Standard errors on respiratory mechanical parameters obtained by forced random excitation

Equations describing the standard errors of forced random impedance data and derived parameters in terms of various data collection and data processing factors were developed and verified. The equation indicate that to obtain reliable estimates: 1) 16 ensembles are adequate when coherence is greater than 0.9, and that 32 ensembles are adequate when the coherence is between 0.8 and 0.9; 2) the impedance of the bias tube should be at least two times the impedance of the respiratory system in the bandwidth of the applied noise; and 3) the spectrum should include at least 20 frequencies with at least 2 and preferably more below 10 Hz. Fortunately, all of these constraints can be satisfied with most subjects. This analysis also provides a basis for using weighted regression in estimating resistance, inertance, and compliance parameters, and for separating observed parameter variability into methodological and physiological components.