Effects of common anesthetic agents on electroencephalograms

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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Effects of common anesthetic agents on electroencephalograms

R. Doris Wang, MD

The electroencephalogram (EEG) is a depiction of the electrical activity occurring at the surface of the brain. This activity appears on the EEG recording as waveforms of varying frequency and amplitude measured in microvolts. Four categories of frequencies are the most clinically relevant (Table 43-1, Figure 43-1).

Table 43-1

Categories of the Most Clinically Relevant Electroencephalographic Frequencies

Wave Pattern	Frequency Range (Hz)	Level of Consciousness
Delta	0.5-4	Deep sleep
Theta	4-8	Drowsiness (also first stage of sleep)
Alpha	8-14	Relaxed but alert
Beta	14-30	Highly alert and focused

Figure 43-1 Examples of alpha, beta, theta, and delta electroencephalographic frequencies.

Intraoperative monitoring that employs raw EEG data is one of the traditional methods used to assess the adequacy of cerebral perfusion. Many electrical changes associated with increased doses of intravenously administered or inhalation anesthesia can mimic EEG changes associated with inadequate cerebral perfusion (i.e., brain ischemia).

Inhalation anesthetic agents

Inhalation anesthetic agents consistently increase slow-wave EEG activities. At doses lower than minimum alveolar concentration (MAC) levels, alpha rhythm disappears and the voltage of beta activity increases and becomes more widespread. As the end-tidal concentration of the inhalation agent increases, the EEG progressively increases in amplitude and decreases in frequency. During deep-inhalation anesthesia, burst suppression occurs. EEG changes vary somewhat depending on the specific anesthetic agent used, whether the agent is used alone or in combination with other anesthetic agents, whether hyperventilation is employed, and the patient’s age. In addition, patients with increased baseline delta (slow-wave) activity resulting from underlying intracranial pathology can show relatively little or no change in the EEG with increasing anesthesia depth, but burst suppression may eventually become evident.

Halothane

Halothane produces 10-Hz to 20-Hz activity that persists until the patient loses consciousness. Theta and delta activity increase in a dose-dependent manner up to moderate doses of halothane (e.g., 1.5 MAC), at which point there is a modest decrease in theta rhythm. At 1 MAC of halothane (0.75%) and normocarbia, the dominant EEG frequencies are between 10 and 15 Hz. At 2 MAC, the predominant background rhythm is 7.5 Hz; at 2.5 MAC, it is 6 Hz. The use of halothane for EEG burst suppression is not practical because of the cardiovascular depression associated with the use of high-dose halothane (3-4 MAC).

Isoflurane

In patients receiving 0.5% isoflurane with 70% N₂O and O₂, 15-Hz to 20-Hz activity of low to moderate voltage is evident. With 2% isoflurane and O₂ or 1% isoflurane with 70% N₂O and O₂, the EEG exhibits 2-Hz to 6-Hz moderate-voltage to high-voltage activity with superimposed 10-Hz to 15-Hz activity. A burst-suppression pattern or total electrical silence usually occurs with isoflurane concentrations of greater than 2% in O₂ or 1.5% in 70% N₂O.

Sevoflurane

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