The demographic of the cardiac surgery population has progressively changed such that there is a greater proportion of older patients and females who by virtue of their co-morbidities and smaller average size have an increased risk of receiving blood products during and after surgery. Cardiopulmonary bypass (CPB) is the major contributing factor to this adverse event, as the technique is not only associated with coagulation and platelet abnormalities, but also by necessity, significant hemodilution as a consequence of the pump prime and the cardioplegia solution used to induce cardiac arrest. The hemodilution and the resultant decreased hematocrit (Hct) may trigger transfusion by the surgical team and there is evidence that platelet and coagulation function may be further compromised, resulting in worsening of postoperative bleeding. Modified ultrafiltration (MUF), a technique commonly utilized in pediatric cardiac surgery to reverse the effects of hemodilution after CPB, utilizes a circuit with an inflow attached to the aortic cannula, an ultrafiltration device, and outflow of the hemoconcentrated product directed back into the right atrium. Although MUF has been demonstrated to be effective at increasing the Hct, this technique has not received general acceptance as a blood conservation technology in the adult cardiac surgery population for several reasons. First, the necessary prolongation of the operative time with MUF may be perceived to be inconvenient by the operating staff and this may dissuade their use of this technique. Second, the results from the limited adult trials utilizing MUF, though positive in terms of blood conservation, have not shown changes of a magnitude that would routinely impact standard of care. It is possible that the perceived benefits of MUF in these trials have been less than impressive primarily due to the inclusion of patients at low risk of bleeding and the lack of blinding, thus contributing to the concern of treatment bias. Finally, none of these trials have been blinded and thus treatment bias may have contributed to the results. We believe that ultimately the use of MUF in this target population must be tested in a double-blind randomized controlled trial. However, the logistics of carrying out a trial of this magnitude must be established beforehand in a pilot trial that convincingly demonstrates the feasibility of blinding of this operative intervention, as well as the reproducibility of the model in several clinical centers. We are proposing to carry out a two phase multi-centre (University of Ottawa Heart Institute \[OHI\], Kingston General Hospital, Royal Victoria Hospital, University Hospital London) randomized controlled trial to evaluate the feasibility of an experimental model testing MUF versus a "sham" circuit in low-body weight (= 65 kg) patients at high risk to receive transfusion. In Phase I (n = 16 patients - 4 patients/center), the model will be optimized, and in Phase II (n = 64 patients - 16 patients/center) the project will be expanded and the blinding tested. The primary outcome will involve the success of blinding of the treating physicians (surgeon, anesthetist, and intensivist) in Phase II as measured by a novel Blinding Index. The information derived on the study model will be used to support a proposal in the subsequent definitive multi-center trial for which we will seek funding in the following year (n = 278/group). The primary outcome of the definitive trial will address the mean number of units of blood transfused in patients treated with or without MUF. There is also evidence in an animal model of the benefit of MUF to limit neurologic injury related to cardiac surgery. This aspect has not been assessed in a clinical model and therefore we intend to address this by evaluating the impact of MUF on early and late neurocognitive dysfunction related to CPB as a secondary outcome in the definitive trial. The proposed pilot and the definitive trial have the potential to address the role of MUF in adult cardiac surgery as a blood conservation technology. The potential application has significant implications as the procedure is inexpensive, easily teachable and physiologically sound. Further, it has very considerable relevance to women undergoing heart surgery due to their relative small body weight and increased current transfusion risk. As such, we believe that the magnitude of the findings from these trials will greatly impact current care in cardiac surgery worldwide.