Neurophysiological Signs of Brain Death (eeg & Evoked Potentials): Are They Safe & Reliable?*

ElectroEncephaloGraphy (EEG) was introduced by Hans Berger in the 20s and 30s. This technique allowed for the first time to record from the scalp the electrical activity of a living brain. The signal picked-up via surface electrodes reflects the sum of the post-synaptic potentials of the underlying cortical neurons. In order to increase the signal-to-noise ratio (neurons produce very small electrical signals in the order of microVolts, that is a million times smaller than 1 Volt!) differential amplifiers were created – that is amplifiers which make an electronic subtraction of the signals entering grid 2 from those entering grid 1 at the same instant (if they are of the same polarity they go therefore to 0, while if they are of opposite polarity they double in amplitude) – with the need of having two different electrodes (one exploring and one referential) for each explored brain region which corresponds to one recording channel. In order to cover simultaneously the whole brain surface, multichannel EEG machines have been developed up to 250 channels of the modern ones utilized for research purposes. However, for clinical applications, 8 to 16 recording channels are routinely employed. Since pioneering days, it was shown that the EEG signal is quite sensitive to the state of the neural cells producing it: moreover, it was shown that complete deprivation of blood flow provokes in a few minutes the rapid deterioration of the EEG signal, followed by electrical failure and cell death with complete electrical silence. In the 50s French researchers clearly demonstrated that in comatose patients with complete brain destruction the EEG was isoelectric or flat. When this EEG pattern was present for a sufficiently prolonged time, prognosis for survival was unfavourable. In the following years the concept of brain-death clinical condition was progressively introduced and it was demonstrated that – when present – it is invariably associated to an isoelectric, flat EEG pattern.