24 Processed EEG as a Measure of Brain Activity During Anaesthesia
373
This chapter focuses exclusively on this widely used hypnotic drugs that elicit
common well-known patterns on the EEG. Other drugs such as opioids operate under
very different mechanisms, and agents such as nitrous oxide, dexmedethomidine and
ketamine among others, possess an ambivalent effect, both hypnotic and analgesic,
all inducing distinct effects on EEG outside the scope of this chapter, see [29, 32]
for more details.
The two main EEG induced effects observed under propofol, similarly for sevoflurane, desflurane and isoflurane, are shown from an example case under a Total
Intravenous Anaesthesia (TIVA) procedure with propofol and remifentanil (opiod).
Figure 24.1 shows segments of EEG collected at different propofol Ce concentrations.
Clearly, the changes in the EEG activity regarded to the hypnotic drug concentrations
are not following a continuous transitional pattern [30, 32], but two major changes
regarded to the preservation of the statistical stationary properties of the signal,
1
which will require consequently two different major signal processing approaches.
The two main type changes are:
EEG changes preserving signal stationary. In these circumstances, there is
a continuous wise modification of the EEG signal spectral components related to
the drug concentration. For lower and moderate propofol concentrations the EEG
induced changes vary continuously without affecting signal stationary properties
(strict sense) in mean and variance. See some EEG example segments in Fig. 24.1a,
for several propofol Ce levels (Ce = 2.2 µg/ml and Ce = 3 µg/ml) respect to the awake
state, Ce = 0 µg/ml. As drug concentration increases the EEG becomes “smoother” in
time, with its corresponding translation in the frequency representation, shifting the
spectrum content to lower frequencies, diminishing the beta and gamma components,
see Fig. 24.1b. Additionally, at some concentrations, propofol induces strong periodic
oscillations around 10 Hz (α-band) and low frequencies, δ-band.
2 Similar effects
occur under other agents such as sevoflurane, desflurane and isoflurane.
Non-stationary time preservation induced patterns. At higher concentrations
of propofol, a strong disruption from the previous “smoothing“ transition pattern
takes place, see Fig. 24.1c for Ce = 5.8 µg/ml. High doses induce major temporal
changes in the EEG waveform, losing the time stationary properties since the signal
variance changes abruptly along time. The EEG activity alternates with periods of no
activity, low signal amplitude periods consistently less than 5 μV for more than 500
ms (clinical definition), a pattern known as Burst Suppression (BS). Traditionally,
the BS is quantified coarsely with the Burst Suppression Rate (BSR), defined as the
percentage of periods with no activity, isoelectric EEG, respect to the total time, a
measurement proportional to the drugs Ce [13]. As before, the BS takes place at high
concentrations under other GABA agents such as sevoflurane and isoflurane.
1 A stationary process (strict sense), is a stochastic process preserving the unconditional joint probability distribution across shifts in time. Under second-order statistics, this implies that mean and
variance do not change over time [41].
2 Despite, GA agents (such propofol) provoke a broad neurodepression, the activity in some frequency bands may increase due to the networks deviation from their natural dynamics, altering
(thalamocortical) loops and leading to synchronous (α and δ) activities [9].
373
This chapter focuses exclusively on this widely used hypnotic drugs that elicit
common well-known patterns on the EEG. Other drugs such as opioids operate under
very different mechanisms, and agents such as nitrous oxide, dexmedethomidine and
ketamine among others, possess an ambivalent effect, both hypnotic and analgesic,
all inducing distinct effects on EEG outside the scope of this chapter, see [29, 32]
for more details.
The two main EEG induced effects observed under propofol, similarly for sevoflurane, desflurane and isoflurane, are shown from an example case under a Total
Intravenous Anaesthesia (TIVA) procedure with propofol and remifentanil (opiod).
Figure 24.1 shows segments of EEG collected at different propofol Ce concentrations.
Clearly, the changes in the EEG activity regarded to the hypnotic drug concentrations
are not following a continuous transitional pattern [30, 32], but two major changes
regarded to the preservation of the statistical stationary properties of the signal,
1
which will require consequently two different major signal processing approaches.
The two main type changes are:
EEG changes preserving signal stationary. In these circumstances, there is
a continuous wise modification of the EEG signal spectral components related to
the drug concentration. For lower and moderate propofol concentrations the EEG
induced changes vary continuously without affecting signal stationary properties
(strict sense) in mean and variance. See some EEG example segments in Fig. 24.1a,
for several propofol Ce levels (Ce = 2.2 µg/ml and Ce = 3 µg/ml) respect to the awake
state, Ce = 0 µg/ml. As drug concentration increases the EEG becomes “smoother” in
time, with its corresponding translation in the frequency representation, shifting the
spectrum content to lower frequencies, diminishing the beta and gamma components,
see Fig. 24.1b. Additionally, at some concentrations, propofol induces strong periodic
oscillations around 10 Hz (α-band) and low frequencies, δ-band.
2 Similar effects
occur under other agents such as sevoflurane, desflurane and isoflurane.
Non-stationary time preservation induced patterns. At higher concentrations
of propofol, a strong disruption from the previous “smoothing“ transition pattern
takes place, see Fig. 24.1c for Ce = 5.8 µg/ml. High doses induce major temporal
changes in the EEG waveform, losing the time stationary properties since the signal
variance changes abruptly along time. The EEG activity alternates with periods of no
activity, low signal amplitude periods consistently less than 5 μV for more than 500
ms (clinical definition), a pattern known as Burst Suppression (BS). Traditionally,
the BS is quantified coarsely with the Burst Suppression Rate (BSR), defined as the
percentage of periods with no activity, isoelectric EEG, respect to the total time, a
measurement proportional to the drugs Ce [13]. As before, the BS takes place at high
concentrations under other GABA agents such as sevoflurane and isoflurane.
1 A stationary process (strict sense), is a stochastic process preserving the unconditional joint probability distribution across shifts in time. Under second-order statistics, this implies that mean and
variance do not change over time [41].
2 Despite, GA agents (such propofol) provoke a broad neurodepression, the activity in some frequency bands may increase due to the networks deviation from their natural dynamics, altering
(thalamocortical) loops and leading to synchronous (α and δ) activities [9].