After a brief period of probe stabilisation, 2 ml of blood will be withdrawn for blood gas analysis (including glucose), and the probe readings will be calibrated against this value. Subsequent calibration will take place 6 hours later, and then at 12 hourly intervals. A maximum total of 30ml of blood will be required for each patient for blood oxygen content analysis. Clinically necessitated blood sampling for blood gases will continue, and these values will be collected post-hoc from our E-hospital clinical records system.