Corpses make noise

Question to the brain

Prof. Dr. med. Gerhard Schneider, Director of the Clinic for Anaesthesiology, University of Witten / Herdecke, Helios Clinic Wuppertal, and spokesman for the Neuroanesthesia Working Group at the German Society for Anesthesiology and Intensive Care Medicine:

Among other things, hearing serves as a warning system: if we are in danger, a sound can wake us up from deep sleep so that we can escape. You also notice this when you sedate or anesthetize someone: the hearing function is retained for a relatively long time. We address the patient at the beginning of anesthesia, then again louder - and sometimes a patient actually still reacts. So we know that we need to deepen the anesthesia before the operation. This test is also sufficient in everyday clinical practice. But you can check even more precisely: To do this, you measure the brain waves and then look for so-called acoustic evoked potentials, AEP for short, in the EEG.

Usually, brain waves show spontaneous electrical activity. Evoked potentials are not spontaneous, but reactions to a stimulus - acoustically evoked means “caused by noises”.

The anesthetized patient is put on headphones and an acoustic stimulus is played, for example a click. If the patient can still hear and process this, the brain waves change. But I can't see that with the naked eye. Therefore the sound has to be repeated dozens, hundreds of times and then the brain waves are averaged by a computer. Because the spontaneous electrical activity is always different, the mean value approaches zero. For this purpose, pointed spikes and waves are visible in the EEG, which occur at certain time intervals after the acoustic stimulus. These are the acoustically evoked potentials. And these can then be assigned to specific locations. The early component comes after a few milliseconds and it indicates the brain stem, which is quite close to the ears. A little later there are AEP components that rather indicate that other areas of the cerebral cortex are gradually activated: first the stimulus processing, then the conscious processing of the content. If one no longer recognizes such acoustically evoked potential in the EEG, then the person no longer hears anything. This is the case with anesthesia - and can happen with cardiac arrest.

In the event of a cardiac arrest, ie "clinical death", the brain is no longer supplied with enough oxygen and also no longer with enough sugar. Then the nerve cells stop functioning. The remaining energy that the nerve cells still have is used to ensure their survival. And so complex brain functions are stopped and you become unconscious. But hearing can still work. At least you can hear something, even if you no longer necessarily perceive its meaning. And only for a short time. In this short period of time, “clinical death” is reversible: this is where resuscitation measures must be taken. Because after two to five minutes, the nerve cells begin to die. So if you are not successfully resuscitated quickly, there is a risk of permanent brain damage up to and including brain death and biological death. Hearing is not what dies last.

Recorded by Franziska Badenschier