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High frequency oscillatory ventilation


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McEvoy, R D(Ronald Douglas) 1984 , 'High frequency oscillatory ventilation', MD thesis, University of Tasmania.

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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
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

Item Type: Thesis - MD
Authors/Creators:McEvoy, R D(Ronald Douglas)
Keywords: Respirators, Pulmonary function tests, Blood gases, Ventilation-perfusion ratio
Copyright Holders: The Author
Copyright Information:

Copyright 1984 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Bibliography: leaves 103-118. Thesis (Ph.D.)--University of Tasmania, 1985. "These studies were carried out in the Department of Medicine, University of California, San Diego, La Jolla, California, USA"

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