Narcotic potency of helium

[Basil Mavros]

We were told in our advaced trimix school that helium is about four times less narcotic than nitrogen. This means that we expect to feel the narcotic effects of helium at four times the partial pressures of nitrogen. But that is not the case because we were also told that helium can be breathed down to great depths (1500MSW),without showing any inert gas narcosis effects.
Is there another mechanism that takes place apart from the lipid solubility approach?

 

[BillP]

Howdy Basil:

You'll get different answers on how narcotic a gas is based on who you ask, but I'd say that claiming "helium is about four times less narcotic than nitrogen" would be a gross overstatement of helium's narcotic potential. I'm sure that many (if not most?) diving medicine authorities would likely claim that helium is not narcotic at all under any partial pressure.

The idea of the relationship between the lipid solubility of an anaesthetic and its narcotic potential was put forth by Meyer, Overton, and Hopf. The hypothesis holds well for most anesthetics, but not all. For example, argon is often said to be about twice as narcotic as nitrogen, but it has fairly similar lipid solubility. The lipid solubility association is an interesting (if not always accurate) "rule of thumb", but it is not the only explanation for the narcotic potential of inert gases. The exact cause of the narcosis isn't known to the best of my knowledge.

HTH,

Bill

 

[Dr Deco]

Hello Basil:

Pressure Reversal

Curiously enough, some aspects of the gases can be reversed by hydrostatic pressure. This pressure can be applied with helium. Anesthesia reversal and movement of protoplasm in cells are effects of pressure antagonism to inert gas effects.

High Pressure Nervous Syndrome

This is a presentation in humans of the effect of pressure. It takes the form of uncontrollable shaking of the extremities to the extent that useful work cannot be performed. The method employed to reverse this is to add nitrogen (an anesthetic). Thus we se that not only is there no narcosis with helium, but it actually can reverse the effects of nitrogen.

Lipid Solubility

At one time, it was thought that lipid solubility reflected the tendency of gaseous anesthetics to dissolve in the cell membranes. Today it is thought that this simply reflects some aspect of the molecule, as BillP said.

Dr Deco :doctor:
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