Patients with critical illness often develop anaemia due to frequent blood sampling, gastrointestinal blood loss as a result of stress ulcers or gastric erosions, blood loss from intravascular lines and haemodialysis or haemofiltration circuits, impaired erythropoeitin production and direct marrow suppression by cytokines. Such patients often develop shock and multiple organ failure. Organ failure is probably partly due to an inadequate supply of oxygen to cells.
It is important to ensure sufficient oxygen supply to organs by maintaining adequate cardiac output, haemoglobin concentration, and haemoglobin saturation. These three factors determine the oxygen delivery. It used to be thought that survival could be improved by maintaining very high levels of oxygen delivery by transfusing red cells and giving drugs that increased cardiac output. This was called goal-directed therapy and a frequently used goal was a haemoglobin concentration > 100g/l.
It is now known that in most critically ill patients this level of haemoglobin concentration is not necessary. Most intensivists transfuse critically ill patients if their Hb falls below 80 g/l and maintain a concentration of 70−90 g/l(PMID9971864.)(PMID15383011)
A possible exception to this guideline is for patients with known ischaemic heart disease. In this group many clinicians maintain a Hb > 90−100 g/l. ICU doctors no longer aim to achieve a predetermined oxygen delivery, but assess whether the oxygen delivery is adequate in individual patients by monitoring urine output, skin temperature, and the severity of lactic acidosis. Up to 50% of ICU admissions receive transfusion, and the ICU accounts for 5−6% of all red cells transfused.
Transfusion management: Critically ill patients often need transfusion because surgery or medical treatment is undertaken when the patient has a lowered Hb concentration and also a poor endogenous response to anaemia. Intravenous administration of iron may allow correction of Hb level in some cases (see Treatment of iron deficiency).