Processed EEG simulator

BIS

EMG

ST0:00:00

SR0

25 mm / sec. EEG 25 µV / division

0:00

SQI --%EMG -- dBSR 0%SEF -- HzTP --BC 0/min--Suppression 0:00BIS<40 0:00

Anaesthetic

Depth target

AwakeSedatedGADeepFlat

Model BIS --

Adjust current EEG

Older patient (over 70) Facial muscle activity (EMG)

One-sided EEG slowing

Diathermy (electrical interference)

Display options

Teaching labels (zone, newest epoch, colour key, target band)

DSA colour range (power → blue … red) Floor (blue)-3 dB Ceiling (red)31 dB

Raise the floor and the DSA turns bluer, lower the ceiling and it turns redder. Teaching strips keep the default scale.

Playback 1×

Awake

Anaesthesia teaching · processed EEG

From the BIS sensor to the DSA

The BIS number is one summary of a frontal EEG. The density spectral array (DSA) shows power at each displayed frequency over time. Read the pattern alongside the index and raw EEG; the DSA alone does not encode the full proprietary index algorithm. Move the drug and depth controls and watch the raw EEG, the number and the DSA change together.

Choose a monitor to compare display layouts. On BIS Advance, time runs left to right, frequency increases upwards (0–30 Hz) and colour shows power. SEF95 is the frequency below which 95% of measured spectral power lies; MF divides it in half. Check each screen’s legend because orientation, colours and labels vary. SQI indicates signal quality and EMG indicates muscle contamination. The waveforms and indices here are synthetic teaching examples, including the PSi display; they are not validated device outputs.

How one DSA column is made

In this simulator, each new second contributes a column calculated from the latest 2 seconds of EEG. This animation uses the live EEG from the monitor above, so change the drug and the next column changes too.

Pattern library

Each strip is 2½ minutes of simulated DSA at a steady state. Compare the shapes with the raw EEG. The displayed BIS ranges are model examples, not diagnostic thresholds.

Name that DSA

A fresh strip is simulated each time, so no two are identical.

0–30 Hz2½ min

What is this?

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When the number misleads

Ketamine and nitrous oxideKetamine can increase fast power and BIS without emergence. Nitrous oxide’s anaesthetic contribution may not be fully reflected in BIS. Effects depend on dose, co-administered drugs and the transition being observed.Source: Purdon 2015, Anesthesiology; Akeju 2016.
Muscle (EMG) and paralysisFrontalis activity can contaminate high frequencies and increase BIS. Review the raw EEG, EMG, signal quality, stimulus and hypnotic delivery together. Neuromuscular blockade can lower BIS even in awake volunteers and removes movement as a warning sign. Paralysis provides neither hypnosis nor analgesia; a low index does not exclude awareness.Source: Schuller 2015, BJA; Messner 2003.
Older brainsAgeing is associated with lower EEG power, weaker and slower alpha, and greater susceptibility to suppression. These are group trends, not a fixed correction for every patient. Weak alpha alone does not establish light anaesthesia; inspect the raw EEG and SR.Source: Purdon 2015, BJA.
Read SR, not just BISSuppression can occur while BIS remains in the 30s–40s. Inspect the raw tracing and signal quality when SR rises: a single average index can hide alternating bursts and suppressed intervals.Source: Bruhn 2000, J Clin Monit Comput.
LagBIS is smoothed (10, 15 or 30 s), and the whole delay before it settles at a new level ranged from about 15 s to over 2 minutes in testing (Pilge 2006). The DSA and raw EEG change first.Source: Pilge 2006, Anesthesiology.
An unexplained low indexHypothermia, severe metabolic disturbance or impaired cerebral perfusion can slow or suppress the EEG. Artefact, reduced EMG after paralysis and delta arousal can also lower the index through different mechanisms. An abrupt unexplained change warrants prompt assessment of the patient, circulation, oxygenation, drug delivery and signal quality, with temperature and glucose as indicated.Source: Mathew 2001; Schuller 2015; García 2021.
InterferenceOscillating ventilators and pacing or ECG spikes add high-frequency noise that can push BIS up; diathermy swamps the signal. Check the raw EEG, EMG bar and SQI.Source: Hemmerling & Fortier 2002; BIS Complete operator’s manual.
Age and neurological contextThese examples represent adult EEG patterns. Infants, especially under about 4 months, may lack the frontal alpha pattern seen in older children under sevoflurane. Adult pattern-to-index assumptions should not be extrapolated to them; neurological disease and psychoactive drugs also alter interpretation.Source: Cornelissen 2015, eLife.

What the trials say

Processed EEG is a tool, not a target. The main evidence in brief:

Study	Finding
B-Aware 2004	In high-risk patients, BIS-guided anaesthesia cut awareness (2 vs 11 cases) compared with routine care.
B-Unaware 2008 / BAG-RECALL 2011	BIS was not better than an end-tidal anaesthetic gas alarm protocol at preventing awareness.
NAP5 2014	Spontaneously reported accidental awareness was about 1:19,600 overall and 1:8,200 with neuromuscular blockade; most reports concerned induction or emergence. This reporting method differs from structured postoperative interviews.
ENGAGES 2019	EEG-guided anaesthesia in older patients did not reduce postoperative delirium.
BALANCED 2021 (delirium sub-study)	Delirium, the primary outcome of this sub-study, occurred in 19% with a BIS target of 50 versus 28% with a target of 35. The larger parent trial found no significant difference in its primary outcome, 1-year mortality (Short 2019, Lancet).
AAGBI/SIVA TIVA guideline 2018	Use processed EEG when TIVA is given with a muscle relaxant.
NICE HTG292 (formerly DG6)	Depth-of-anaesthesia monitors are an option in patients at higher risk of awareness or excessively deep anaesthesia, and in all patients having TIVA.