Gordon's thesis explored post-HBO reductions in oxygenation in mechanically ventilated ICU patients. 

Hyperbaric oxygen (HBO) is recognised as treatment in a variety of conditions for which recipients may require concurrent mechanical ventilation. Subsequent to HBO, some mechanically ventilated intensive care unit (ICU) patients may experience reduced oxygenation. Reductions in oxygenation may result in mild to severe physiological effects; however published data on this phenomenon in the context of HBO remain limited.

The aim of this study was to explore post-HBO reductions in oxygenation within the context of mechanically ventilated ICU patients. Specifically incidence and magnitude of changes in oxygenation, changes in ventilator settings necessitated to maintain oxygenation, the patient characteristics associated with any reductions in oxygenation, the possibility of a protective effect against reductions in oxygenation from previous HBO treatments and any association of mechanical ventilation parameters or routinely collected physiological information with changes in oxygenation.

This was a prospective, single-centre, observational study in one metropolitan teaching hospital in Melbourne, Australia. The study sample comprised of 25 consecutive mechanically ventilated ICU patients, from which data were collected for 61 HBO treatments. All data were collected pre-HBO, immediately post-HBO and at 1, 2, 3 and 6 hours. The primary outcome measure of oxygenation was arterial partial pressure for oxygen (PaO2) against level of inspired oxygen (FiO2) the PF ratio. Secondary measures of oxygenation and a range of mechanical ventilation and physiological data were also collected, all of which indicated reduction in oxygenation for 2 hours in the majority of patients. Post-HBO changes in oxygenation were associated with adjustments to mechanical ventilation settings in many patients, typically increasing FiO2 for approximately 2 hours.

Patients who received post-HBO alterations in ventilation were more likely to be male and have lower pre-HBO oxygenation parameters, inspiratory pressures and mean corpuscular haemoglobin concentrations. No evidence of a protective effect upon post-HBO reductions in oxygenation was found between first and second HBO treatments. Associations were shown between changes in the PF ratio with collected physiological and treatment parameters. These data have the potential to guide ventilation protective strategies and future research to determine causation.

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