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

Abstract

We measured pressure excursions at the airway opening and at the alveoli (PA) as well as measured the regional distribution of PA during forced oscillations of six excised dog lungs while frequency (f12-32 Hzj), tidal volume (VT 15-80 ml]), and mean transpulmonary pressure (PL 125, 10, and 6 cm H20) were varied. PA'S were measured in four alveolar capsules glued to the pleura of different lobes. The apex-to-base ratio of PA'S was used as an index of the distribution of dynamic lung distension. At low ; there was slight preferential distension of the lung base which was independent of VT, but at higher f, preferential distension of the lung apex was found when VT's were small, whereas preferential distension of the lung base was found when VT's approached or exceeded dead space. These V1-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 VT/VD*, where VD* is an index of dead space. Furthermore, they imply substantial alteration in the distribution of ventilation during high frequency ventilation as f VT, and PL 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 PA's, which has been described previously for the case of very small

Interplay between Geometry and Flow Distribution in an Airway Tree

“…Unambiguous experimental and numerical evidences of inertial effects have been observed in several studies on flow though branched structures, with special emphasis on the bronchial tree [2][3][4][5][6][7][8][9][10][11][12][13][14]. Such phenomena exists in real lungs but they are more simple to study in a symmetric geometry [15,16].…”

Dependence of Intrapulmonary Pressure Amplitudes on Respiratory Mechanics during High-Frequency Oscillatory Ventilation in Preterm Lambs

“…At least in theory, HFOV should reduce the potential barotrauma and volutrauma that are commonly associated with more conventional ventilatory modalities and which may be vitally important in the structurally and functionally immature preterm lung. The experimental evidence supporting the generation of small ⌬P A during HFOV was obtained in healthy adult rabbits (1) and excised adult dog lungs (2). Only one study has obtained direct measurements of ⌬P A in both a normal and abnormal lung model (3).…”

Effect of Frequency and Tidal Volume on Pleural Pressure in Rabbits during High Frequency Ventilation

“…In relating differences in phase between Paw and Pes or Ppl to impedance properties of the respiratory system and chest wall, we assumed that flow was uniform through the lung and chest wall so that regional flow remained in phase throughout the thorax. This assumption is at best a rough approximation, as previous studies have shown [1,2,5,6,10,20,23]. Accordingly, a more complex model with multiple pathways within the pulmonary airways to the mediastinum and rib cage is needed to understand fully the effect of a nonuniform chest wall impedance on esophageal and costal pleural pressures during high frequency ventilation.…”

“…Studies in dogs and rabbits during HFV using inductance plethysmography indicated that when the tidal volume approached or exceeded the dead space, the nonhomogeneous mechanical behavior of the chest wall dominated any potential nonhomogeneous mechanical tendency of the lungs [5,6]. By contrast, studies in intact animals [4,25] have found a ventilation distribution similar to that found in excised lungs [1,3]. The preferential ventilation of the caudal lung regions as indicated by alveolar pressure in excised lungs during HFV with large tidal volumes [3] was produced with the use of tracer gas studies [4,25] and parenchymal markers in intact animals [20].…”