Respiratory impedance and multibreath N₂ washout in healthy, asthmatic, and cystic fibrosis subjects

We measured forced expiratory volume in 1 s (FEV₁), respiratory impedance (Zrs) from 4 to 60 Hz, and a multibreath N₂ washout (MBNW) in 6 normal, 10 asthmatic, and 5 cystic fibrosis (CF) subjects. The MBNW were characterized by the mean dilution number (MDN) derived by a moment analysis. The Zrs spectra were characterized by the minimum resistance (Rmin), the drop in resistance (Rdrop) from 4 Hz to Rmin, and the first resonance frequencyh (F(r1)). Measurements were repeated after bronchodilation in three normal and all asthmatic subjects. Before bronchodilation, six of the asthmatic subjects showed close to normal FEV₁. The Zrs in the normal subjects showed low Rmikn (1.9 ± 0.7 cmH₂O · l^-1 · s), Rdrop (0.4 ± 0.4), and F(R1) (10 ± 2 Hz). Four of the mildly obstructed asthmatic subjects had normal Zrs but elevated MDNs (i.e., abnormal ventilation distribution). The other six asthmatic subjects had significantly elevated Rmin (4.1 ± 0.8), Rdrop (6.3 ± 5.8), and F(R1) (34 ± 0.4 Hz) and elevated MDNs. The CF patients had elevated Zrs features and MDNs. After bronchodilation, no changes in FEV₁, MDN, or Zrs occurred in the normal subjects. All asthmatic subjects showed increased FEV₁ and decreased MDN, but the Zrs was unaltered in the four asthmatic subjects whose base-line Zrs was normal. For the other six asthmatic subjects, there were large decreases in the Rmin, Rdrop, and F(r1). Finally, there was a poor correlation between the MDN and the Zrs features but high correlation between the Zrs features alone. These results imply that significant nonuniform peripheral airway obstruction can exist such that ventilation distribution is abnormal but Zrs from 4 to 60 Hz is not. Abnormalities in Zrs from 4 to 60 Hz occur only after significant overall obstruction in the peripheral and more central airways. Combining Zrs and the MBNW may permit us to infer whether the disease is predominantly in the lung periphery or in the more central airways.