The association between early arterial oxygenation and mortality post cardiac surgery

Recently, many studies have been conducted to investigate the relationship between hyperoxia and mortality in various cohorts of intensive care unit (ICU) patients. Results from these studies have been varied and often contradictory. The first of a series of studies by Kilgannon et al found arterial hyperoxia was independently associated with increased in-hospital mortality compared with either hypoxia or normoxia in patients admitted to ICU post cardiac arrest. Further analysis of these patients revealed a dose-dependent association between supranormal oxygen (O2) tension and risk of in-hospital death. However, a similar study in Australia and New Zealand demonstrated no robust or consistently reproducible association between hyperoxia and mortality in a cohort of patients admitted to ICU post cardiac arrest. Such contradictions have also been seen in cohorts of mechanically ventilated patients and patients with acute ischaemic stroke. As a result, the impact of early hyperoxia on mortality remains uncertain in many ICU cohorts. Hypoxaemia is believed to carry significant risk and is typically carefully avoided. Similarly, hyperoxia provides a buffer of safety in some high-risk ventilated patients, yet may also be injurious. For example, in the lungs, hyperoxia may cause histopathological injury, induce interstitial fibrosis, atelectasis, tracheobronchitis, alveolar protein leakage and infiltration by neutrophils. Systemically, it can decrease cardiac output and generate free radicals in various organs. To date, there is limited data regarding the effects of hyperoxia in patients post cardiac surgery, though it is accepted that hyperoxia during cardiopulmonary bypass should be avoided. One small study * MB BS (Hons), Anaesthesia Registrar † PhD, MSc, BSc (Hons), Statistician, Australian and New Zealand Intensive Care Research Centre, School of Public Health and Preventive Medicine, Monash University, Melbourne, Victoria ‡ MB BS, MD, FRACP, FCICM, PGDipEcho, Co-Director, Australian and New Zealand Intensive Care Research Centre, School of Public Health and Preventive Medicine, Monash University and Director of Intensive Care Research, Department of Intensive Care, Austin Health, Melbourne, Victoria § RN, BN (Hons), GradDipNsg (Crit Care), PhD, ICU Research Manager, Department of Intensive Care, Austin Health, Heidelberg, Victoria ** MBBS, MRCP, FRACP, FCICM, Intensive Care Specialist and Chair, Australian and New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation, Melbourne, Victoria