Fluid shifts associated with gas-induced osmosis.

It has been proposed that equilibration of nitrous oxide with blood plasma increases osmotic pressure and thereby causes hemodilution. However, calculations show that the 250-torr osmotic gradient produced by 0.7 atmosphere of nitrous oxide dissolved in blood plasma (separated from the other body fluids by a membrane permeable only to water) would be eliminated by a water shift which would dilute the plasma by only 4 percent. Permeability of the membrane to nitrous oxide would further reduce the shift. In vivo measurements confirmed the smallness of any osmotically induced shifts by demonstrating no significant transient changes in hematocrit value when 0.7 atmosphere of nitrous oxide was added to or removed from an inhaled anesthetic mixture in man. These results cast doubt on the suggestions that gas-induced osmosis is an important factor in dysbarism or in clinical anesthesia.