30 adult patients admitted to Aberdeen Royal Infirmary ICU, requiring invasive mechanical ventilation and with impaired gas exchange will be recruited and the following baseline demographic data will be recorded: cause of respiratory failure, APACHE II score, cardiovascular shock (presence / absence), duration of mechanical ventilation prior to inclusion, duration of respiratory failure prior to inclusion. Also the following baseline physiological data will also be recorded: FIO2, SaO2, PEEP, arterial blood pressure, central venous pressure, haemoglobin, PaO2, PaCO2, arterial pH and inotrope / vasopressor infusion rates.