The IFCN proposes a unified electrode nomenclature and electrode placement, called the 10–20 system (Klem et al. 1999), which includes 21 electrodes providing a good scalp coverage. This international and widely adopted system should be used on every scalp EEG in the ICU. The electrodes are named with a letter, representing the anatomical region (Fp = frontopolar, F = frontal, C = central, P = parietal, T = temporal, and O = occipital), and a number (even numbers on the right and odd numbers on the left; midline electrode is called z (zero)) (Fig. 2.2). The system is called 10–20 because each electrode is separated with 10 or 20 % of an anatomical distance.

The guidelines edited by the IFCN provide detailed methods of the electrode placement based on anatomical landmarks (Klem et al. 1999). The first step is to place the midline chain of electrodes, Fpz, Fz, Cz, Pz, and Oz (Fig. 2.3), by measuring the distance between the nasion and the inion through the vertex. Of note, Fpz and Oz positions will not be covered by an electrode. The next step is to measure the coronal distance from the left preauricular point to the right one through Cz. This will provide the position of T3 (also named T7), C3, C4, and T4 (also named T8) (Fig. 2.4). Then, in order to obtain the position of Fp1, F7, T5 (also named P7), and O1 on the left and Fp2, F8, T6 (also called P8), and O2 on the right, a circumferential measurement of the head through Fz, T3, Oz, and T4 should be obtained (Fig. 2.5). Finally, F3 should be placed at the intersection of Fp1–C3 and F7–Fz, F4 at the intersection between Fp2–C4 and Fz–F8, P3 at the intersection of C3–O1 and Pz–T5, and P4 at the intersection between C4–O2 and Pz–T6. A cerebral or extracerebral reference electrode, a ground, and an electrocardiogram should be placed eventually.

The types of montages are highly variable in every center. The same montages used for routine EEG should be used. Some bipolar and referential arrangements are proposed by the ACNS (American Clinical Neurophysiology 2006a) and can be easily applied. Of note, the most popular is the traditional longitudinal bipolar montage (also known as “the double banana”) (Fig. 2.6). The use of the 21 electrodes provided by the 10–20 system offers good scalp coverage, but, in selected cases, montages with fewer electrodes can be used. To determine brain death, for example (this only applies in certain countries), the distance between electrodes should be at least 10 cm. The ACNS recommends a bipolar montage using 10 electrodes, for example, F7-T5, F8-T6, F3-P3, F4-P4, and Fz-Pz (American Clinical Neurophysiology 2006b).

Each electrode’s wire will be plugged in a jackbox (Fig. 2.9). Many devices often surround the ICU beds; therefore, a small jackbox that can be placed near the patient’s head is recommended. The jackbox should also be fixed and possibly sealed into a waterproof packing to avoid damage (Herman 2013). The jackbox contains the amplifiers; according to the IFCN (Ebner et al. 1999), at least 23 connectors (amplifiers) are required for clinical practice. However, larger capacity input systems are available (for higher special resolution and extracerebral electrodes).

EEG amplifiers have two functions: discrimination and amplification (Fisch 1999). Discrimination is the ability to amplify the difference between two electrical potentials: the amplifier will subtract the signal of two different input EEG electrodes, between two adjacent scalp electrodes on a bipolar montage or between one electrode and a common reference for a referential montage, for example. Of note, the subtraction implies that signals common to both subtracted electrodes will be suppressed (this may also give rise to the “electrical bridge,” when an analogous signal between two electrodes is seen as a flat line in a bipolar montage). Artifacts that are common to both electrodes will also be suppressed. In this way, an amplifier can sort out true EEG brain signals and artifacts. This discrimination power can be measured with the “common mode rejection ratio”: the higher this ratio, the higher the discrimination power. With current EEG machines, the specified common mode rejection ratio should be at least >80 dB, typically 100 dB (Ebner et al. 1999). Then, the amplifier will increase the potential difference; this effect is called “gain” or sensitivity. A typical setting for EEG is 7–10 uV/mm.

Filters are used to exclude frequencies that are not generated by the brain. In clinical practice, frequencies of interest are between 1 and 30 Hz. Therefore, the low-frequency filter (synonymous with high-pass filter) should be set at 0.5–1 Hz and the high-frequency filter (or low-pass filter) at 70 Hz. A notch filter of 50 or 60 Hz (depending on the country) for power line artifacts should be used when needed. Of note, it is advisable to start the recording without the notch filter, as electrodes with loose contact to the scalp will tend to show a prominent notch artifact.