Patent ID: 7681574

Claim:
A method for determining the functional residual capacity of a subject's lungs using a washout/wash-in technique in which a concentration of a tracer gas in the inspired breathing gases for the subject is altered from an initial concentration, said method comprising the steps of: (a) providing a functional residual capacity (FRC) measuring circuit having a central processing unit (CPU); (b) determining in the CPU, for each of a plurality of breaths of a series of breaths, a value of a volume dilution factor k n , where the volume dilution factor is calculated using tidal volume and lung volume for each breath; (c) determining in the CPU, for each breath of the plurality of breaths, a value of the functional residual capacity of the subject's lungs; (d) establishing a predetermined value for the volume dilution factor k n ; (e) determining in the CPU when the value of the volume dilution factor k n determined in step (b) stands in a predetermined relationship to the predetermined value for the volume dilution factor k n ; (f) establishing a relationship in the CPU between the determined value for the volume dilution factor k n and the functional residual capacity of a subject's lungs over the plurality of breaths; and (g) estimating the functional residual capacity of the subject's lungs in the CPU at the predetermined value for the volume dilution factor k n based upon the established relationship of step (f).