Alveolar pressure nonhomogeneity during small-amplitude high-frequency oscillation

In six excised canine lungs, regional alveolar pressures [P(A)] were measured during small-amplitude high-frequency oscillations applied at the airway opening. Both the regional distribution of P(A)'s and their relationship to pressure excursions at the airway opening (Pao) were assessed in terms of amplitude and phase. P(A) was sampled in several capsules glued to the pleural surface and communicating with alveolar gas via pleural punctures. Pao and P(A) were measured over the frequency (f) range 1-60 Hz, at transpulmonary pressures [P(L)] of 5, 10, and 25 cm H₂O. The amplitude of P(A) excursions substantially exceeded Pao excursions at frequencies near the resonant frequency. At resonance the ratio |P(A)/Pao| was 1.9, 2.9, and 4.8 at P(L)'s of 5, 10, and 25 cm H₂O, respectively. Both spatial homogeneity and temporal synchrony of P(A)'s between sampled lung regions decreased with f and increased with P(L). Interregional variability of airway impedance [(Pao-P(A)/V̇ao] and tissue impedance [P(A)/V̇ao] tended to be larger than differences due to changing P(L) but not as large as between-dog variability. These data define the base-line nonhomogeneity of the normal canine lung and also suggest that there may be some advantage in applying high-frequency ventilation at frequencies at least as high as lung resonant frequency.