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Abstract

A method for determining pulmonary functional residual capacity (FRC). A given amount of indicator gas is delivered into the breathing gases flowing into the lungs of a subject in a selected number of sequential breaths. The amounts of indicator gas delivered during the selected number of breaths are summed to provide a cumulative total (.SIGMA.V.sub.in). The amount of indicator gas exhaled in the number of sequential breaths is summed to provide a cumulative total (.SIGMA.V.sub.out). An indication of the concentration of indicator gas in the lungs of the subject (F.sub.ET) is obtained for said two or more breaths. Using the quantities (.SIGMA.V.sub.in), (.SIGMA.V.sub.out), and (F.sub.ET) as measured variables, at least two measured value data sets are formed in which the product of the indicator gas concentration (F.sub.ET) and a regression coefficient comprising the functional residual capacity (FRC) plus the product of the cumulative total of exhaled indicator gas (.SIGMA.V.sub.out) and a regression coefficient K equals the cumulative total of the delivered indicator gas (.SIGMA.V.sub.in). Multi dimensional regression analysis is carried out using the data sets to obtain values for K and FRC fitted to said data sets. The value FRC so obtained is a determination of function residual capacity.

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