A Study of the Effect of Blinding in a Trial of Blood Filtration During Heart Surgery

Cardiac surgery is a major consumer of blood products worldwide. The nature of the surgery when the heart-lung machine is used, is such that the patient's blood is significantly diluted by intravenous fluids. This dilution further compounds the blood's ability to form blood clots to seal the wound, thus increasing bleeding after surgery. The problem of this dilution is particularly severe in patients with low body weight, of whom a large percentage are women undergoing heart surgery. As a result, in this population there is an increased risk of needing a blood transfusion during and after surgery. The effects of this dilution can be potentially reversed by filtering the blood and removing the fluid after the principal part of the procedure has been completed through a process referred to as modified ultrafiltration (MUF). This procedure has been shown to be effective in several small clinical studies, however the interpretation of the results has been complicated by the fact that the studies were not "blinded". In other words, because the physicians and surgeons making the decisions about blood transfusions knew what treatment the patients received (i.e. MUF or not) it may have biased their judgement and affected the validity of the findings of the studies. The investigators believe that MUF may be a useful procedure to limit blood transfusions, particularly in patients of low body size, and weight undergoing heart surgery. This can only be demonstrated in a large clinical trial, and in the best case scenario, in a trial in which total blinding has been undertaken. However, blinding in this manner is quite difficult to achieve and it is necessary first to demonstrate that this is possible. Further, as the trial will involve several heart centers, it is essential to demonstrate that the relatively complex study interventions can be carried out in each of the centers in a reproducible manner. With this data in hand, the investigators will be able to submit for funding