Regional alveolar pressure during periodic flow: Dual manifestations of gas inertia

Abstract

We measured pressure excursions at the airway opening and at the alveoli (P(A) as well as measured the regional distribution of P(A) during forced oscillations of six excised dog lungs while frequency (f[2-32 Hz]), tidal volume (V(T) [5-80 ml]), and mean transpulmonary pressure (P(L) [25, 10, and 6 cm H2O]) were varied. P(A)'s were measured in four alveolar capsules glued to the pleura of different lobes. The apex-to-base ratio of P(A)'s was used as an index of the distribution of dynamic lung distension. At low f, there was slight preferential distension of the lung base which was independent of V(T), but at higher f, preferential distension of the lung apex was found when V(T)'s were small, whereas preferential distension of the lung base was found when V(T)'s approached or exceeded dead space. These V(T)-related changes in distribution at high frequencies seem to depend upon the branching geometry of the central airways and the relative importance of convective momentum flux vs. unsteady inertia of gas residing therein, which, in this study, we showed to be proportional to the ratio V(T)/V(D)(*), where V(D)(*) is an index of dead space. Furthermore, they imply substantial alteration in the distribution of ventilation during high frequency ventilation as f, V(T), P(L) vary. The data also indicate that alveolar and airway opening pressure costs per unit flow delivered at the airway opening exhibit weakly nonlinear behavior and that resonant amplification of P(A)'s, which has been described previously for the case of very small V(T)'s, persists but is damped as V(T)'s approach dead space values.