In an attempt to standardise the degree of impairment of gas exchange in the lungs of ICU patients a number of indices of oxygen exchange have been developed. These include: the arterial−alveolar oxygen tension ratio (a/APO2), the arterial oxygen tension−inspired oxygen concentration ratio (PaO2/FIO2), the respiratory index (RI), [A−a)DO2/PaO2) and the alveolar−arterial oxygen tension difference [A−a)DO2). These are used clinically despite the fact that they do not accurately predict gas exchange particularly with changing the amount of inspired oxygen or when there is mixing of oxygenated blood with unoxygenated blood in the patient (called shunt). None of the indices reliably reflect the behaviour of the physiological shunt. In a pilot study as inspired oxygen (FIO2) was increased incrementally from 0.30 to 1.00, up to 55 per cent of the measured changes in these indices were opposite in direction to the corresponding changes in the physiological shunt. The investigators have developed a computer algorithm and a slide rule to try and improve on these clinical measures. The investigators wish to test their slide rule on ventilated patients by altering the inspired oxygen to see if it will predict what the actual change in oxygen will be.