Consciousness is not lost during anaesthesia

You probably know what it’s like to have an anaesthetic. You seem to lose consciousness during anaesthesia and don’t recall what went on when you wake up. But the question on most scientists’ minds is if consciousness is fully lost during anaesthesia or if it exists in the brain — but in an altered state?

To answer this question, a group of scientists conducted a study that monitored the changes caused by anaesthetics using electroencephalogram (EEG) and positron emission tomography (PET).

The responses in the EEG showed that the brain cannot differentiate between normal and weird sentences when under anaesthesia.

Forty-seven healthy voluntary participants were randomised to be anaesthetised with either dexmedetomidine or propofol. The drugs were administered with computer-driven target-controlled infusions until the participants nearly lost responsiveness. The participants could be woken up from this state with light shaking or a loud voice, without changing the drug infusion. An attempt was then made to arouse the participants to regain responsiveness while keeping the drug infusion constant. Immediately after regaining responsiveness, the participants were asked about their experience during the anaesthesia period.

Eighteen (78 per cent) and 10 (42 per cent) of subjects were aroused during the constant drug infusion in the dexmedetomidine and propofol groups, respectively. Almost all of the participants reported having dream-like experiences sometimes mixed with reality.

During anaesthesia, the participants heard Finnish sentences — half of which ended as expected (congruent) and the other half ended in an unexpected (incongruent) word. Under normal circumstances when a person is awake, the incongruent sentence will trigger the brain to process the meaning of the sentence and the word — which shows up as a response in the EEG. But in this case, the responses in the EEG showed that the brain cannot differentiate between normal and weird sentences when under anaesthesia. When dexmedetomidine was used, the congruent words created a significant response, indicating that the brain was trying to interpret the meaning of the words. However, after the participants woke from the anaesthesia, they did not remember the sentences they had heard. The results were the same with both drugs.

Next, unpleasant sounds were played during the anaesthesia. When the participants woke up, the sounds were played again. The participants reacted quicker to these sounds than to new sounds which they had not heard before. Participants who were given dexmedetomidine also recognised the sounds better, even though they could not recall them spontaneously.

The study shows that the brain processes sounds and words, even though the participants don’t recall this when they wake up. It seems that consciousness is not fully lost during anaesthesia, but it is sufficient to just disconnect the patient from the environment.

Source: Anesthesiology