Blood Conservation in Cardiac Surgery
Cardiac surgery is traditionally a heavy user of blood and blood products. Until recently, the benefits of transfusion have been largely assumed and the risks relatively ignored. This has prompted us to examine new ways of minimising patient exposure to donor red blood cells (RBC's). At the present time, most clinical guidelines for RBC transfusion are based mainly upon haemoglobin concentration ([Hb]). As [Hb] may be artificially depressed by the haemodiluting effect of the heavy clear fluid load associated with cardiac surgery, transfusing based upon [Hb] alone may overestimate the requirement for RBC's. Where such haemodilution is present, systemic oxygenation may be maintained through a viscosity mediated patho-physiological response. The work reported in this thesis attempts to explore the relative contribution of both red cell volume (RCV) and plasma volume (PV) to the anaemia encountered following cardiac surgery while also examining factors that may be associated with a low post-operative RCV. In addition, we have explored on a theoretical basis what [Hb] would represent a critical level of systemic oxygen delivery (DO2Crit). Taken together, this has allowed us to develop an RCV based transfusion guideline aimed at reducing the incidence of unnecessary (and potentially counter-productive) RBC transfusion. As RBC's may be associated with pulmonary endothelial damage, we have also studied the impact of the RCV guideline developed on post-operative acute lung injury (ALI). Finally, in a separate study, the merits of a simple activated clotting time (ACT) based system of anti-coagulation management for cardiopulmonary bypass (CPB) versus that of an individualised heparin management system (HMS) are described.