Oxygen delivery and hemoglobin concentration in cardiac surgery: when do we have enough?

For many years it has been assumed that patients undergoing cardiac surgery utilizing cardiopulmonary bypass accumulate an "oxygen debt" that requires a higher postoperative hemoglobin concentration for its reversal. Much of this evidence has now been discredited due to mathematical error with recent research suggesting critical levels of oxygen delivery are lower than previously thought. This article aims to explore the relationship between observed and critical oxygen delivery with an estimation of the minimal hemoglobin required. This was a single-center observational study. Nineteen adult elective cardiac surgery patients were recruited to participate with four subsequently excluded. Observed measurements of oxygen delivery were recorded and compared with calculated "critical" values adjusted for temperature. The hemoglobin value that represented critical oxygen delivery was compared with the observed value to identify any "hemoglobin reserve." At no perioperative time point did observed oxygen delivery or critical hemoglobin concentration significantly approach its corresponding critical value. Current transfusion practice in noncritically ill cardiac surgery patients may be considered excessive if systemic oxygen requirement is the sole parameter considered.