High frequency ventilation. Study of the mechanisms of action

Dried lungs and isolated bronchial trees dissected from large animals were submitted to high-frequency oscillation and jet-ventilation. The pattern of intrapulmonary pressure distribution and CO2 diffusion were measured through transalveolar chambers fixed to the perforated pleural surfaces and through airbags pasted on the isolated bronchial trees. Under oscillating conditions, the pressure profiles in different lung and bronchial compartments were inhomogeneous and frequency dependent; the pressure-wave amplitude was proportional to the oscillation frequency. On the other hand, the inhomogeneities found with jet-ventilation were mostly dependent on the airflow direction and position of the intratracheal cannula. Since these inhomogeneities were similar on dissected lungs as well as on isolated bronchial trees, it was concluded that they were essentially dependent on endobronchial aerodynamic effects. But the absence of the in vivo pulmonary and bronchial elastic recoil certainly modified the effects of these ventilation modes with respect to accepted clinical findings. Also results were shown to vary between individuals and within individuals, probably explaining the divergent results obtained by different authors.