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Effects of sequential changes from conventional ventilation to high-frequency oscillatory ventilation at increasing mean airway pressures in an ovine model of combined lung and head injury

Published online by Cambridge University Press:  01 May 2007

J. O’Rourke*
Affiliation:
Beaumont Hospital, Department of Anaesthesia and Intensive Care Medicine, Dublin, Ireland
P. Sheeran
Affiliation:
Our Lady’s Hospital for Sick Children, Department of Anaesthesia and Intensive Care Medicine, Crumlin, Dublin, Ireland
M. Heaney
Affiliation:
Royal Perth Hospital, Department of Intensive Care Medicine, Perth, Western Australia
R. Talbot
Affiliation:
St James’ Hospital, Department of Anaesthesia and Intensive Care Medicine, James Street, Dublin, Ireland
M. Geraghty
Affiliation:
St James’ Hospital, Department of Surgery, James Street, Dublin, Ireland
J. Costello
Affiliation:
Our Lady’s Hospital for Sick Children, Department of Anaesthesia and Intensive Care Medicine, Crumlin, Dublin, Ireland
C. McDonnell
Affiliation:
The Hospital for Sick Children, Department of Anaesthesia, Toronto, Ontario, Canada
J. Newell
Affiliation:
National University of Ireland, Department of Mathematics, Galway, Ireland
D. Mannion
Affiliation:
Our Lady’s Hospital for Sick Children, Department of Anaesthesia and Intensive Care Medicine, Crumlin, Dublin, Ireland
*
Correspondence to: Dr James O’Rourke, Department of Anaesthesia and Intensive Care Medicine, Beaumont Hospital, Dublin 9, Ireland. E-mail: [email protected]; Tel: +353 1 809 2773; Fax: +353 1 809 3345
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Summary

Background

The objective of this study was to determine the intracranial, cardiovascular and respiratory changes induced by conversion to high-frequency oscillator ventilation from conventional mechanical ventilation at increasing airway pressures.

Methods

In this study, 11 anaesthetized sheep had invasive cardiovascular and intracranial monitors placed. Lung injury was induced by saline lavage and head injury was induced by inflation of an intracranial balloon catheter. All animals were sequentially converted from conventional mechanical ventilation to high-frequency oscillator ventilation at target mean airway pressures of 16, 22, 28, 34 and 40 cm H2O. The mean airway pressure was achieved by adjusting positive end expiratory pressure while on conventional mechanical ventilation, and continuous distending pressures while on high-frequency oscillator ventilation. Cerebral lactate production, oxygen consumption and venous oximetry were measured and analysed in relation to changes in transcranial Doppler flow velocity. Transcranial Doppler profiles together with other physiological parameters were measured at each airway pressure.

Results

Cerebral perfusion pressure was significantly lower during high-frequency oscillator ventilation than during conventional mechanical ventilation (CMV: 45, 34, 22, 6, 9 mmHg vs. HFOV: 33, 20, 19, 5, 5 mmHg at airway pressures mentioned above, P = 0.02). Intracranial pressure and cerebrovascular resistance increased with increasing intrathoracic pressures (P = 0.001). Cerebral metabolic indices demonstrated an initial increase in anaerobic metabolism followed by a decrease in cerebral oxygen consumption progressing to cerebral infarction as intrathoracic pressures were further increased in a stepwise fashion. Arterial PaCO2 increased significantly after converting from conventional mechanical ventilation to high-frequency oscillator ventilation (P = 0.001). However, no difference was observed between conventional mechanical ventilation and high-frequency oscillator ventilation when intracranial pressure, metabolic and transcranial Doppler indices were compared at equivalent mean airway pressures.

Conclusions

The use of high positive end expiratory pressure with conventional mechanical ventilation or high continuous distending pressure with high-frequency oscillator ventilation increased intracranial pressure and adversely affected cerebral metabolic indices in this ovine model. Transcranial Doppler is a useful adjunct to intracranial pressure and intracranial venous saturation monitoring when major changes in ventilation strategy are adopted.

Type
Original Article
Copyright
Copyright © European Society of Anaesthesiology 2007

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