Respired Gases in Patients Post Cardiac Surgery

To allow the OGA to acquire certain physiological data during the study it will be necessary to slightly vary the tension of oxygen and carbon dioxide for short periods. The changes involved will be of a lesser magnitude than those often seen due to natural variation over time in critically ill patients. The FiO2 will be increased by around 20% from baseline for several minutes; this is a far more modest increase than is seen with the practice of pre-oxygenation - a transitory increase in fraction of inspired oxygen ( FiO2) to 100% - performed regularly in ICU patients to make certain routine interventions safer. The end-tidal CO2 level will also briefly (1-2 min) be altered by around 1 kPa by transient adjustment of the ventilator settings.