24 Processed EEG as a Measure of Brain Activity During Anaesthesia
377
electrode-to-skin impedance is measured and included in the SQI calculation. The
EMG, which also acts on the SQI, is also provided as a monitor parameter since per
se since it is an indicator of potential significant biases on the DoA estimation as
well as a continuous indicator of the patient muscular relaxation under neuromuscular blocking agents (NMBAs). It is worth mentioning that EMG activity measured
from the EEG recording refers to the facial muscular activity which is not entirely
linked to the body muscular relaxation. Following the artifact rejection and the preprocessing stage, feature extraction is performed to characterize the EEG activity. At
this point any algorithm such as the ones in Table 24.2 extracts the necessary features
to be integrated into an index. The BS is also quantified at this step, commonly as
the percentage of time where the EEG is isoelectric, BSR. Despite the simplicity of
this measure, it works nicely with the anaesthetic agents under deep anaesthesia [6,
52], and it is easy to interpret and visually inspect by the anaesthesiologists. Anaesthesiologists generally try to avoid deep anaesthesia levels, where the BSR is high,
since lighter monitor-guided depth of anaesthesia level conducted to improved postoperative outcomes [8, 25, 28, 55]. Finally, the latest step integrates the extracted
features into an easy and fast to interpret scale correlated with: (1) the clinical assessment, e.g. scores like the Observer Assessment of Alertness/Sedation scale (OAA/S)
or Ramsay scale [32], and (2) the pharmacological information obtained from the
pharmacokinetic/pharmacodynamic models (PK/PD) models.
Figure 24.2b shows the time evolution of the propofol Ce under the Schnider
PK/PD model [48, 49] on a patient during GA and under remifentanil (analgesic).
The panel below shows the Density Spectral Array (DSA) for the whole intraoperative
procedure where it is possible to observe the characteristic effects of propofol on the
EEG. A strong elicited α-band activity with a decrease of β and γ activities respect
to the awake state. In lower panel, it is shown two simultaneously recorded Processed
EEG (pEEG) neuromonitoring indices, BIS and qCON, lower panel. Clearly, both
EEG indexes follow relatively well the Ce. The anaesthesia induction, in intravenous
and volatile anaesthesia, usually is performed inducing high concentrations in a short
time; a fast induction is generally more pleasant and less problematic in the hemodynamic stability. Independently of the anaesthesiologist procedure any monitor index
must be fast enough to track fast changes in the patient hypnotic state. Fast inductions
induce periods of BS, as it is shown in this example, with BSR reaching 32. After the
induction, the common practice is to keep the patient into a stable adequate balanced
anaesthesia level, until the end of the surgery.
Current example shows a novel index, qNOX
® (Fresenius Kabi, GmbH, Germany)
[18], linking the EEG to the patient probability of response to noxious stimulation.
This index was formulated, similarly to the qCON hypnotic index with those spectral
features of the EEG which integrated into a model produced the best correlation with
the remifentanil Ce (analgesic) given by the Mintos‘ PK/PD model [33] and the
response of the patients to noxious stimuli, in this case, different types of airway
intubation: laryngoscopy, LMA insertion and tracheal intubation. See [18, 19] for
more details.
377
electrode-to-skin impedance is measured and included in the SQI calculation. The
EMG, which also acts on the SQI, is also provided as a monitor parameter since per
se since it is an indicator of potential significant biases on the DoA estimation as
well as a continuous indicator of the patient muscular relaxation under neuromuscular blocking agents (NMBAs). It is worth mentioning that EMG activity measured
from the EEG recording refers to the facial muscular activity which is not entirely
linked to the body muscular relaxation. Following the artifact rejection and the preprocessing stage, feature extraction is performed to characterize the EEG activity. At
this point any algorithm such as the ones in Table 24.2 extracts the necessary features
to be integrated into an index. The BS is also quantified at this step, commonly as
the percentage of time where the EEG is isoelectric, BSR. Despite the simplicity of
this measure, it works nicely with the anaesthetic agents under deep anaesthesia [6,
52], and it is easy to interpret and visually inspect by the anaesthesiologists. Anaesthesiologists generally try to avoid deep anaesthesia levels, where the BSR is high,
since lighter monitor-guided depth of anaesthesia level conducted to improved postoperative outcomes [8, 25, 28, 55]. Finally, the latest step integrates the extracted
features into an easy and fast to interpret scale correlated with: (1) the clinical assessment, e.g. scores like the Observer Assessment of Alertness/Sedation scale (OAA/S)
or Ramsay scale [32], and (2) the pharmacological information obtained from the
pharmacokinetic/pharmacodynamic models (PK/PD) models.
Figure 24.2b shows the time evolution of the propofol Ce under the Schnider
PK/PD model [48, 49] on a patient during GA and under remifentanil (analgesic).
The panel below shows the Density Spectral Array (DSA) for the whole intraoperative
procedure where it is possible to observe the characteristic effects of propofol on the
EEG. A strong elicited α-band activity with a decrease of β and γ activities respect
to the awake state. In lower panel, it is shown two simultaneously recorded Processed
EEG (pEEG) neuromonitoring indices, BIS and qCON, lower panel. Clearly, both
EEG indexes follow relatively well the Ce. The anaesthesia induction, in intravenous
and volatile anaesthesia, usually is performed inducing high concentrations in a short
time; a fast induction is generally more pleasant and less problematic in the hemodynamic stability. Independently of the anaesthesiologist procedure any monitor index
must be fast enough to track fast changes in the patient hypnotic state. Fast inductions
induce periods of BS, as it is shown in this example, with BSR reaching 32. After the
induction, the common practice is to keep the patient into a stable adequate balanced
anaesthesia level, until the end of the surgery.
Current example shows a novel index, qNOX
® (Fresenius Kabi, GmbH, Germany)
[18], linking the EEG to the patient probability of response to noxious stimulation.
This index was formulated, similarly to the qCON hypnotic index with those spectral
features of the EEG which integrated into a model produced the best correlation with
the remifentanil Ce (analgesic) given by the Mintos‘ PK/PD model [33] and the
response of the patients to noxious stimuli, in this case, different types of airway
intubation: laryngoscopy, LMA insertion and tracheal intubation. See [18, 19] for
more details.