Alveolar pressure magnitude and asynchrony during high-frequency oscillations of excised rabbit lungs

One possible advantage of high-frequency ventilation (HFV) over conventional mechanical ventilation is that adequate pulmonary ventilation may be established with lower pressure swings. Pressure swings measured at the airway opening may not accurately reflect pressure swings in the alveoli, however. Furthermore, little is known about the synchrony of alveolar filling during HFV. We have assessed the magnitude of alveolar pressure swings (PA) relative to those at the airway opening (Pao) and investigated asynchrony of alveolar filling during small tidal volume (less than 1.0 ml), high-frequency (1 to 60 Hz) oscillations (HFO) in 8 excised rabbit lungs. The PA was measured in several capsules glued to the pleural surface and communicating with alveolar gas via pleural punctures. The peak value of the ratio |PA/Pao| occurred near the resonant frequency and was 1.90, 1.45, and 1.0 at distending pressures of 25, 10, and 5 cm H₂O₂, respectively. Temporal asynchrony of PA between sampled lung regions was quantitated by measuring the interregional standard deviation of alveolar pressure phase angles, ΔΦ. The ΔΦ increased with increasing frequency and decreasing transpulmonary pressure. The maximal observed ΔΦ was 30 degrees. These results, when compared with earlier results on excised canine lungs, show that the amplification of PA during HFO is lung-size dependent. The observed degree of phase differences in pressure swings between peripheral alveolar locations implies substantial asynchrony of alveolar filling. This in turn suggests interregional gas transport as an important contributor to gas mixing during HFV.