Relationship between PaO2 and Lung Volume during High Frequency Oscillatory Ventilation

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Abstract
The relationship between oxygenation and lung volume during high frequency oscillatory ventilation (HFOV) was studied. We ventilated anesthetized, tracheostomized adult rabbits that were rendered surfactant-deficient by lung lavage. Lung volume was measured by the 'disconnection technique'. In the first experiment, HFOV was commenced after conventional mechanical ventilation (CMV) for 1 hr. In the absence of sustained inflation (SI), oxygenation improved with time during HFOV. In the second experiment, HFOV was instituted after CMV for 4 hr. In the absence of SI, all animals expired during the experimental period. In the third experiment we ventilated rabbits for 4 hr and then switched to HFOV. We applied SI first and increased mean airway pressure (MAP) by increments of 2 cmH2O every 15 min. However, there was little improvement in PaO2 despite the use of repeated SI and the increase in MAP. We conclude that oxygenation has a linear relationship to lung volume during HFOV, and that secondary lung injury due to long-term CMV impairs the response to HFOV. Therefore, it is important to minimize the risk of such secondary injuries before instituting HFOV.