Anaemia and cardiac surgery

Patients undergoing cardiac surgery are at risk of developing ischaemic episodes, which may progress, resulting in adverse effects on organ function. In considering the oxygen balance of vital organs, the main determinants on the supply side of the equation are organ blood flow, arterial oxygen saturation and the amount of haemoglobin available for the carriage of oxygen. Thus, anaemia may be a significant factor in the development of adverse outcomes. In this issue, Klein et al. 1 have reported on the incidence and importance of anaemia in patients undergoing cardiac surgery in the UK. The study represents a national audit undertaken by a specialist society – the Association of Cardiothoracic Anaesthetists (ACTA). Although not all the UK centres took part, 12 centres provided usable data over a 3 year period from 19,023 patients; a sizeable cohort and one from which useful lessons can be learnt. The study defined anaemia according to established WHO criteria. The first point to note is the variability of the incidence of pre-operative anaemia. Many other studies have noted an incidence of pre-operative anaemia of around 30%, but this study in the UK the noted an incidence of 23-45%. Why was there such marked regional variability? There are a number of possible explanations – a significantly older and sicker patient cohort in one region, the variable implementation in primary care of pre-operative measures to improve haemoglobin levels, perhaps even simple nutritional factors in different parts of the UK, although as the authors note, the most likely cause is functional iron deficiency anaemia. Nonetheless it raises the possibility that almost half the surgical population in one region of the country was suffering from anaemia. There have been numerous studies showing an association between anaemia and adverse surgical outcomes. Many of these are in cardiac surgery patients, and a number of them have been cited by the authors. Previous thinking was that anaemia may be a marker of significant disease and perhaps not the primary cause of an adverse outcome. Thus ‘anaemia of chronic disease’ (ACD) was established as an important cause of poor outcome, associated as it is with proven risk factors including: age; diabetes; hypertension; cardiac failure; and other systemic diseases that are well recognised peri-operative risk factors, many of which are incorporated into modern risk stratification scores including EuroSCORE 2. Klein et al. have shown that, in their cohort, pre-operative anaemia was an independent predictor of an adverse outcome. Furthermore, although there is a predictable relationship between the severity of anaemia and mortality, even mild levels of pre-operative anaemia have an adverse effect. Thus, anaemia itself is an important cause of adverse outcomes and needs to be dealt with, notwithstanding any other associated risk factors. Although the importance of pre-operative anaemia has long been recognised, often it would not be regarded as a condition that was serious enough to postpone surgery. Moreover, a reduction in haemoglobin is seen as an inevitable part of cardiac surgery under cardiopulmonary bypass. It is standard practice to reduce the haematocrit on bypass by haemodilution, ideally to between 21-25% 3. This may represent a halving of the pre-bypass haematocrit. Haemoglobin levels of 60-70 g.l-1 on bypass are not only common but seen as desirable. In mild and even moderate anaemia, to delay surgery for such a common condition seemed unnecessary, particularly when the situation could simply be dealt with by treating the pre-operative anaemia with peri-operative blood transfusion. The alteration in haematocrit described above and the possibility of significant surgical blood loss meant that allogenic blood transfusion was viewed with equanimity, and if extra blood was required peri-operatively to increase the haemoglobin value to an acceptable level, that was often viewed as acceptable practice. Modern cardiac surgery is frequently undertaken without autologous blood transfusion. However, the nature of the surgery and obligatory anticoagulation may result in significant blood loss. Thus, allogeneic blood cross-match is standard practice unless there is a specific contraindication to its use, for example religious belief. However, the use of allogeneic blood transfusion seems to be enormously variable, both between 4, 5 and within 6 institutions. With transfusion rates varying from 0% to > 90% in comparable centres, in those circumstances it is difficult to imagine what is being used as an indication for blood transfusion and what are the goals of treatment. Klein et al. have described the need for blood transfusion, clearly an effective treatment for anaemia, as an adverse outcome. They are not alone in this. Many authors have also described the hazards of blood transfusion in the cardiac surgical patient, and Klein et al. have cited some of them in their article. In many cases, the exact aetiology of the adverse effects is not clear, but there is clear evidence that blood transfusion is not a benign treatment in the surgical population and we should be cautious with its use. This is, of course, irrespective of the resource implications, which are well known. Allogeneic blood transfusion may be neither cheap nor benign. Thus, it would appear that, in the cardiac surgical patient, both pre-operative anaemia and allogeneic blood transfusion carry significant risk and both anaemia and transfusion should be avoided wherever possible. Anaemia corrected by transfusion may be more hazardous than anaemia without transfusion, at least up to a point. However, the point at which blood transfusion itself becomes desirable is still controversial. Although the prevailing opinion seems to be with a more restrictive transfusion trigger, there are still important studies that continue to raise the question. 7. A continuing debate is probable, but even if it achieves nothing more than a more rational patient-based approach to blood transfusion in this setting, it will be valuable 8. As an audit, this study was not designed to compare transfusion triggers, and no doubt clinical trials in this area will continue. With a patient population of over 19,000 in this observational study, it must rank amongst the largest of its kind, and it would be interesting to know if it has any useful information to impart regarding the role of different transfusion triggers on the outcomes measured. Like many good audits, this study poses many interesting questions for the future. An effective process for managing the anaemia/transfusion paradox has been identified in the concept of ‘patient blood management’ 9, and this includes strategies for dealing with pre-operative anaemia, including functional iron deficiency anaemia 10. Although acute treatments are available, the management of pre-operative anaemia seems to be as much an organisational as a clinical problem. We have a good idea what we should do, but often we do not have the time to do it. A recent meta-analysis of studies involving over 950,000 patients concluded that pre-operative anaemia was associated with poor outcomes after surgery 11. With anaemia being recognised as an important determinant of outcome, perhaps it is also time to look more closely at strategies designed to minimise blood loss, or rather blood wastage. Avoidance of cardiopulmonary bypass, for instance by the use of off-pump coronary artery bypass surgery has not had the ubiquitous use once predicted, but further attention could be given to reduction in bypass priming volumes and other measures to reduce excessive haemodilution. Pharmacologic treatments to reduce peri-operative bleeding are useful, but some have had a chequered history. The use of cell salvage with appropriate haemo-concentration processes could be better utilised. More exact strategies for anticoagulation and its reversal may be beneficial. Many of these questions have already been addressed, but often in small scale or observational studies, when what is needed are randomised controlled trials powered to generate the answers to specific questions. Such studies take time, organisation and money. One of the advantages of the involvement of specialist societies in this work is that it is possible both to organise the studies and then to disseminate best practice, as ACTA have recently shown with their study on mortality 12. They are well placed to carry the burden, and it gives me much pleasure and much hope for the future to see ACTA so active in this role. Dr Feneck was Chairman of ACTA from 1993-96; he has no other competing interests to declare.