Thresholds for red cell transfusion in surgery and critical care (Figure 8)
The critical Hb concentration is defined as the level below which organ ischaemia occurs due to inadequate delivery of oxygen. This level is different for different organs, but the heart may be most susceptible to very low haemoglobin levels because its basal oxygen extraction is high. On a whole body level, a variety of studies show that the critical Hb level may be about 50 g/l in healthy adults, children with acute − on chronic anaemia and in elderly patients. In parts of the world where blood for transfusion may be in limited supply or of uncertain safety, clinicians tend to use a Hb level around 50 g/l as a threshold for transfusing red cells.
Even where blood is safe and readily available, red cell transfusion should be used conservatively in the young because for them it is especially important to avoid any risk of long-term complications, and because young patients without cardiorespiratory problems generally tolerate low Hb levels.
A large randomised clinical trial in critically ill adult patients in Canada evaluated the use of a restrictive transfusion policy (a single unit of red cells when the Hb fell below 70 g/l and maintaining Hb in the range 70−90 g/l) or a liberal policy (transfusing when Hb fell below 100 g/l and maintaining Hb in the range 100−120 g/l) during intensive care treatment. Overall, 30-day mortality was similar in the two groups. However among patients who were less severely ill or were less than 55 years of age, mortality was significantly lower with the restrictive transfusion strategy.(PMID9971864)
The restrictive red cell transfusion policy appeared to be as effective as (and possibly better than) a liberal transfusion strategy in critically ill patients. Overall, the patients in the liberally transfused group had more cardiac complications, including myocardial infarction, suggesting that anaemia was not a major risk factor for these events. However, in the sub-group of patients who had ischaemic heart disease at study entry, there was a non-significant trend towards better outcomes in the liberally transfused group. Most of these patients had chronic rather than acute cardiac disease. Based on systematic review of this and other smaller trials ,(PMID12076437) current clinical guidelines generally suggest the following.
The safe lower haemoglobin level for patients with acute coronary syndromes is uncertain. A recent observational study suggests that in patients with acute coronary syndrome who develop bleeding, anaemia, or both during their hospital course, early mortality may be higher in patients who were transfused with nadir hematocrit values above 25%. The authors suggest caution in using transfusion to maintain arbitrary hematocrit levels in patients with acute coronary syndromes until evidence from appropriate randomised trials is available.
Figure 8 - Thresholds for red cell transfusion in the critically ill patient in absence of bleeding
[Figure 8 resources: View large format, download as gif, pdf or Word™ document]
For patients who are critically ill or undergoing surgery, and who do not have evidence of ischaemic heart disease, a haemoglobin concentration of 70 g/l is a reasonable threshold for transfusion. For those with evidence of ischaemic heart disease it may be safer to maintain the haemoglobin concentration at 90−100 g/l (see also Thresholds for red cell transfusion).
Intra- and post-operative blood conservation
Intra-operative blood salvage
Blood aspirated from the operative field can be re-infused to the patient. Blood may be returned as collected or it may be processed to remove plasma constituents. If large volumes of shed blood are returned without processing, the patient may experience coagulation problems that could cause more bleeding. Blood salvage procedures have been evaluated by clinical trials in cardiac and orthopaedic surgery; systematic reviews of these studies indicate that salvage can reduce the proportion of patients who receive allogeneic red cell transfusion in orthopaedic surgery. In cardiac surgery, trials show only a slight reduction in transfusion of allogeneic red cells. This may be due to the inclusion of trials in which unprocessed blood was re-infused. It should be noted that, although unproven by clinical trials, many clinicians believe that patients with major surgical blood losses do better when salvaged blood is reinfused to reduce transfusion requirements.(PMID14583940)(PMID10512256)
Post operative blood salvage
Blood from wound drains can be collected and re-infused using special equipment. This procedure is used in the belief that it can reduce transfusion requirements in some operations such as knee replacement. The re-infusion of unprocessed blood from wound drains may cause coagulation problems, so some authorities recommend that blood is processed by washing before it is re-infused. The effectiveness of this procedure in reducing allogeneic transfusion has not been proven by adequate clinical trials.
Post-operative anaemia
After a major operation, provided iron stores are adequate and the patient has normal bone marrow function, the reticulocyte count begins to rise after a week or so and by one month after operation, about 60% of the perioperative fall in haemoglobin will typically have been regained.
Iron stores equivalent to a ferritin of 15−20 micromol/l are required for each gram/dl of haemoglobin recovered. Post-operatively, many patients become iron deficient as a result of the operative blood loss and utilisation of iron stores for recovery.
Management: Iron given preoperatively is only likely to be effective if the patient’s iron stores are low (ferritin below 30 micromol/l). Post-operatively, absorption of iron is probably good in patients without other chronic disease.
A three to four week course of iron supplements started on post-operative day seven should be considered for a patient with low preoperative iron stores and large perioperative loss of haemoglobin. (see Treatment of iron deficiency − oral iron replacement.)