List the potential causes of anaemia in critically ill patients, and outline how you would determine which factors were contributory.
Common problems encountered related to the lack of an organised approach (eg. to history, examination and investigation). Blood loss can be occult!
Anaemia in critically ill patients is usually multifactorial. Potential causes can be categorised into decreased production (as a small proportion [approx 1%] of circulating RBCs are destroyed each day), increased destruction, loss of RBCs and haemodilution. Decreased production includes problems with nutrients (eg. iron, folate, B12), disease involving bone marrow (eg. infiltration, myelodysplasia), depressant effects of drugs (eg. chemotherapy) or irradiation, and low levels of stimulatory hormones (eg. EPO in renal failure, thyroid hormones). Increased destruction can occur in haemolytic anaemias: either congenital (eg. thalassaemia major, sickle cell) or acquired (eg. Coomb’s positive auto-immune, TTP-HUS, infection with malaria or clostridiae etc). Increased RBC loss can occur via injuries, bleeds into viscera or organs (eg. GI tract, GU tract, lungs) and iatrogenic (procedures, blood samples for testing). Dilutional anaemia usually occurs in the context of rapid or extensive non-blood fluid resuscitation.
Evaluation of cause includes obvious but essential role of history (trauma, drugs and therapies, nutrition, chronic disease, infection, review of blood tests and procedures etc) and examination (trauma, sites of potential blood loss [including PR], jaundice, hepato-splenomegaly etc.). Simple investigations include morphological assessment of blood (eg. MCV, blood film: red and white cell mophology), reticulocyte count, electrolytes and renal and liver function tests. More specific tests
as indicated include assays for folate/B12/ferritin, indicators of haemolysis (eg. haptoglobin, Coomb’s test), Hb electropheresis, cultures for infection (±thick/thin film) etc.
Important causes in ICU, which the candidate should emphasise, include:
a) blood samples for testing
b) EPO suppression and lack of marrow responsiveness in sepsis c) trauma
d) stress ulcer bleeds
e) extracorporeal circuits blood loss
f) hemodilution from resuscitation
g) chemo/oncology patient groups
This question is best approached systematically. For this purpose, anaemia can be divided into groups by function (rather than the conventional classification, by RBC morphology - into normocytic, microcytic and macrocytic).
The conventional approach is by far the better approach, because 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. The Mayo Clinic Proceedings have an excellent article from 2003 which does this topic justice.
Dilution of RBC concentration
Increased loss of RBCs
Decreased production of RBCs
Thus, a panel of investigations which might differentiate between the abovementioned differentials should include the following:
Walsh, T. S. "Anaemia during critical illness." British journal of anaesthesia97.3 (2006): 278-291.
Vincent, Jean Louis, et al. "Anemia and blood transfusion in critically ill patients." Jama 288.12 (2002): 1499-1507.
Tefferi, Ayalew. "Anemia in adults: a contemporary approach to diagnosis."Mayo Clinic Proceedings. Vol. 78. No. 10. Elsevier, 2003.