ALVEOLAR PRESSURE MAGNITUDE AND ASYNCHRONY DURING HIGH-FREQUENCY OSCILLATIONS OF EXCISED RABBIT LUNGS

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Abstract
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. The magnitude of alveolar pressure swings (PA) relative to those at the airway opening (PAO) and asynchrony of alveolar filling during small tidal volume (< 1.0 ml), high-frequency (1-60 Hz) oscillations (HFO) were investigated 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 H2O, respectively. Temporal asynchrony of PA between sampled lung regions was quantified by measuring the interregional standard deviation of alveolar pressure phase angles, .DELTA..vphi.. The .DELTA..vphi. increased with increasing frequency and decreasing transpulmonary pressure. The maximal observed .DELTA..vphi. was 30.degree.. 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.

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