Determining functional residual lung capacity (FRC) by changing a subjects inspirium FiO2 by a predetermined amount, and a) for each breath in a series of breaths subsequent to changing the FiO2, determining expiratory tidal volume, determining expiratory fractional N2 tidal volume, multiplying the expiratory tidal volume by an absolute difference between the expiratory fractional N2 tidal volume of the breath and that of an immediately preceding breath for a first multiplication result, dividing the first multiplication result by the sum of the differences for a first division result, and multiplying the fractional N2 tidal volume by the sum of the first division results of the breaths for a second multiplication result, and b) dividing the sum of the second multiplication results of the breaths by the absolute difference between the fractional N2 tidal volume of the first and last breaths to produce a measurement of the subjects FRC.
