General Anesthesia

[Deslaar]

I posted this at PF but I haven't much of a response so I thought I'd post it here as well.

 

Could someone explain to me the pharmacological and biological mechanisms involved in general anesthesia?

 

It would appear that under general anesthesia consciousness is completely "turned off". To what extent is brain function reduced? Does general anesthetic target particular areas of the brain?

 

Thanks in advance.

 

BTW and on a philosophical note, is being under a general anesthetic indistinguishable from and, in a temporal and cognitive sense, equivalent to death?

[Glider]

General anaesthetics usually involve a cocktail of drugs, each serving a different function. Basically, GA will include a muscle relaxant (similar to curare) which is why patients need to be intubated and put on a respirator (the muscles controlling respiration are also knocked out).

 

The other main component of GA induces unconsciousness (and thus prevents the experience of pain, arguably the most important part). Generally, it does this by universally decreasing brain activity (i.e. no particular areas are targeted). The mechanism of action is that it opens potassium gates, allowing potassium to flow out of neurons of the central nervous system, effectively hyperpolarising them and decreasing the probability of action potentials. Thus the neurons become unresponsive to most stimuli.

 

This mechanism of function also means that level of unconsciousness can, to a degree, be controlled, from light anaesthesia (barely responsive) to deep anaesthesia (not at all responsive), depending on the reasons for giving GA (short, simple procedures or long complex ones). An interesting thing is that for neurosurgery, GA is not used, only a local anaesthetic. The patient has to be awake to report the effects of what the surgeon is doing. It's the only way the surgeon knows exactly where he/she is.

 

On the philosophical note, I'd have to say that going under a GA is pretty much like dying. You can feel the 'numbness' creeping over you. It's an inexorable loss of awareness and 'being', that you can't fight, and that won't stop. It sounds unpleasant, but it really isn't. I quite like it.

 

Is being under a GA like being dead? Who knows? Cognitively, it's like being in a dreamless sleep, and temporally, well, you wake up unaware of what time has passed, and with no memory of events during the period of unconsciousness, the same as when you wake up from natural sleep (except more groggy and you tend to be in and out for a bit). Residual GA can persist in your system for quite a while. You can taste it in your mouth for hours afterwards as it gasses off and you breathe it out. However, the first night after GA, you can expect to sleep well, and have some excellent dreams.

[Deslaar]

Thanks Glider, I appreciate your response. That's pretty much what I thought.

[Radical Edward]

I didn't realise the mechanisms were that well understood Glider, particularly with there being such a wide range of substances that cause GA, from the noble gasses to the more familiar alcohol and more complicated chemicals.

Going under though is wierd, I have experienced it many times, as I had alot of trouble with my ears when I was younger, and also a strange growth on the side of my head that had to be removed... enough about my medical history now, one interesting thing is that I always had a craving for buttered toast afterwards, and still get that if I drink too much. heh.

[Deslaar]

Originally posted by Radical Edward

one interesting thing is that I always had a craving for buttered toast afterwards, and still get that if I drink too much. heh.

 

That is interesting. That sort of outcome would imply some sort of learning process had occured during the GA which conflicts with the contention that all cognitive processes are suspended.

[Radical Edward]

I think it is something more to do with the chemical effect on the brain maybe. as Glider said, Residual GA exists for a while after recovery.

[Glider]

Radical Edward: You're right, there are many different anaesthetics; halothane, cyclopropane etc. etc., and they all have slightly different mechanisms of action. A general hyperpolarisation is one of the more common mechanisms (e.g. ether and choloroform both result in hyperpolarisation), and most modern GAs try to do what they did, more controllably and without the harmful effects.

 

It's interesting what Deslaar said about possible learning processes under GA though. There is in fact evidence that learning at some level can take place under GA. It's one of the pieces of evidence for the contention that preattentive processes outweigh conscious action. Others are such phenomena as blind-sight, automaticity and the automatic evaluation effect.

 

It's all very interesting stuff, and the basic contention is that conscious awareness (volitional cognition) is in fact merely a kind of 'cognitive hitch-hiker' doing little more than moderating and explaining actions and behaviours that are initiated on a non-conscious level. E.g., from this perspective, when we say "I decided to do that", what we are really doing is explaining an action the conscious part of ourselves observed us doing.

 

It boils down to a deterministic approach which implies that we are nowhere near as in control as we thought. Nevertheless, there is a large (and growing) body of empirical evidence supporting it.

[Radical Edward]

so it is more like conscious justification than conscious action?

[blike]

Going under though is wierd, I have experienced it many times

 

Tell me about it. I couldn't seem to find the floor last time I woke up from GA. I remember walking bent over with my hands feeling for the ground all the way out ot the car. Actually, I don't think I was intubated, so there probably wasn't a muscle relaxant. I was only getting my molars removed.

[blike]

GA will include a muscle relaxant (similar to curare) which is why patients need to be intubated and put on a respirator (the muscles controlling respiration are also knocked out).

 

Why doesn't this affect the heart as well?

[Glider]

Originally posted by Radical Edward

so it is more like conscious justification than conscious action?

 

That's the theory, yeah.

 

You can see a good example of this in hypnosis. If the hypnotists suggests to the participant that they get down and crawl about on all fours, the participant will do so. If you then ask the person why they are doing that, they will come up with some rational explanation, e.g. "I'm looking for my contact lense" or "I dropped my pen". Our minds don't cope well with randomness. It actively searches for pattern and reason. So (the theory goes) when we observe ourselves doing something, our minds take current circumstances into account, and provides us with a rationale, which we can then use to justify what we observed ourselves doing.

 

Originally posted by Blike

Tell me about it. I couldn't seem to find the floor last time I woke up from GA. I remember walking bent over with my hands feeling for the ground all the way out ot the car. Actually, I don't think I was intubated, so there probably wasn't a muscle relaxant. I was only getting my molars removed.

That sounds like intravenous valium to me. That doesn't paralyse you, it just puts you in a shallow state of unconsciousness. Two reasons I think it was IV valium 1) it's commonly used by dentists for removal of wisdom teeth etc.. 2) That rubber floor thing...hehehe...unique to valium. Your legs don't quite seem long enough to reach the floor somehow. That's how I remember it anyway.

 

Why doesn't this affect the heart as well?

The heart is autonomous. It will continue beating regardless of paralysis. In fact, if you provide it with oxygenated Wringer's solution, it will continue beating even if you remove it from the body. All other motor functions are prone to paralysis as muscle relaxants affect motor neurone transmission. The heart doesn't depend on any motor nerves to function (the vagus nerve only controls rate).