Nitrogen washout method is used for estimating:

B i.e. Functional residual Capacity- Residual volume is the amount of air left inside the lungs at the end of forced expiration. It cannot be measured by simple spirometerQ because the air in the residual volume of lungs can not be expired into spirometer. Because spirometer cannot measure RV, it can't be used to measure lung capacities requiring RV such as functional residual capacity (RFC =ERV + RV) and total lung capacity (TLC = VC + RV). Spirometery can not be used in a direct way to measure - Functional residual capacity (FRC), residual volume (RV) and total lung capacity (TLC)Q. Helium dilution method is used to measure them.Spirometry can measure tidal volume (TV), inspiratory reserve volume (IRV), inspiratory capacity (IC), expiratory reserve volume (ERV) and vital capacity (VC).