The present invention discloses a method to determine the lung clearance index (LCI) or other indices of ventilation inhomogeneity of the lungs by combining two pulmonary gas exchange techniques Inert gas rebreathing (IGR) is used for rapid wash-in of the inert tracer gas and this is followed by multiple-breath wash-out (MBW). The functional residual capacity (FRC) can either be determined from the tracer gas concentration and the gas flow inhaled and exhaled during multiple-breath wash-out or by gas analysis alone from the inert gas rebreathing. The cumulative expired volume (VCE) required to clear the inert tracer gas from the lungs is determined from the multiple-breath wash-out, and LCI is calculated as the ratio between VCE and FRC. The advantages of the method are i) significant reduction of required test time, ii) significant reduction of consumed gas mixture for wash-in of tracer gas, iii) potential for further reduction of the use of tracer gas, and iv) potential for more accurate determination of the FRC by gas dilution alone. Furthermore the present invention relates to a corresponding system and computer-readable medium.
