Anaemia in Critical Care
The investigations and management of anaemia in ICU have been explored to a great depth in Question 9 from the second paper of 2005 and the identical Question 4 from the first paper of 2003. There have been numerous other questions about anaemia (eg. Question 18.1 and Question 18.2 from the first paper of 2015) but they have generally been more concerned with the interpretation of iron studies and morphological abnormalities of red blood cells.
Probably the best single reference for this topic is this excellent article from the Mayo Clinic Proceedings (2003). In addition to this generic anaemia chapter, one may also wish to look at the chapter on haemolytic anaemia broadly, and autoimmune haemolysis specifically: these disorders seem to come up rather frequently in the exam papers, more so than one might expect from their community prevalence.
Classification of anaemia
Together with the low haemoglobin result one usually also gets a whole panel of RBC volume and Hb content indices, which immediately allows one to classify the anaemia into groups. These groups are organised by corpuscular haemoglobin content and RBC size. Alternatively, one could classify anaemia according to the pathophysiological mechanism, as in this article by Berlot et al (2014).
Dilution of RBC concentration
- dilutuonal anaemia with fluid replacement
- Artifactual anaemia due to sampling of a vessel with diluted content (i.e. the vein that has the fluids running through it).
Increased loss of RBCs
- Extravascular loss
- Traumatic blood loss
- Upper or lower GI blood loss, eg. gastric erosion/ulceration
- Iatrogenic loss through samling and surgery
- Intravascular loss
- Intravascular haemolysis, eg. autoimmune hemolytic anaemia
- Mechanical haemolysis, eg. due to extravascular perfusion circuits, dialysis filters, or mechanical heart valves
- Appropriate reticuloendothelial sequestration of abnormal haemoglobin
- Sickle cell anaemia
Decreased production of RBCs
- Decreased hematinics
- B12/folate deficiency
- Drugs which interfere with B12/folate metabolism
- Iron deficiency
- Decreased proliferation signals
- Decreased EPO in renal failure
- Anaemia of chronic inflammatory states
Investigations for anaemia
- History of fluid resusictation or trauma
- Resampling to confirm the diagnosis
- Imaging to look for obvious blood loss
- Faecal occult blood test and/or endoscopy to exclude GI blood loss
- Direct Coombs test, formal blood film, conjugated/unconjugated bilirubin, LDH, haptoglobin and reticulocyte count to investigate haemolysis
- RBC folate and B12 levels
- Iron studies
- Coagulation screen (looking for DIC and MAHA)
- Renal function tests and/or EPO levels
Management of anaemia in ICU
It is important to consider whether there is a genuine iron deficiency, or whether the inflammatory state has decreased access to the otherwise normal iron stores. Obviously, in a perfect world the iron-deficient patient would have some iron supplements, and the patient with inflammatory anaemia would have transfusions of blood and some sort of specific management aimed at the cause of inflammation.
However, in critical illness it is possible to have both issues simultaneously.
It is therefore difficult to say conclusively that any given critically ill patient will or will not benefit from a blood transfusion. Obviously some patients trend to one group or another. For instance, trauma patients can be expected to develop iron deficiency associated with their blood loss. A study which focused on trauma patients (Pieracci et al, 2014) enrolled 150 patients, of whom the majority demonstrated iron studies consistent with iron deficiency. Some were then randomised to receive 100mg of iron sucrose. In them, serum ferritin increased (presumably reflecting that the iron stores were replenished), albeit not quite back to the normal range. Unfortunately, nothing else happened. Neither did the haemoglobin increase, nor did the red cell count, nor was the need for transfusion affected. Mortality and ICU stay were also unaffected.
The IRONMAN trial is mentioned in Question 18.2 from the first paper of 2015; it also aims to answer the question of whether IV iron administered to anaemic ICU patients reduces their need for blood transfusion. The hypothesis is that yes, it will (unless you are severely septic, erythropoiesis should be stimulated by the infusion of 500 mg of ferric carboxymaltose). In contrast to Pieracci et al, the investigators are enrolling
Question 18.1 and Question 18.2 from the first paper of 2015 reported erythropoietin levels alongside conventional iron studies. This is odd, because it is not a part of the normal panel. The glorious RPCA Manual lists it as one of the tests "occasionally indicated" to discriminate primary from secondary erythrocytosis.
Erythropoietin, however, is one of the potential treatments for anaemia. Recombinant human EPO was used in this manner by Corwin et al (2002), whose group was able to achieve a 19% decrease in RBC transfusions. Without transfusions, the treatment group still ended up with a higher haematocrit than the placebo group. This had no impact on mortality outcome or duration of ICU stay. This decrease in blood transfusion seems to come at the cost of a significantly increased risk of thrombotic events (according to a 2013 meta-analysis by Mesgarpour et al). That is not to mention the actual dollar cost of EPO, which is comparable with the cost of RBC transfusion (if not greater). In any case, EPO is off-label for anything other than the chronic anaemia of renal failure.
Blood transfusion is of course the last option. As such, the pros and cons of blood transfusion in the ICU are discussed in greater detail elsewhere.