EEG Signature(s): Propofol
By: Alex Papangelou M.D. & Priscilla Nelson M.D.
Alex Papangelou M.D
The administration of anesthetic hypnotic drugs leads to unconsciousness and the development of a predictable electroencephalogram (EEG) signature, which is measured at the frontotemporal regions of the brain. The purpose of this article is to highlight the drug signature of propofol on the EEG color density spectral array (DSA). This will be the first of several highlighted drugs with a focus on the EEG signature to improve understanding and EEG utilization in the operating room. A brief drug explanation will be followed by a case to demonstrate the visualization of the signature.
EEG measures electrical potentials transmitted through the scalp and generated by cortical postsynaptic currents. The raw EEG is composed of many different frequencies. To quantify the power of the different frequencies, we use the DSA.
Most of the EEG activity with anesthetics occurs in the 1-30Hz range. This is categorized into 5 broad categories: Gamma (>30Hz), Beta (14-30hz), Alpha (8-14 Hz), Theta (4-8Hz), and Delta (0.5-4Hz).
Propofol potentiates γ-Aminobutyric acid (GABA) receptors throughout the brain and spinal cord to enhance inhibition and leads to a state of thalamocortical synchrony that is associated with unconsciousness. Unconsciousness under propofol is characterized in the EEG by alpha (8–12 Hz) oscillations that are coherent across the frontal cortex, delta (1–4 Hz) oscillations, and high amplitude incoherent slow (0.1–1Hz) oscillations.
Case example: 36-year-old female underwent a hysteroscopy, dilation and curettage, polypectomy, and endometrial ablation. The case started with a propofol infusion and a natural airway, and after several episodes of patient movement a laryngeal mask airway (LMA) was inserted, and the anesthetic was changed to sevoflurane.
In the spectrograms, time is listed on the x-axis and frequencies are displayed on the y-axis. The power of the frequencies is indicated by color on a decibel (dB) scale. Red indicates higher power, with blue indicating lower power. For tracking brain states under anesthesia, we utilize the frontal EEG.
Figure 1: 8:20 – 8:40
Anesthesia was initiated. In Figure 1, From 8:20 to 8:30 you see the classic propofol EEG signature with two red bands on the spectral density corresponding to increased activity at alpha ~10-12Hz and delta 0.5-4hz. This is followed by two periods where the alpha activity diminishes (8:32, 8:36). At these instances there is patient movement and two boluses of propofol were administered.
Figure 2: 8:35 – 8:55
In Figure 2, from 8:35-8:50 there is diminished alpha activity overall when compared to Figure 1. There are three distinct periods where there the alpha activity is almost completely absent and corresponds to patient movement/stimulation. At 8:50 a propofol bolus is given and an LMA is inserted. On the spectral analysis, this is noted by a period of decreased power across the spectrogram and then a resumption of the classic propofol signature.
Alpha dropout between 8:43 and 8:48 likely represents pain, and this will be addressed in future articles.
Figure 3: 8:50 – 9:07
Figure 3 demonstrates the period following the bolus of propofol and LMA insertion at 8:50. Although the anesthetic maintenance was transitioned to sevoflurane (propofol off), the propofol signature was maintained as expected, given the large amount of propofol that had been administered. Note the signature of propofol with the red bands at alpha and delta frequencies.
Figure 4: 8:56 – 9:13
Sevoflurane turned off followed by emergence at 9:10 in Figure 4. At emergence, you see the alpha power diminishes with increased power at the higher frequencies.
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