High frequency oscillatory ventilation
Creators: name: McEvoy, R D
Publisher: name: University Of Tasmania
Year: 2023

description: This work was designed to investigate several aspects of a new type of mechanical ventilation known as high frequency oscillatory ventilation (HFOV) which employs frequencies of 5-40 Hz. The subject has been the focus of a great deal of recent interest and research. There have been two main reasons for this. First, it was shown with HFOV that normal gas exchange could be achieved using tidal volumes that were less than the anatomical dead space, indicating that the mechanisms of gas transport must be quite different from those at physiologic breathing frequencies and tidal volumes. Second, it was suggested that HFOV may offer a useful alternative to artificial ventilation by conventional positive pressure devices. It was considered that, because of lower distal airway pressures, HFOV might lead to less barotrauma, and because of its different gas exchange properties, might be a more effective means of artificial ventilation in the presence of severe lung disease. These hopes for HFOV, however, rested to some extent on its actions on other physiologic systems in the lung. The first part of the study reported herein was undertaken in attempt to elucidate mechanisms of gas exchange. The latter two parts were designed to test the effects of HFOV on lung mucociliary transport and respiratory mechanics. The results can be summarized as follows: 1. Gas Exchange. HFOV was found to markedly alter the pattern of elimination from the lung of 6 simultaneously infused trace gases. The result raised the possibility of a marked change in lung blood flow with altered ventilation-perfusion ratios. However, further studies that were undertaken to investigate this possibility revealed a normal lung blood flow distribution. It was found instead that the conducting airways were participating actively in the elimination of higher solubility gases. ' Additional studies in an airway model suggested that this property of HFOV was dependent on the presence of a wet internal airway surface. This observation led to the development of new theory for gas transport during HFOV which involves a reciprocating exchange of gas with the surface of conducting airways. The mechanism would seem to be most important for very high solubility gases but it might also facilitate C02 removal from the lung. A similar pattern of inert gas elimination was observed in dogs ventilated with a conventional positive pressure ventilator at frequencies 3 to 4 times normal. In this case, however, it appeared to be the result of an increase in alveolar dead space and not enhanced high-solubility gas transport. 2. Mucociliary Transport. A radiolabel surface marker was used to test the effects of HFOV on mucociliary function in the dog lung. HFOV was found to significantly depress the clearance of label from most lung regions over 4 hours, and also appeared to result in the formation of excessive central airway secretions. Radiolabel which was placed on tracheal secretions was noted to move rapidly toward the lung periphery under the influence of HFOV, suggesting one possible explanation for the overall depression of mucous clearance. 3. Respiratory System Mechanics. The effects on lung stability and respiratory system compliance were compared for HFOV and conventional positive pressure ventilation (CMV) in rabbits. Neither form of mechanical ventilation resulted in a change in respiratory compliance or lung volume after 4 hours. The tendency for alveoli to collapse, as judged by repeated arterial P02 measurements, was also not different between the two methods of ventilatory support.