Nitrous oxide and air space volume changes

Jonas Johansson, M.D. discusses nitrous oxide and closed spaced.  Compliant gas-filled spaces in the body such as intestinal air or a small pneumothorax in a patient with a rib fracture may expand with time if nitrous oxide (N₂O) is a component of the anesthetic.  The underlying principle is that N₂O (blood/gas partition coefficient of 0.47) is 31 times more soluble in blood than the nitrogen (blood/gas partition coefficient of 0.015) present in the preexisting gas pocket, which means that N₂O can diffuse more rapidly into such gas pockets than nitrogen can diffuse out. The volume of the gas pocket will therefore increase as a function of both time and the inspired N₂O concentration. For example, if the patient is breathing 50% N₂O, the volume of the gas-pocket will double (the final concentration of N₂O in the gas pocket will be 50% at equilibrium); while with 75% N₂O in the inspired gas mixture a four-fold increase in volume will, in theory, be achievable (the final concentration of N₂O in the gas pocket will be 75% at equilibrium).

For well-perfused tissue such as lung, the volume of a pneumothorax can double in ten minutes. In less well perfused tissue such as intestine, volumes of gas pockets may double over the course of an hour. Air-filled balloons on endotracheal tubes and Swan-Ganz catheters are also susceptible to volume expansion with exposure to N₂O. Of note is that xenon which is somewhat more soluble in blood than N₂O should have the same effects on gas-filled spaces in the body. It is possible to describe this relationship using the simple equation V_F/V₀ = 1/(1-F_N2O), where V_F and V₀ are the final and initial volumes of the gas pocket, respectively, and F_N2O is the fraction of the inspired gas mixture that is composed of N₂O. For example, a 50% inspired N₂O concentration would translate into a F_N2O of 0.5, while a 75% inspired N₂O concentration would be equivalent to a F_N2O of 0.75.

Reference: Eger EI, Saidman LJ. Hazards of nitrous oxide anesthesia in bowel obstruction and pneumothorax. Anesthesiology 26:61-66, 1965

Dr. Johansson is Associate Professor of Anesthesiology and Critical Care

Comments

[andrew kofke]

In neuroanesthesia there seems to have been widespread abandonment of N2O. One reason is concern about pneumocephalus. I have an impression however of some really significant pneumocephalus in the post op ct scans of late, despite seldom seeing n2o anymore. I am wondering if postop pneumocephalus is, in fact, decreased with n2o use. if n2o is in the head, rather than N2, after surgery one would expect to see much smaller size pneumocephalus.

needs studied

[RichardH]

Overall in prolonged surgery there now appears to be good evidence that N2O is detrimental. See http://www.thelancet.com/journals/lancet/article/PIIS0140673604163009/abstract