MBNW has been utilized in a number of clinical studies, and is considered to be a gold standard in the measurement of lung volume18-21. Currently, the most accurate way to measure the volume of the lung is through dilution of a known amount of a gas with low solubility be rebreathing in a closed system. The changes in concentration with sequential breaths allow a calculation of the volume of distribution of the gas. One gas which has been utilized for this purpose is nitrogen22, appealing due to its ubiquitous presence in the environment. Measurement of nitrogen gas concentrations, however, is available only using gas chromatography or mass spectrometry, neither of which is clinically practical. Recently, a technique has been validated by which the partial pressure of nitrogen is calculated as the residual of partial pressures of oxygen gas, carbon dioxide gas and nitrogen gas, which together comprise the only three important gases in a ventilator circuit. The former two gases are readily measured in a ventilator circuit in real time, but of course vary widely with the metabolic state of the patient. Stenqvist has developed the NMBW technique to calculate FRC using the changes in exhaled O2 and CO2, manipulating inspired oxygen concentration to alter fraction of inspired nitrogen23. FRC is calculated as follows: