This is your brain on anesthesia: New light shed on how brain reacts during anesthetic induction and emergence

"The process of anesthetic state transitions -- such as loss of consciousness or return of consciousness -- is still unclear," said George A. Mashour, M.D., Ph.D., from the Department of Anesthesiology at the University of Michigan. "Certain theories suggest that the process of going in and out of consciousness is gradual, while others suggest that we return from sleep or anesthesia in a sudden flip-flop of behavioral states."

Resolving the question of how we go in and out of consciousness would help anesthesiologists be able to detect or predict return to consciousness in the O.R. and would aid in the development of more sophisticated brain function monitors.

Specifically, Dr. Mashour and his research team isolated two brain network properties in their study: the structure of brain networks and the connection strength of brain networks. Loss of consciousness was consistently associated with sudden structural disruption of brain networks, rather than with the strength of connections. Recovery of consciousness, though, was often associated with a sudden increase in the strength of connections after the structural changes had returned to normal.

Dr. Mashour and his colleagues concluded that network structure might be more reliable in monitoring changes in the anesthetic state because connection strength appeared to have a variable behavior, showing two distinct responses in the population studied.

"Our data also support recent evidence suggesting that anesthetic induction and emergence are not mirror images of one another, as previously thought, but rather have a distinct underlying neurobiology," said Dr. Mashour.

In the study, 20 healthy young males were given I.V. infusions of the anesthetic propofol, while their brain activity was recorded with electroencephalography, or EEG.

The researchers also investigated how propofol affects different sections of the brain, particularly the frontal and parietal lobes. They found that network structure was maintained in the frontal region, but significantly disrupted in the parietal region. Connection strength decreased somewhat in the frontal region, but was more significantly affected in the parietal region.

"It has been suggested that the parietal lobe is a critical point of integration for neural information and that its disruption is important for anesthetic-induced consciousness," said Dr. Mashour. "Our data support this hypothesis."