Observations¶

A general fact about the dataset's timing or recording, with its evidence and what it means.

Observation	Status	Summary
About 20 ms of signal lost at every SNUADC reset	established	At each step-mode reset the acquisition pauses for about 20 ms inside the last record before the new timing, and that time is never sampled: 17.7 ms on the sample count (Signal lost at a SNUADC step-mode correction), once the filler sample is counted. Every record still holds 100 samples, so nothing in the file shows it.
All SNUADC channels share one clock	established	Every SNUADC waveform track of a case (ECG leads, ART, PLETH, CVP) has the same first stamp, the same record timing and the same corrections. They are stamped together.
An extra zero sample after most resets	established	Most SNUADC resets (335 of 392) contain one sample that reads 0 in every channel at once, a little after the reset. It carries no time of its own: the signal either side runs on as if it were not there.
Anaesthesia record times on 5-minute marks are rounded up	established	An anaesthesia start or end entered on a 5-minute mark was rounded up to it: the event happened in the 5 minutes before, and on average a little earlier still. Clear for anaesthesia end; for anaesthesia start the margin is small, and the value model (How the anesthesia start time was charted) prefers rounding to the nearest mark.
Arterial timing steps cluster at line events	established	About a quarter of the steps in ECG→arterial-pressure timing fall within a minute of a line access or a cuff occlusion, against 4.5% at random times.
Case ids carry no time order	established	Dates are shifted and case ids are shuffled: neighbouring ids are no more alike in device drift or port than random pairs.
Device clock drift depends on the setup	established	The drift of the SNUADC's and the BIS's timestamps against the recorder PC is largely set by the setup: knowing a recording's PC key (the serial ports of its four core devices) puts the SNUADC's drift within 3 ppm of its setup's median in 97% of cases and the BIS's in 83%, against 34% and 17% for the release's median. On the whole release's correction intervals the same test gives 71% and 60% (Identifying the recording setup of every case).
Eight recordings store whole messages twice	established	In 8 recordings 15–48% of the waveform records are byte-for-byte copies of the record before them, with the same timestamp: case 15, case 364, case 1946, case 3865, case 4857, case 5414, case 5595 and case 5619. SNUADC, Primus and BIS are affected equally, so the recorder, not a device, wrote them twice.
Every case starts at 2100-01-01 00:00 UTC	established	The release shifts each case to the same start, removing the date and the time of day. Within a case all times keep their spacing.
Every time is quantised, and fiducials inherit it	established	A time on a grid of step Δ carries an error of Δ/√12 (1 SD). In VitalDB that is 0.58 ms for a 500 Hz sample, 2.3 ms for BIS EEG, 0.8 ms for the arrhythmia annotations' 1/360 s grid, and up to ±17 ms for PPG fiducials taken on raw samples, which lock to the 33.3 ms frame.
Five records out of order at every SNUADC reset	established	At each step-mode reset the five records after it are written in the wrong order: two continue the old timing and three start the new, interleaved. Read in file order, or sorted by stamp, one second of signal is scrambled.
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.
Op end was filled in from the recording end in 537 cases	established	In 537 cases (8.4%) the op end is exactly 10 minutes before the end of the recording, so it was filled in rather than charted. These cases are concentrated in thoracic surgery and under sedation or spinal anaesthesia.
Primus waveform gains are wrong in every recording, for CO₂ or for AWP	established	Recordings store the Primus waveforms with one of three sets of scale factors. Checked against the Primus's own ETCO₂ and PIP numerics in all 6,388 recordings, CO₂ fits only 0.075 mmHg per code and AWP only 0.1 mbar per code with offset −20, so most recordings overstate CO₂ by a third, and the others overstate AWP by about 3% (950 recordings) or understate it about fourfold (26).
Recording pauses of about 20 s	established	In 29% of files every device jumps forward together, by a median 19.6 s: the recorder paused. Data are missing there, and the stamps show it honestly. The pauses run on a 5-hour timer: consecutive pauses in a recording are 5 h apart (plus the pause) in 86% of cases, and the first comes at any time after the recording starts.
SNUADC sends no time; the recorder stamps by count	established	The SNUADC board streams samples with no time stamps. Every SNUADC time in VitalDB is the recorder's model of the board's clock: the sample count at the nominal rate, pulled back when it drifts too far.
SNUADC stamps run −0.95 ppm against the sample count	established	Between corrections each 0.2 s record stamp increment is about 0.19 µs short: stamps run −0.95 ppm from the sample count in 88% of files (both SNUADC modes). It is arithmetic, not clock drift. The other 12% sit between −0.90 and −0.55 ppm.
Small positive SNUADC gaps are real data loss	established	Forward jumps of under a second in SNUADC stamps are missing data, not clock corrections: the beat interval across them fits the gap better than the samples butted together.
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 ECG→PPG delay is a sawtooth	established	Inside the oximeter the PPG content creeps 18–52 ppm against its 30 Hz frames; every 11–31 minutes a frame repeats and the delay jumps up by 33 ms. The rate is fixed per recording setup.
The EEG–ECG offset is fixed by the SNUADC correction mode	established	After the stamp model, BIS EEG still lags the SNUADC ECG by a constant: −804 ms in step mode and −660 ms in frequent-correction mode over the whole release, tight within a port or unit. The mode, a property of the setup, sets the constant.
The EMR clock's offset cannot be separated from documentation lag	established	Both routes between the EMR clock and the recorder run through the case start, so any EMR clock offset cancels. Without a time from outside the release it is confounded with how late entries are made.
The EVENT track repeats the clinical table	established	In 411 random cases the recorder EVENT track's four entries equal the clinical table's case start, operation start, operation end and case end exactly: the release generated them from the table.
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 Primus AWP stops for about 21 s every 7,230 s	established	In 5,502 recordings the Primus airway-pressure waveform has gaps of 19–23.5 s (whole messages of 0.48 s, mostly 21.12 s) that recur every 7,230 s, the Primus's correction interval; a clock correction follows about 21.6 s after. The CO₂ waveform carries on through 90% of them. The 21 s of AWP are lost, not merely re-stamped.
The SNUADC correction mode belongs to setups, not dates	established	The frequent-correction mode is confined to a few setups, can switch either way within a case, and marks a period in at most one setup. It is not a global software change.
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 blood pressure cuff often sits on the arterial line's arm	established	Many "lost pulse" events in the arterial pressure are an NIBP cuff inflating on the same arm: the pulse fades and pressure decays to 35–50 mmHg, then returns as the cuff cycle ends.
The lab time is neither the draw nor the result	established	VitalDB's lab timestamp marks the specimen (logged at the lab, or entered from a theatre analyser): minutes after the draw and before the result, whose time is not recorded.
The oximeter's frame clock is a fingerprint of the setup	established	The rate of the oximeter's 30 Hz frame clock against the SNUADC sample clock is fixed per recording setup (+7 to +92 ppm across setups) and repeats within about 2 ppm inside a recording, so it identifies the hardware a recording was made on.
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.
Three ART tracks carry the PLETH conversion factors	established	In case 818, case 5175 and case 5693 the SNUADC/ART track holds arterial pressure, but its header gives the PLETH conversion (0.394983 per code, offset −198.2504) instead of the ART one (0.987457 mmHg per code, offset −495.626). Converted as stored, the pressure reads about 2.5 times too low.
Time zero depends on the tracks loaded	established	A file's time zero is its first record of any track. Loading only some tracks moves the zero to their first record: seconds later, and up to hours.
Twelve recordings sample the SNUADC waveforms at 100 Hz	established	In 12 of 6,388 recordings every SNUADC waveform (ECG II, ECG V5, PLETH, and ART or CVP where present) is stored at 100 Hz instead of 500 Hz: case 299, case 803, case 1150, case 1693, case 2362, case 2920, case 4458, case 4582, case 5066, case 6119, case 6133 and case 6375.
CardioQ FLOW uses three scales (a segmented code)	likely	CardioQ FLOW values step by 1 from 0 to 255, by 2 from 256 to 510 and by 4 from 512 to 1016: 509 values, a 9-bit code whose step doubles in each upper segment, as in µ-law and A-law companding or a small floating-point format (an exponent and a 7-bit mantissa). Below 256, even values outnumber odd ones by 40 to 230 times.
SNUADC channels are read one after another	likely	The SNUADC firmware reads its 8 channels in turn, about 0.1 ms each, so within one sample the last channel is read up to about 0.8 ms after the first. The channel order is not known.
SNUADC values are truncated to the count below	likely	The SNUADC firmware truncates each calibrated value instead of rounding it, so every value sits half a count low on average (about 0.5 mmHg for arterial pressure). Values clamp at counts 0 and 1023.
Some ICU stays are longer than the postoperative stay	likely	In 37 cases the ICU day count reaches or exceeds the days from surgery to discharge, though the release describes it as the postoperative ICU stay. Most are emergencies with long stays before surgery, so the count probably includes ICU days before the operation.
Stamp-based loaders lose samples at overlaps	likely	Loaders that place samples by their stamps onto a grid overwrite samples where stamps overlap: about 0.48 s at every SNUADC reset, and one sample about every 35 minutes from the stamp arithmetic.
The BIS EEG carries an averaged cardiac artefact	likely	Averaged on the ECG's R peaks, the BIS EEG shows the heartbeat (median 4.3 µV), near-simultaneous with the R wave. Single beats are too weak to match one by one.
Three lab analytes stop at a reporting limit	likely	Lactate is capped at 15.0 mmol/L, aPTT at 400.0 s, and ESR does not go below 2 mm/h: the values at the limit stand for "this or beyond", not for the number itself.
A heartbeat-locked trace in the CO₂ waveform that ignores the Primus clock	open	Averaged on the ECG's R peaks, the Primus CO₂ waveform shows a heartbeat-locked signal in 60% of 10-minute windows (30-recording pilot), tight to about 20 ms. Unlike the breath, its lag does not drift with the Primus clock: it stays fixed on the SNUADC timeline.
In 34 files the SNUADC records overlap throughout	open	In 34 files the SNUADC records step back over their neighbours far more often than resets explain: in 20 of them 4–95% of all record junctions overlap, by up to about 100 ms. The reset-order rule matches there by coincidence.
In some recordings two unrelated streams are interleaved in one track	open	In 7 recordings 64–95% of the records alternate between two streams: each record continues the one two before it, not the one before, and the two streams do not match each other. SNUADC and BIS in case 2219, case 2379, case 2640, case 4836, case 5327 and case 5617; SNUADC in case 1939. In case 94, case 945, case 1132, case 3195 and case 5700 4–7% of SNUADC records interleave.
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).
SNUADC has two event inputs	open	The firmware sends two event-jack flags with every sample. If any case used them, they would be markers on the SNUADC's own clock.
SNUADC values are 10-bit codes, though the device is documented as 12-bit	open	Every SNUADC channel in the release stores codes 0–1023 and nothing above: 10 bits, a step of 0.0098746 V. The VitalRecorder device list describes the SNUADC as a 12-bit converter.
The PPG sawtooth sometimes stops	open	In some stretches, and some whole recordings, no frame slip comes although several were due. The delay there is flat and the frame clock keeps running; what stops the content's drift is not known.
The BIS EEG carries no heartbeat	withdrawn	Claimed after a first search found no consistent cardiac trace in the BIS EEG. The search window was in the wrong place: the BIS stamps sit about 0.8 s off the ECG's.