Selenium and oxidative stress modulation during extracorporeal circulation

Abstract

Oxidative stress occurs when there is an imbalance between reactive oxygen and nitrogen species (RNOS) and the antioxidant system. In cardiac surgery patients and the critically ill there is an association between oxidative stress and morbidity and mortality. Extracorporeal circulation (ECC) encompasses cardiopulmonary bypass (CPB) which is used for cardiac surgery; extracorporeal membrane oxygenation (ECMO) which is used for cardio-respiratory support in the critically ill and haemodialysis which is used in patients with intermittent or end-stage renal failure. During ECC, the patient’s total blood volume is in direct contact with an artificial surface for extended periods. In addition, there is often pre-existing inflammation, surgical trauma and/or ischemia-reperfusion (I-R) injury, all of which are major activators for the increased production of RNOS. Concurrent decreases in the level of essential antioxidants and trace elements (such as selenium, zinc and copper) occur, which upset redox balance in favour of oxidative stress. The trace element selenium is required for the normal function of the antioxidant enzyme glutathione peroxidase (GPx). Despite in vitro and biochemical data suggesting that reinforcement of antioxidant activity by selenium supplementation might reduce excessive oxidative stress damage, there is conflicting evidence in clinical studies. The reason for failure of antioxidant supplementation in the clinical setting is currently unclear. We postulated that the ECC and its associated interventions such as transfusion, might have a larger impact on trace element loss, antioxidant function and the oxidative stress response than previously recognised. This led to the following questions: • What is the selenium level in healthy Queensland residents, and is this level similar to that reported in other parts of the world? • Does a patient’s pre-CPB or pre-ECMO circulating selenium level influence their outcome? • Does the ECC independently alter circulating selenium levels and the oxidative stress response? • Do other interventions during ECC, such as transfusion, alter circulating selenium and oxidative stress levels? This thesis focussed on oxidative stress and the trace element selenium, because oxidative stress is associated with poorer outcomes in cardiac and critically ill patients, and decreased selenium is independently associated with poorer outcomes. To answer the research questions outlined above, a series of experiments were designed and conducted, and they generated the following new information. The average selenium level in a Queensland blood donor population was defined at 1.09 µmol/L (Chapter 3). In addition 88.5% of participants had a selenium level lower than the critical level required for maximal glutathione peroxidase activity, which has previously been determined to be between 1.14 – 1.27 µmol/L. Furthermore, this study generated novel information that packed red blood cells and buffy coat pooled platelets have very low levels of selenium (≤ 0.3 µmol/L). The effect of cardiopulmonary bypass circuits on circulating trace element levels was reported in Chapter 4. Reductions in selenium (17.1%), zinc (29%) and copper (18.2%) were recorded over a two-hour period. Coating the CPB circuit with albumin did not reduce the loss of trace elements. The effect of transfusing fresh or aged packed red blood cells (PRBC) on selenium and oxidative stress levels was compared using an ovine model (Chapter 5). Transfusion of PRBC (with low selenium content), into a healthy host reduced selenium and GPx activity and increased oxidative stress markers. The separate and combined effects of lung injury and ECMO on selenium levels and oxidative stress were investigated using an ovine model (Chapter 6). Our results revealed that the combination of acute lung injury and ECMO resulted in a 72% reduction in selenium after 24 hours, (1.36 to 0.38 µmol/L); however, oxidative stress markers were no worse than that associated with each separate intervention. Finally the association between selenium and post-operative atrial fibrillation (POAF) in low and high-risk cardiac patients was investigated (Chapter 7). Selenium levels were significantly lower pre-operatively and at all post-operative time points in high-risk patients and also in those who developed POAF. There was an association between longer intensive care stay at pre-operative selenium levels ≤ 0.7 µmol/L. Overall the key findings of this thesis were: • Selenium levels in healthy Queensland blood donors were lower than the previously reported levels for other regions of Australia. • PRBC have very low levels of selenium. • CPB and ECMO independently reduced selenium levels. • The addition of ECMO to a host with lung injury compounded selenium reductions but did not increase oxidative stress. • Low selenium levels before cardiac surgery were associated with POAF. • Selenium levels lower than 0.7 µmol/L in cardiac surgical patients were associated with longer ICU stay. Collectively, the findings of this thesis answered the original questions. The knowledge and experience gathered from this journey has significantly improved my ability to design and conduct research, and my future research aspirations are detailed in chapter 8.