Delays and averaging windows¶
Work out how long after the event a value is stamped, and what span of signal it summarises, by recomputing monitor numbers from the waveforms they come from.
Studies¶
| Study | Date | Status | Summary |
|---|---|---|---|
| The SNUADC clock through the Solar8000 numerics, whole release | 2026-10-19 | current | On the SNUADC's own sample count, the delay from the waveform to the PC-stamped Solar8000 heart rate and mean arterial pressure ramps at the SNUADC's drift against the PC, measured without its stamps or resets. In recordings with whole reset intervals it agrees with them (−63 ppm; median difference +1.5 ppm, 581 recordings). It is too noisy per recording, and in recordings under about 4 h its rate is biased toward zero, so it cannot yet speak for recordings without resets. |
| One device delay for the PLETH across the release | 2026-10-18 | current | The stored ECG-to-PLETH delay was measured in 5,536 recordings, to the foot and to the steepest upstroke of each pulse. It has one peak (655 ms to the foot, 715 ms to the upstroke, in the median recording), no clusters by recording setup or SNUADC port, and no shift along the case order: one global device delay, 385 ms with an adopted R-to-foot finger PAT of 270 ms, fits the whole release, as Ruffolo et al. found in their own data. |
| How the BIS index trails its EEG | 2026-10-13 | current | Modelling the BIS index as a lagged mix of EEG features shows a trailing average starting about 3 s after its EEG window ends: about 16 s long in half the cases (the 15 s smoothing setting) and 32–61 s in most others. Half the response weight is older than 30 s. |
| Two heart-rate delay groups, one poll apart | 2026-10-13 | current | Solar8000 heart rate reaches the recorder either about 1.9 s or about 4.2 s after its window ends: one 2 s poll apart. The group belongs to the recording PC, stays fixed through a case, and on some PCs varies from case to case. |
| What a BIS-mimic project says about the index's smoothing | 2026-10-13 | current | The openeeg project reproduces the BIS index from the raw EEG and infers each case's smoothing setting: about 80% of its cases come out at 15 s. Its alignment cannot serve as a timing reference. |
| Rare devices against SNUADC | 2026-10-13 | current | CardioQ, EV1000, Vigileo and Vigilance each get a link to the SNUADC timeline: beat matching for the CardioQ, mean arterial pressure alignment for the EV1000 and Vigileo, and a long-window fit for Vigilance's averaged heart rate. |
| Numeric delays are fixed within a case, different between cases | 2026-10-12 | current | With each numeric's algorithm held fixed, delays were fitted for every case and every 30-minute block. They do not drift within a case, but 40–70% of their variance lies between cases, so a single population delay is wrong by up to 2 s for HR and 1 s for the others. Each case gets its own delay label. |
| How the monitors compute their numbers | 2026-10-12 | current | Recomputing each monitor numeric from its own waveform finds a short, simple algorithm for every one (a median, an exponential average or a time mean over 3–30 s) and a delay of 2–9 s behind the newest data it uses. No value is stamped before its data exist. |
| The PPG hardware delay and the adopted pulse arrival time | 2026-10-12 | current | VitalDB's raw ECG→PPG pulse arrival time is about 0.4 s longer than physiology. Splitting it into a hardware delay (one value for the release; One device delay for the PLETH across the release) and a physiological part needs an adopted value from studies that sampled ECG and PPG on one clock: about 270 ms for anaesthetised adults. |
| SpO₂ and pulse rate latency from step events | 2026-10-12 | current | SpO₂ cannot be recomputed from one plethysmogram, so its timing comes from events that change the PLETH abruptly. SpO₂ lags the recorded PLETH by about 4 s and then smooths over roughly 10–20 s. |
Observations from these studies¶
| Observation | Status | Summary |
|---|---|---|
| Monitor numerics describe the past, never the future | established | Every monitor numeric describes a window of 3–30 s or more that ended 1–9 s before its stamp. None is stamped before its data exist, so the stamps already respect the availability rule. |
| Numeric delays are fixed within a case but differ between cases | established | A numeric's delay does not drift through a recording, but 40–70% of its variance lies between cases. One delay per case describes it; one delay for the whole dataset is wrong by up to about 2 s. |
| Solar8000 ST records are written twice | established | Every Solar8000 ST_II record is duplicated at the same stamp (0–20 ms apart). |
| Solar8000 heart rate comes one poll early or late | established | Heart rate reaches the recorder about 1.9 s or about 4.2 s after its window ends, one 2 s poll apart. The group belongs to the recording PC and stays fixed through a case. |
| Solar8000 heart rate is truncated, not rounded | established | The Solar8000 heart rate drops its fraction (floor), in 24 of 24 pilot cases; the respiratory rate rounds. |
| SpO₂ freezes in low perfusion | established | During low perfusion SpO₂ mostly holds its last value rather than blanking (55–70% of 5–20 s episodes); when the probe comes off it blanks within about a second. |
| The BIS index is slow | established | The BIS index averages 15–60 s of EEG ending about 3 s before its stamp, with half its weight older than 30 s. It cannot time anything faster than about a minute. |
| The PLETH arrives about 0.4 s late | established | Raw R-to-PPG-foot pulse arrival time in VitalDB has a median of 0.64–0.71 s, about 0.4 s more than physiology: the monitor's buffering and filtering. One delay fits the whole release: no clusters by setup, SNUADC port or case order (One device delay for the PLETH across the release). |
| The Solar8000 relays the Primus's ventilation numbers | established | Solar8000 ETCO₂, RR_CO2, VENT_PIP and VENT_RR change only every 6–8 s: they are Primus values passed on through the Solar8000's 2 s polling, not measured by the monitor. |
| The plethysmogram is built in 30 Hz frames | established | The SNUADC PLETH waveform comes from the oximeter in 33.3 ms frames, smoothed to 500 Hz. Its content changes once per frame, on the oximeter's own clock, about 0.4 s after the finger. |
| Long SNUADC recordings with no correction at all | open | 1,066 cases of 2.5 h or longer show no SNUADC correction, although at the step-mode median of −63 ppm a 0.49 s step falls due within about 2.2 h. Slower units, a third recorder mode, or both; shared bursts point to slower units (about −36 ppm where the BIS rate is known, The stamp sawtooth model for SNUADC and BIS, due only after about 3.8 h). The Solar8000 route cannot settle it: its rates shrink toward zero in recordings under about 4 h (The SNUADC clock through the Solar8000 numerics, whole release). |
The approach¶
The idea. A heart rate, a mean pressure or a depth-of-anaesthesia index describes a window of signal that ended before its stamp. Recompute the number from the raw waveform with candidate algorithms and windows, and find the window and the delay that best reproduce the stamped values.
What it gives. For each numeric: the window it averages, the delay from the window's end to its stamp, and whether that delay is fixed within a case and between cases (and so whether it needs a per-case label).
How. A uniform model of time and value: each stamped value is the candidate statistic over a window ending one delay before the stamp, plus noise; fit the delay per case by likelihood over a grid, with the availability rule enforced (a value cannot precede its window). Step events (a sudden desaturation) give onset latencies.
Pitfalls. A longer window with a smaller delay can mimic a shorter one with a larger delay; the availability rule removes the impossible half. Device settings (an averaging option) are not recorded and must be inferred.
References¶
- Goodwin AJ, Eytan D, Dixon W, Goodfellow SD, Doherty Z, Greer RW, McEwan A, Tracy M, Laussen PC, Assadi A, et al. Timing errors and temporal uncertainty in clinical databases – a narrative review. Front Digit Health. 2022;4:932599. ↗