Low-frequency vs. high-frequency respiratory mechanics after methacholine challenge in artificially ventilated...

Respiratory system resistance (Rrs) and elastance (Ers) were estimated by two methods, before and after methacholine in six anesthetized, paralyzed, and artificially ventilated rabbits. Rrs and Ers were obtained (1) by multiple linear regression analysis of the relationship between tracheal pressure and tidal volume and flow [Rrs(mlr)], Ers(mlr), and (2) by analysis of the Fourier transforms of tracheal pressure and flow resulting from 4 to 30 Hz pseudorandom pressure oscillations delivered by an Infant Star respirator [Rrs(os), Ers(os)]. Rrs(os) was significantly lower than Rrs(mlr). For instance, Rrs(os) at 20 Hz [Rrs(os)20] was (mean +/- SD) 17.3 +/- 3.5 vs 21.4 +/- 3.6 cm H2O x L-1 x s (P < 0.01) for Rrs(mlr). Ers(os) was significantly higher than the respective value obtained by multiple linear regression (718.2 +/- 81.0 vs 403.7 +/- 43.0 cm H2O L-1; P < 0.01). After methacholine, the changes of respiratory mechanics were similar with both methods. Rrs(mlr) and Rrs(os)20 increased respectively by 131 +/- 45 and 134 +/- 76%, and Ers(mlr) and Ers(os) increased respectively by 63 +/- 7 and 54 +/- 13%. A significant correlation was observed between Rrs(mlr) and Rrs(os)20 (r = 0.97) and between Ers(mlr) and Ers(os) (r = 0.96). We conclude that positive response to methacholine may be detected by forced oscillation as well as by multiple linear regression. However, the identified physiological components of the lung response (alteration in lung viscoelastic properties, increased lung inhomogeneity or increased intrathoracic airway shunt) are likely to be different with each method.