Effect on lung volumes of oxygen concentration when breathing is restricted

We have examined the effect of the fractional concentration of nitrogen (FIN2) on the decrease in lung volumes which occurs during restricted breathing with oxygen-nitrogen mixtures. Conscious human subjects breathed one of five gas mixtures of oxygen and nitrogen for 15 min on each of five occasions. For the final 5 min of each 15-min period, functional residual capacity (FRC) was reduced by one tidal volume, by external pressurization. After return to normal breathing, the subjects showed a statistically significant decrease in FRC and residual volume (RV), which became larger as FIO2 increased from 30% to 100%. This reduction in lung volumes was resistant to early reexpansion. The maximum decrease in both FRC and RV was found with 100% oxygen, and was 10% of the control lung volumes. The use of a mixture of 75% nitrogen and 25% oxygen prevented this decrease in lung volumes. Nitrogen 50% reduced the decrease in FRC, but did not affect the decrease in RV. The difference in effect on lung volumes between FIO2 25% and 30% was statistically significant, indicating a watershed area for FIN2 between 70% and 75%.