EEG Equipment

Fig. 1

Schematic of typical setup for ICU EEG monitoring. The headbox and camera/audio recording equipment are located in the patient room. A bedside monitor is also located in the room. The actual EEG machine may be located in the room (dashed lines) or at a networked location. Data is stored on an EEG server which allows for local and remote review of data as well as archiving

Technical Requirements of Amplifiers

The technical specifications of the individual amplifiers are the same as that for routine EEG/scalp recording: full-scale input range of ±2 mV. Filters available (as a combination of analog/hardware filters and digital/software filters) should include high-pass (low frequency) filters of 0.1–0.3 Hz and low-pass (high-frequency) filters of 35–100 Hz and notch filters (60 Hz). The input impedance of the amplifier should be at least 10 MΩ with a common-mode rejection ratio (CMRR) of at least 100 dB. The analog-to-digital converters should have an input range ±1–2 V with a sampling rate of at least 200 Hz per channel and a resolution of at least 12 bits (>16 bits preferred). Commercially available systems on the market today will typically exceed these requirements.

In order to record a full EEG, a minimum of 16 channels with system reference is needed. Most systems on the market currently will have 32 or more channels; additional channels are often useful for recording from other electrode locations on the scalp as well as cardiac rhythm and electromyographic (EMG) activity from selected muscle groups (to help correlate with artifact or subtle clinical activity). In addition, DC-coupled input channels may be useful for recording other physiological parameters such as oxygen saturation, respirations, intracranial pressure, and blood pressure. Such parameters may be useful in interpreting changes in EEG, especially those associated with changes in cerebral perfusion or medication effects. Most equipment today has eight or more DC-coupled input channels, including some that are specifically configured for oxygen saturation or end-tidal CO₂ measurement.

The capability to simultaneously record video and audio that is synchronized with the EEG data is extremely useful in ICU EEG monitoring. It can be useful for correlating subtle clinical activity with EEG patterns. For example, recognition of subtle twitching of the face/arm in combination with a periodic pattern on the EEG may suggest an ictal event, whereas without the clinical activity it may not be possible to comment whether the EEG activity is ictal or not. Another role for video is in the recognition of artifacts. For example, artifacts due to ventilators, suctioning, bed percussion, and others can be much more easily recognized with video. Lastly, changes in the background EEG or events may be related to stimulation (e.g., stimulus-induced rhythmic, periodic or ictal discharges, SIRPIDs [2]); review of associated video is the only way to distinguish these from spontaneous activity.

ICU patients should be considered as high risk for potential injury from electrical equipment due to the common presence of indwelling catheters/lines and connection to multiple pieces of equipment. Thus, it is essential that a common ground be used for all equipment attached to a patient. Also, the equipment must be routinely inspected by clinical engineering to ensure that leakage currents are low, ideally below 10 μA [3].

Hardware Requirements

Physical Configuration of Equipment

The actual physical configuration of the EEG equipment is another consideration. In the past, most equipment was portable, mounted on carts, and wheeled into the desired location. Cameras and microphones were typically mounted on poles in the cart. The main advantage to such equipment is flexibility; it can be taken essentially anywhere in the hospital and used for multiple purposes, including for routine EEG and long-term video monitoring. The main disadvantages include suboptimal and variable placement of equipment in the room, especially with respect to video and audio recording. Another is the physical demand put on both technologists and the equipment itself. Lastly, extra network connections are often not available, especially in older ICUs, making the study offline and requiring repeated downloads of data for review or physically going to the bedside to review data.

Fixed-/wall-mounted units have the main disadvantage of a lack of flexibility/availability – they can only be used in the room where they are located. However, their location can be optimized for higher quality video/audio and EEG recording. Also, the placement is typically sturdier, with less danger of damage to the equipment or accidental disconnection. A hybrid approach involving permanently mounted cameras and microphones in each ICU room with portable amplifiers brought in for the recordings may be ideal. In this approach, software/networking is used to select the specific camera to associate with a given amplifier. While a dedicated camera/microphone is still needed for each ICU room, the number of amplifiers needed is reduced.

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