BIS monitor¶

Medtronic BIS: raw EEG at 128 Hz and the BIS index.

Stub

Imported from the time-source table; the full page is still to be written.


Type	monitor
Clocks	BIS clock

Device documentation¶

Variables¶

Variable	Type	Stamped by	Spaced by
BIS EEG	waveform	Recorder PC clock	BIS clock
BIS index	numeric	BIS clock	—

Scale factors and resolution¶

Waveform scale factors. Physical value = offset + stored code × gain, one row per stored configuration. A factor shown wrong has its correction in Corrected to.

Track	Unit	Stored as	Rate	Gain / offset	Recordings	Corrected to
`EEG1_WAV`	uV	float32	128 Hz	1 / 0	5,566	—
`EEG2_WAV`	uV	float32	128 Hz	1 / 0	5,566	—

Waveform resolution. Observed span is what the data reach, and the whole bits needed to store it; converter is our estimate of the device's resolution (never less than the span), with its evidence. The step is in the physical unit, with the corrected gain where the stored one is wrong.

Track	Step	Observed span (codes used)	Bits to store span	Converter (estimate)	Evidence
`EEG1_WAV`	0.05	codes -1476.7–1800.05 (65,536 used); 400 corrupt samples outside	16	16 bits	all 65,536 steps of 0.05 µV between −1476.70 and +1800.05 used, none outside
`EEG2_WAV`	0.05	codes -1476.7–1800.05 (65,536 used); 400 corrupt samples outside	16	16 bits	all 65,536 steps of 0.05 µV between −1476.70 and +1800.05 used, none outside

Numerics. Stored as floats with gain 1. Grid is the coarsest decimal step holding 99% of records; the share on the next coarser grid shows heaping (by chance about 1/2, 1/5 or 1/10).

Track	Unit	Stored as	Interval	Recordings	Grid	Share one step coarser	Rounding	Observed range	Note
`BIS`	—	float32	1 s	5,566	0.1	0.27	—	0 to 97.7	corrupt values to -1.33e+03
`EMG`	dB	float32	1 s	5,566	0.01	0.2	—	18.67 to 81.21	—
`SEF`	Hz	float32	1 s	5,566	0.01	0.17	—	2.63 to 29.4	—
`SQI`	%	float32	1 s	5,566	0.1	0.39	—	0 to 100	corrupt values to -2.15e+08 and 1.34e+07
`SR`	%	float32	1 s	5,566	0.1	0.89	—	0 to 100	corrupt values to -1.66e+03 and 2.12e+03
`TOTPOW`	dB	float32	1 s	5,566	0.01	0.2	—	24.98 to 82.5	—

Observations¶

Observation	Status	Summary
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.
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.
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 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 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.
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.
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.

Studies¶

Study	Date	Status	Summary
Gaps and unbroken segments in every waveform	2026-10-19	draft	Every gap and every unbroken segment in the 6,388 recordings, per waveform. The curves are not smooth: a cliff at about 18 s in every device marks the recorder's pauses, SNUADC segments stop at 5 hours, Primus segments at about 2.0 hours and BIS segments at about 2.3 hours (each device's correction cycle), and BIS's short gaps grow with the time since the last one (clock resynchronisations, not lost data).
When each device's clock is corrected, in every recording	2026-10-19	draft	From the steps between records in all 6,388 recordings: the BIS is corrected every 8,192 s and the Primus every 7,230 s, on their own timers (the first correction comes at any time after the device starts); the SNUADC is reset when 0.49 s of drift has built up. With the full rerun of the correction fit, it gives each device's drift against the recorder PC in 4,728 BIS and 2,728 SNUADC recordings, and shows that the Primus's 20–23 s AWP pauses belong to its correction cycle. Every rate here is relative to the recorder PC, whose own clock drifts and is corrected.
Message sizes and the steps between records	2026-10-19	draft	Every record in every waveform: SNUADC writes messages of 100 samples (0.2 s), Primus 30 (0.48 s), BIS 16 (0.125 s), CardioQ no fixed size. Backward steps outnumber gaps by about a hundred to one and come from a few recordings with duplicated or interleaved records. Primus gaps are whole lost messages; most SNUADC and BIS gaps are not.
What each break in the stamps is	2026-10-19	draft	Every step between waveform records in the release, sorted by mechanism: written twice, a SNUADC clock reset, interleaved streams, a pause shared by every device, whole messages, less than a message, or a fraction of a message beyond whole ones. A continuity test with controls shows which classes lose data: shared pauses and most longer SNUADC gaps do; steps under one message, and SNUADC's lone one-message steps, mostly do not.
Value resolution of every signal	2026-10-19	draft	How finely every waveform and numeric track in the release records its values. The SNUADC waveforms are 10-bit codes, though VitalRecorder's documentation describes the SNUADC as 12-bit; Primus waveforms use about 10 bits of 12; BIS EEG is a 16-bit grid of 0.05 µV. Numerics are 32-bit floats on decimal grids of 1, 0.1, 0.01 or 0.001, one per track; settings sit on coarser grids of 10 or 100.
Identifying the recording setup of every case	2026-10-18	open	Recognising which equipment made each recording, from its serial ports and, where those fall short, from clock fingerprints, so that per-setup values can be used for every case.
How often each device is corrected	2026-10-17	current	The devices re-synchronise in different ways: SNUADC when its drift reaches about half a second, BIS on a fixed 8,192 s timer, and the Primus by small steps about every two hours that do not keep up with its drift (+104 ppm, measured later by breath matching).
Recorder PC clock steps and recording pauses	2026-10-17	current	A step of the recorder PC clock would move every device's stamps at once. Searching all 6,388 files for such simultaneous jumps found none of 0.5 s or more; what moves everything together is recording pauses.
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.
Drug effects as a clock check	2026-10-13	current	Infusion pumps, the rapid infuser and the cerebral oximeter share no waveform with the reference. Drug effects that follow infusions at plausible delays (propofol to BIS in about 2 minutes, fluid to CVP in about 30 s) bound their clocks to well within a minute of the recorder.
Closing loops in the timing graph	2026-10-13	current	Where two independent routes reach the same clock, the offsets around the loop must sum to zero. Three measured loops close within their noise; the BIS loop closes up to a constant latency; the proposed EMR loop is not a loop at all.
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.
The cardiac artefact in the BIS EEG	2026-10-12	current	The BIS EEG carries a weak electrical trace of every heartbeat. Too weak for beat-by-beat matching, it shows clearly when averaged on the ECG's R peaks, and gives a second SNUADC–BIS timing reference that agrees with the bursts to about 16 ms.
Anaesthesia record times are rounded up	2026-10-12	current	Anaesthesia start and end entered on a 5-minute mark sit 90–290 s later, against device markers, than entries typed to the minute. Rounding up to the next 5 minutes fits every one of 11 markers better than rounding to the nearest or down.
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.
Shared bursts between the ECG and the EEG	2026-10-12	current	Electrocautery and muscle bursts appear in both the SNUADC ECG and the BIS EEG. Cross-correlating their envelopes gives the offset between the two devices' stamps every 10 minutes: about −0.8 s, in a sawtooth that follows both clocks' corrections.
The stamp sawtooth model for SNUADC and BIS	2026-10-12	current	Fitting each track's record stamps against its sample count as a drift with corrections (a sawtooth) puts SNUADC and BIS on one timeline without any artefact. Checked against shared bursts, the EEG–ECG timing error falls from about 790 ms on the recorded stamps to 33 ms with one constant per SNUADC mode in the pilot, and 42 ms over the whole release.
Clock corrections in every file	2026-10-11	current	A scan of all 6,388 files for the jumps where the recorder re-synchronises a device's stamps. Each device type has its own correction signature; only SNUADC and BIS correct clearly enough to measure their drift.
Physical units, shared drift and time order	2026-10-11	current	Device drift rates cluster into units: about 8 SNUADC boards and 8–18 BIS monitors. Part of the drift is shared by both devices of a case and changes between cases: the recorder PC's own rate. Case order carries no time information.

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