Amazing experience so unlike sleep

 

MEDICAL MATTERS:Studying anaesthesia effects may help coma patients, writes MUIRIS HOUSTON

THE INTRAVENOUS line went in at the back of my hand as I chatted with the anaesthetist and his assistant. The syringe was then attached to the cannula. Because I prefer to look away I wasn’t quite sure when the anaesthetic agent began its journey into my vein. I was just about to ask if they had started when I find myself in the recovery room with a kind person saying: “Hi Muiris, the operation is over now.”

It’s an amazing experience that is so unlike sleep. You go under so fast as your brain is placed into a state that is, essentially, a reversible coma.

A fully anaesthetised brain is much closer to the deeply unconscious low-brain activity seen in coma patients, than to a person asleep. Essentially, general anesthesia is a coma that is drug-induced, and, as a consequence, reversible. The states operate on different time scales – general anesthesia in minutes to hours, and recovery from coma in hours to months to years, if ever. Understanding that these states have more in common with each other than differences – that they represent a continuum of activity with common circuit mechanisms being engaged across the different processes of awakening from sleep or emerging from coma or general anesthesia is central to modern research about anaesthesia.

Knowing more about the brain circuit mechanisms may also help researchers develop therapeutic agents to “tweak the circuits as needed, to help us in the areas where we don’t do well, such as abnormalities of sleep and, especially, emergence from a coma,” says professor of neuroscience at Weill Cornell Medical College Dr Nicholas Schiff. “And while use of general anesthesia is an incredibly safe technique, it can have effects on the elderly, such as slower recovery time and impaired cognitive function afterwards.”

Specific brain areas involved in these circuits include the cortex, which is made up of layers of neural tissue at the outer edge of the brain, and the thalamus, a ball of neural tissue at the centre of the brain. Also part of the circuit is the basal ganglia, within the front of the brain, which is used to control certain actions. It does this in part by setting up two feedback loops. One is a negative feedback release on behaviour, and that part of the circuit is active when overall brain activity is reduced.

The second feedback loop, however, releases the brake imposed by the first, the researchers say. Certain drugs, such as propofol, a powerful general anaesthetic, can trigger that loop to function, producing what is known as “paradoxical excitation”. Most people given propofol become agitated and confused shortly after falling unconscious until the brake is switched back on – giving way to sedation and deeper sleep. There is another phenomenon – emergence delirium – that results from this circuit. So when bringing a person out of general anesthesia, the brain is woken up enough to be active, but it is not coherent or organised, which can explain the slower recovery time seen in some patients.

It is these two areas – losing consciousness and returning to consciousness – that researchers believe they might be able to target to provide better therapies for emergence from coma, and to develop general anesthesia with fewer side effects. And it is by studying general anesthesia that researchers will likely make progress in understanding all three states of mind, according to Schiff. Because coma patients each have individualised damage to their brains due to injury or stroke or haemorrhages, studying recovery from general anesthesia may help identify ways of helping these patients.

“Understanding this circuit will help us understand the relationship of brain function to consciousness in general – what it is, how it is produced, and what the variety of brain states truly are,” says Schiff. It may also help further our understanding of anaesthetic awareness, a rare but upsetting event. In my recent experience I could not recall the words and actions of anyone during my operation. Unlike going to sleep, being anaesthetised was an abrupt process. Emerging from the deep of Morpheus was almost as sudden. What happened in between will thankfully remain a mystery.

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