eu-central-1.amazonaws.com/688079f8e88a4c3023543f31/j_msr-2025-0024_fig_001.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=ASIA6AP2G7AKIB2JSJJ2%2F20260308%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260308T101941Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEEYaDGV1LWNlbnRyYWwtMSJHMEUCIQCLe1lGb8SwkctduyNtYJhr6ROTh5oAl1wmkUG5guUA8QIgbszPwmM1HZxYiF5%2FlA%2F1d6OxTBPD0q0D4x0JNLKV2ZgqvQUIDxACGgw5NjMxMzQyODk5NDAiDMGzi58KF9LbNDt3liqaBav%2FDKhe32KUyMD37OBgsdv513c9StNZEXhRLPkuKo%2BtO2%2FRrx85FSTz79nLwAKMDndA0QyHUK5yagdAK42gK3yTD86yfDbT49UmLk8PfWZcoLuXNG%2BxdTamlHMPhS7kItZjqa4u89CsBFcKpsxMT8dOMD2%2F0RT50QzEUKoOl01%2Fvj703Drsvke9f7oVU0tB66hG%2Bw%2FNVGOkzVZ2uAyTfXokp8qFFswPWpWRVoEYK82s4qsdamn1VJ5QHohH6YguQfFaF%2BJcYQsbc2FdO6uOCa5M51G7NnO6PaJLXAOhhHzDt7%2BBW2Qv7bUwH9zXqdzpFkdHzjYqj9DH8nJcboFLk9vttLx7mcYF9dlMeGsvpZz9JYxMjHIdXFWRydl3XoG6S2k9RFDzdyKS3K2ETOw%2BLwhxZjA0A0bB4wgpRIfNrRXwX6TgER8hCGvsxXOnb%2BgLWiTNPUKSEp4lfSYPebHYIKlhaWTSJhs2DNQSTM6rdS7kBbXaj5W5kP3AaPwrBYYtPFzXWA4o7WXpIcLC37cUw8ujo6hOIqDatV7JFVdo4cpS3%2BdNf0eNagcy0MhJz4MD8QkDP%2BR%2FqL2W1nVQtJ3hz1hwX5rlMkd55AJNk8e%2FgVMdxyo2L%2FdJUjmJ4IexeBq61D0JRXee%2F8Q%2FQ02m2SrCcPHW8oipSXzsI%2FM2rPu%2F7U%2BD2b4%2FwVQH5ZAIrBRCyqKOBu6xRniPoTUS8mjCqUxN4b6ewZzK6YlgAwmhMcmbD%2BDaCgVz3uhWVRfcX25%2Fsi1MJfp%2FsklzvVwtKkqVbKlJeEEH3pj%2FhiW3jPclGmjrSxhITxVsb6AZ3lzg6v8SZ4m9igEa5szGvvkxRi6eJ3MoS9WLMA9AbA%2Bp8ZA6xclCAQKxlePBydB%2FYockQDCwnbTNBjqxAaU%2FEO0UyogJus3TEsMECrpVo9ite%2BIoBKpqzqU2QcygN2eT6IdY6a%2BljX12fuMgPwJGchjHqyEGvfINJxdg82d4Q%2BH5dhekIwlgdhTAVjn01Vhq3AEqZUgfa4d8cMmYRZW1bwn8JAkpaI%2F4RX7jdLkxTYlz6O3WyXWQEY4BpTh80Lcm3UXLrVFs1%2FsHR61DpgFdw2dYNDV3x0FpJGPtlut7C8yhuroFL5RFxzDhiEFlkA%3D%3D&X-Amz-Signature=918a741830647b3cfe149c616244aa9f3565f321ab735aad9f08501fe884d815&X-Amz-SignedHeaders=host&x-amz-checksum-mode=ENABLED&x-id=GetObject)
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Confusion matrix between RRIV and SSIV in the resting and deep breathing phases_
| Phase | SSIV | RRIV Abnormal [ms] | RRIV Normal [ms] |
|---|---|---|---|
| Resting | Abnormal | 44 | 2 |
| Normal | 8 | 190 | |
| Deep breathing | Abnormal | 44 | 3 |
| Normal | 7 | 190 |
SSIV assessment range of normal cardiac autonomic function in each age group based on PPG_
| Age [y] | 20 ∼ 29 | 30 ∼ 39 | 40 ∼ 49 | 50 ∼ 59 | 60 ∼ 69 |
|---|---|---|---|---|---|
| Rest [ms] | 13 ∼ 45 | 7 ∼ 31 | 7 ∼ 35 | 6 ∼ 22 | 8 ∼ 18 |
| Breath [ms] | 20 ∼ 61 | 10 ∼ 53 | 15 ∼ 47 | 12 ∼ 58 | 9 ∼ 27 |
The sensitivity and specificity of SSIV in different assessment ranges_
| Phase | Rate [%] | Out+2 | Out+1 | RRIV source | In-1 | In-2 | In-3 | In-4 |
|---|---|---|---|---|---|---|---|---|
| Rest | Sensitivity | 36.54 | 63.46 | 84.62 | 96.15 | 96.15 | 100.00 | 100.00 |
| Specificity | 98.96 | 98.96 | 98.96 | 91.67 | 79.69 | 70.83 | 59.90 | |
| Breath | Sensitivity | 54.90 | 70.59 | 86.27 | 94.12 | 94.12 | 96.08 | 96.08 |
| Specificity | 99.48 | 99.48 | 98.45 | 95.85 | 91.19 | 84.97 | 78.24 |
A descriptive listing of PPG signal processing methods_
| No. | Filter name | Description |
|---|---|---|
| F01 | Butterworth | The passband frequency is from 1 Hz to 5 Hz, and the filter order is 2. |
| F02 | Bessel | |
| F03 | Chebyshev | |
| F04 | Elliptic | |
| F11 | Savitzky-Golay | The polynomial order is 3, and the window size 401. |
| F12 | Average | The passband frequency is from 1 Hz to 5 Hz. |
| F13 | Periodic moving | After normalizing each cardiac cycle with the same number of sampled signals, the ith sampled signals of each of the 5 cycles are averaged, where 1 ≤ i ≤ the normalized cycle length. |
| F21 | Daubechies | Use the inverse DWT of Daubechies db6 and Coiflet C3 to reconstruct the selected band from 1 Hz to 5 Hz. |
| F22 | Coiflet C3 | |
| F23 | Morlet (CWT) | Apply the inverse WT of Morlet to reconstruct the selected band from 1 Hz to 5 Hz. |
RRIV assessment range of normal cardiac autonomic function in each age group based on ECG [38]_
| Age [y] | 20 ∼ 29 | 30 ∼ 39 | 40 ∼ 49 | 50 ∼ 59 | 60 ∼ 69 |
|---|---|---|---|---|---|
| Rest [ms] | 12 ∼ 46 | 6 ∼ 32 | 6 ∼ 36 | 5 ∼ 23 | 7 ∼ 19 |
| Breath [ms] | 19 ∼ 62 | 9 ∼ 54 | 14 ∼ 48 | 11 ∼ 59 | 8 ∼ 28 |
Average absolute errors between the RRIV/ECG SDNN/ECG, NN50 at 1000 Hz and the SSIV/PPG SDNN/PPG NN50 at different sampling rates_
| Filter/Sample rates | Phase | Parameter | 1000 [Hz] | 500 [Hz] | 250 [Hz] | 200 [Hz] | 125 [Hz] | 100 [Hz] | 50 [Hz] | 40 [Hz] | 25 [Hz] | 20 [Hz] |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| F01 | R | SSIV [%] | 0.6 | 0.6 | 1.2 | 1.3 | 1.3 | 1.4 | 1.8 | 2.2 | 3.4 | 4.4 |
| F11 | R | SSIV [%] | 1.4 | 1.5 | 2.1 | 2.2 | 2.3 | 2.3 | 2.8 | 3 | 4.1 | 7.3 |
| F23 | R | SSIV [%] | 2 | 2.2 | 3 | 2.5 | 2.9 | 2.8 | 3.2 | 4.7 | 4.6 | 5.8 |
| F01 | B | SSIV [%] | 0.9 | 1.1 | 1.2 | 1.1 | 1.2 | 1.2 | 1.8 | 2 | 2.9 | 3.8 |
| F11 | B | SSIV [%] | 2.5 | 2.3 | 2.3 | 2.5 | 2.6 | 2.7 | 3.1 | 3.3 | 4.2 | 14.4 |
| F23 | B | SSIV [%] | 2.6 | 2.7 | 2.9 | 2.5 | 2.9 | 3 | 3.4 | 5 | 4.3 | 5.6 |
| F01 | R | SDNN [ms] | 0.6 | 0.6 | 0.6 | 0.6 | 0.8 | 0.9 | 2.1 | 3.1 | 6.5 | 9.5 |
| F11 | R | SDNN [ms] | 1.3 | 1.4 | 1.5 | 1.6 | 1.7 | 1.9 | 3.2 | 4 | 7.3 | 12.4 |
| F23 | R | SDNN [ms] | 2.7 | 2.6 | 2.8 | 2.1 | 2.7 | 2.3 | 3.4 | 6.9 | 7.5 | 10.8 |
| F01 | B | SDNN [ms] | 1.2 | 1.2 | 1.3 | 1.3 | 1.3 | 1.4 | 2.2 | 2.7 | 5 | 7.2 |
| F11 | B | SDNN [ms] | 2.2 | 2.2 | 2.2 | 2.4 | 2.4 | 2.6 | 3.3 | 3.9 | 6.3 | 15.3 |
| F23 | B | SDNN [ms] | 2.7 | 2.7 | 2.9 | 2.4 | 2.7 | 2.6 | 3.3 | 6.1 | 5.6 | 8.1 |
| F01 | R | NN50 [%] | 0.8 | 0.9 | 0.8 | 1 | 1 | 1.3 | 1.7 | 1.7 | 3.6 | 3.1 |
| F11 | R | NN50 [%] | 1.7 | 1.6 | 1.8 | 1.6 | 1.9 | 2 | 3.5 | 2.2 | 5.1 | 4.3 |
| F23 | R | NN50 [%] | 1.5 | 1.5 | 1.5 | 1.6 | 1.6 | 1.8 | 2.2 | 2.1 | 4.1 | 3.3 |
| F01 | B | NN50 [%] | 0.8 | 0.8 | 0.8 | 0.9 | 0.9 | 1.1 | 2.2 | 1.8 | 3.6 | 3.1 |
| F11 | B | NN50 [%] | 2.4 | 2.4 | 2.8 | 2.3 | 2.7 | 2.3 | 4.9 | 2.8 | 5.9 | 5.3 |
| F23 | B | NN50 [%] | 1.6 | 1.6 | 1.7 | 1.6 | 1.7 | 1.9 | 3.1 | 2.5 | 4.8 | 4.1 |