Site of airway obstruction: Effects on the acoustic impedance of excised pig lungs

We investigated the relationship between the site of airway obstruction and the frequency dependence (FD) of lung acoustic impedance (ZL). The real (RL) and imaginary (XL) parts of ZL were measured by forced random noise in excised left pig lungs, before (base line) and after 1) no airway obstruction (controls, n = 10), 2) insufflation of 1-mm (B₁, n = 5) or 2-mm (B₂, n = 7) beads, and 3) partial reversible obstruction of lower lobar (LL) and then main-stem (MS) bronchus (n = 4). The beads caused both partial and total obstruction of airways with internal diameters of 2 mm (B₁) and 2-6 mm (B₂). Compared with base line, a negative FD of RL appeared from 4 to 10 Hz in LL, B₁, and B₂ obstructions. The FD of XL >20 Hz increased in MS and LL obstruction exclusively and was the ZL feature that most clearly differentiated central from peripheral obstruction. In this experimental model, the anatomic limit distal from which obstruction no longer causes the 'central' type of ZL changes lies in airways with internal diameters notably >2 mm.