Nitrous Oxide

August 08, 2007

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 VF/V0 = 1/(1-FN2O), where VF and V0 are the final and initial volumes of the gas pocket, respectively, and FN2O is the fraction of the inspired gas mixture that is composed of N2O. For example, a 50% inspired N2O concentration would translate into a FN2O of 0.5, while a 75% inspired N2O concentration would be equivalent to a FN2O 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

Posted at 12:05 PM in Nitrous Oxide | | Comments (2) | TrackBack (0)

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