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The ability to reliably measure total respiratory input impedance Zrs from 0.25 to 4 Hz has only recently been reported and only in healthy subjects. The real part of Zrs decreased substantially with frequency. One explanation is provided by the Otis model, which contains parallel resistance-compliance time-constant inhomogeneities. Several investigators have suggested the use of this model at the level of estimating its parameters by fitting the model to data. Such an approach would permit quantification of the functional inhomogeneity of an individual's respiratory system and may be useful diagnostically. In this study, experimental data and a sensitivity analysis are combined to specify the requirements and limitations associated with estimating the parameters. The data acquisition technique was improved to acquire Zrs as low as 0.125 Hz in seven healthy subjects. The Otis model provided an excellent fit to the data with reasonably low intra- and intersubject variability.