Red Blood Cell Transfusion Trigger in Cardiac Surgery

Abstract

In cardiac surgery, the principal aim of red blood cell transfusion is to maintain oxygen delivery and prevent tissue hypoxia in the setting of acute anaemia and severe bleeding. Both these clinical indications are common, and over 50 % of all cardiac surgery patients receive red blood cell transfusion, utilising a significant proportion of blood service resources in developed countries. Severe anaemia accounts for the vast majority of all red blood cells used; however, there is uncertainty as to what constitutes a safe level of anaemia or a trigger for transfusion. There is also uncertainty as to the risks and benefits of transfusion; experimental and early clinical studies suggest that transfusion may promote organ injury. Existing blood management guidelines recommend restrictive transfusion practice, and this is supported by observational analyses in cardiac surgery patients showing strong associations between transfusion and adverse outcome. However, these studies fail to address the important clinical question as to what constitutes the anaemia threshold where transfusion is indicated. They are also beset my multiple sources of bias that confound analysis and contribute to inflated estimates of risk. RCTs in non-cardiac surgery patients do not demonstrate harm from more restrictive thresholds (lower haematocrits) and suggest that this is the best practice. These studies do not reflect the lack of cardiovascular reserve in cardiac surgery patients, however, that is often compounded by the abnormal oxygen utilisation that follows cardiopulmonary bypass. Meta-analyses of RCTs in cardiac surgery appear to support a benefit for more liberal thresholds. These analyses are dominated however by a single large study, the Transfusion Indication Threshold Reduction (TITRe 2) trial, that demonstrated a benefit from a more liberal transfusion threshold of 9 g/dL. We conclude therefore that in the absence of high-quality evidence to the contrary, cardiac surgery patients may be considered a specific high-risk group where restrictive transfusion practice will promote harm.