Respiratory impedance values in young asthmatic children are relatively insensitive to mead model lung compliance and chest wall compliance parameters

Abstract

Impulse Oscillometry (IOS) measurements of respiratory impedance were obtained from young asthmatic children pre- and post-bronchodilator (pre- and post-BD). Mead's model of respiratory impedance was then used to estimate the central inertance (I), central and peripheral resistances (Rc, R p), and lung, chest wall, bronchial, and extrathoracic compliances (Cl, Cw, Cb, Ce) by least-squares-optimal fitting to the IOS data. This typically produced multiple near-optimal solutions, with estimates ofCl and Cw that varied by 2 to 3 orders of magnitude and were also several orders of magnitude larger than expected physiological values; up to 515,000 L/kPa for C l, and 81,138 L/kPa for Cw. We then performed constrained optimization of pre-BD data with Cw fixed at 0.50 L/kPa and C l fixed at 0.38 L/kPa, which yielded averaged least-squares (LS) error 44.1% larger than for unconstrained optimization, while Rc, I, Rp, Cb and Ce changed by an average of 12.5%, 32.2%, 25.1%, 26.7% and 2.6x1011%, relative to unconstrained values; the large change in Ce due to it becoming negligible in about 25% of the results. Next, constrained optimization of post-BD data with Cw = 0.5 L/kPa, Cl = 0.38 L/kPa, resulted in an averaged LS error 39.4% larger than for unconstrained optimization, while Rc, I, R p, Cb and Ce changed by an average of 1.7x1012%, 23.4%, 29.1%, 36.9% and 3.5x1011%, relative to unconstrained values. Neglecting two results with unphysiologically small R c reduced the average change in Rc to 19.1% without significantly impacting the average change of the other parameters. Constrained optimization of pre-BD data and then post-BD data with Cw = 0.5 L/kPa, Cl = 0.33 L/kPa, resulted in similar changes to total LS error and remaining parameters. We conclude that the Mead model's impedance and parameter estimates (except Ce, unimportant in asthma) for young asthmatic children are relatively insensitive to Cl and C w. © 2010 International Federation for Medical and Biological Engineering.