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Dear Bob (and call me Ted – we are far too old to be formal) -

Joan is kind to me (she always has been).  I suspect I know too little to be of help.  But undeterred by ignorance (or prompted by it), I shall ask a few questions.

First, what gases form the bubbles?  Forget about anesthesia for the moment.  Imagine an animal breathing oxygen.  Given sufficient time, breathing oxygen should cause the removal of most other gases (primarily nitrogen).  Bubble formation would be more difficult (what gas would fill the bubble?), especially since the substitute gas (oxygen) would have a low partial pressure (perhaps 40 mmHg)  in tissues because of metabolism.  Carbon dioxide remains as a gas that might be helpful, but the partial pressures are low.  Still, maybe.  Ditto, as you suggest, water-water vapor.  Regardless, wouldn't the removal of the nitrogen be predicted to have some anesthetic effect (it doesn't)?

Second, if water vapor is the bubble source, what would be the predicted effect of changes in body temperature on anesthetic requirement?  Bubbles would more likely form, right?  But anesthetic requirement increases with increasing temperature.

Third, there is a peculiar class of gases called non-immobilizers (Joan named them).  These are volatile compounds (e.g., 1,2-dichlorohexafluorocyclobutane) that have no anesthetic effect (anesthesia in this case is defined by the capacity of all inhaled anesthetics to produce immobility in the face of noxious stimulation – it is the basis for the standard unit of anesthetic potency called MAC) either alone or in combination with a known anesthetic (i.e., they do not decrease the requirement for the known anesthetic).  Yet they would exert a partial pressure that might expand any bubble formed.

Fourth, you focus in your email on noble gases.  Xenon and argon both produce anesthesia (are immobilizers), but neon and helium do not (are non-immobilizers).  I will add that non-immobilizers undermine the old Meyer-Overton hypothesis.

Fifth, if bubbles form in any significant amounts, they should be revealed by ultrasound studies.  Are they?

I don't know if any of the above is useful, or even makes sense.  But you asked!

Be well.

Ted
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Edmond I Eger II, MD
Professor, Dept. of Anesthesia and Perioperative Care
University of California, San Francisco 94143-0464
415-476-6927
egere@anesthesia.ucsf.edu
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