Pressure Reversal Effect?

[cozumelkeith]

Hi There,
I am doing some research for a presentation on narcosis. I am having trouble wrapping my brain around two seemingly opposing phenomena and was wondering if any one could shed some light on this...

As we descend in the water column the ambient pressure increases/along with the pressure at which our regulator delivers gas. As a result the partial pressure of each component gas is increased proportionally. Increased partial pressures of nitrogen at some point cause narcosis. We are all familiar with this.

There is also a phenomena known as the pressure reversal effect in which increased hyperbaric pressure leads to a reduction in anesthetic potency. The mechanisms are not well understood but at least at one point the thinking was hydrophobic inert gases (e.g. N2) dissolve in the hydrophobic region of cell membranes which causes a volume change in the membrane which in turn disrupts membrane protein function. Increasing pressure reduces the volume change in the membrane and the anesthetic effect.

Is is just that at the pressures encountered in normal diving the narcotic effects outweigh the pressure effects?

I know this really isn't a deco question per se but thought there were some similar processes at play (e.g. gradients) that Dr. Deco might be familiar with. Feel free to move.

Thanks!

 

[Duke Dive Medicine]

Is is just that at the pressures encountered in normal diving the narcotic effects outweigh the pressure effects?
Thanks!

Great question. That's the theory - pressure reversal of anesthesia occurs at extremely high ambient pressures.

Best regards,
DDM

 

[cozumelkeith]

OK. So in diving though because the partial pressure of the gas is increasing along with ambient pressure and depth theoretically would it ever be possible for the narcotic effect to be reversed? Need to brush up on my physics but isn't this fundamentally different than say:

An individual breathing a gaseous mixture in a dry chamber at sea level enough to induce narcosis/anesthesia.
Increasing the ambient pressure (not not the partial pressure of the narcotic gas) by pumping in He.

Thanks again!

 

[JohnN]

Great question. That's the theory - pressure reversal of anesthesia occurs at extremely high ambient pressures.

"Extremely" is an understatement. The study I looked at would be relevant if you were a newt diving at 700+ feet on air. . .

yeah sure. . .

 

[Dr Deco]

Hi cozumelkeith:

What you have described is a theory put forward in 1973 by Miller and Smith called the "critical volume hypothesis." It was also called the "lipid bilayer expansion theory." The membrance bilayers of nerve cells would be modified by general anesthetics (eg, nitrogen at pressure) and reverse by the application of large hydrostatic pressures. The reversal portion is experitmentally correct but the "expansion theory" is now considered incorrect. It is thought today that narcosis results if solubilization of the general anaesthetic in the bilayer causes a redistribution of membrane lateral pressures.