Deep hypothermic circulatory arrest · Lobeworks/17

Deep hypothermic circulatory arrest is a surgical method in which the body is cooled to around 18 °C and the heart and circulation are stopped for up to about half an hour, and it is the clearest demonstration that the brain has two levels: its electrical activity can be switched off completely while its structure, and

Deep hypothermic circulatory arrest. Deep hypothermic circulatory arrest is a surgical method in which the body is cooled to around 18 °C and the heart and circulation are stopped for up to about half an hour, and it is the clearest demonstration that the brain has two levels: its electrical activity can be switched off completely while its structure, and with it memory and identity, survives.

Cooling slows the brain's metabolism enough that it can go without blood for tens of minutes. The EEG goes flat (at 18 °C in about 60 % of patients, in nearly all at 12.5 °C; centres confirm the silence on the EEG before stopping the pump), no spike is fired, no transmitter released. Rewarmed, patients wake as themselves, remembering who they are and what they knew. What carried them across the silence was anatomy: which neurons connect to which (the connectome), how strong each synapse is (synaptic plasticity), the size of each spine (structural plasticity), the traces of past experience (engrams).

Activity needs energy every second; structure only needs to be kept. Spikes, release and network rhythms run on the gradients the pump restores, so they stop within seconds without blood; synapses and connections are matter, which cold preserves for a while.

The window is short and costly. Metabolism only slows, so damage grows with time: in children, outcomes worsened beyond about 41 minutes of arrest. Warm, the same silence kills within minutes, as the ischaemic cascade shows.

Memory is kept in the wiring. Sleep and anaesthesia make the same point more gently: the activity changes and comes back while the wiring stays; the running activity is the brain's present, the wiring is what it keeps.

The brain can be switched off and on again because what it knows lives in its wiring, which outlasts the current.

Activity is the music; the connections are the score that survives when the music stops.

Questions: If the brain's electrical activity is stopped for half an hour, what keeps the memories? The structure. In deep hypothermic circulatory arrest the body is cooled to around 18 °C, the circulation is stopped and the EEG goes flat, and patients rewarmed afterwards wake knowing who they are. What crossed the silence is anatomy: which neurons connect to which, how strong each synapse is, the spines that carry them, the sparse ensembles that hold each memory. Activity needs energy every second; structure can be kept for a while, and cold slows its decay.