Packed red blood cells
This chapter is relevant to Section Q4(i) of the 2017 CICM Primary Syllabus, which expects the exam candidates to demonstrate "understanding of the pharmacology of blood and its components, including individual factor replacement". To some extent, it also satisfies Section Q4(iii), which expects them to "understand the process of collection and production of blood and its components". At this stage, the humble bag of the House Red has escaped the attention of CICM examiners, but, considering that it is probably the most commonly prescribed blood product, it is only a matter of time before the trainees are asked something impossibly detailed and obtuse about it.
In summary:
Description: Packed red blood cells is a plasma-depleted cell product for the replacement of haemoglobin.
Presentation: Flexible container with 150-250ml of concentrated red cells.
Preparation of PRBCs:
- Usually collected as whole blood donation
- Citrate is added as an anticoagulant
- Cells are then centrifuged to remove the plasma and buffy coat
- What remains is filtered through a leukodepletion filter to remove any residual white cells
Contents: red cells packed to a haematocrit of 0.50-0.70; each bag will be slightly different:
- 40-50g haemoglobin (i.e. a concentration of about 200g/L
- 25 mmol sodium (i.e. a concentration of about 120 mmol/L)
- 5 mmol potassium (i.e 25 mmol/L)
- 5 mmol lactate (yes, the lactate of red cells is about 25mmol/L)
- about 0.5mmol of citrate (i.e. about 2.5-3.0 mmol/L)
- Water, with a pH of about 6.5
Storage is in the fridge, at 2-6º C; shelf life no longer than 42 days ("storage lesions").
Indications for use:
- To increase oxygen-carrying capacity where there is clinically significant anaemia
- To decrease the myocardial workload
- To replace ineffective red cells (i.e. exchange transfusion)
Dose: one bag of PRBCs will increase the haemoglobin by ~ 10 g/L.
Need for typing and crossmatching: essential to ABO type and crossmatch prior to transfusin
The adverse effects of blood transfusion, discussed elsewhere, in summary, are:
- Circulatory overload
- Lung injury (TRALI)
- Allergic reaction (from urticaria to anaphylaxis)
- Acute or delayed haemolytic transfusion reaction
- Non-haemolytic febrile transfusion reaction
- Iron overload
- Transfusion-associated graft-versus-host-disease (TA-GVHD)
- Post-transfusion purpura
- Risk of bacterial or viral infection
Most of this information comes from transfusion.com.au, which should be viewed as the definitive resource for all things transfusion related (at least for Australian ICU trainees).
Packed Red Blood Cells
Approximately 450 ml of donated blood turns into about 250. This happens in a centrifuge. Prior to being centrifuged, citrate is added to plasma; this results in a chelation of calcium. In the absence of free calcium ions, the coagulation cascade grinds to a halt. The cells are thus separated from their plasma, and put into a bag. In addition to cells, various preservatives are added, and they maintain red cell viability during their long cold storage.
Storage lesions
In storage red cells also modify their own environment by constantly spewing the products of anaerobic metabolism into it. Furthermore, some red cells lyse, and their corpses further pollute the contents of the bag. The milieu inside a unit of packed red cells becomes quite noxious, and the resulting stored product does not really resemble living human blood. For example, here's an arterial blood gas collected directly from a bag of PRBCs:
And so, you're planning to infuse a litre of this into somebody. Looking at the composition of this death-fluid, you can't help but wonder whether that could cause some sort of harm. Indeed, over the years some concerns have been raised that transfusions of "aged" cells (i.e. ones which have been stored over a longer time) have increased the mortality of trauma patients and the elderly. This has not materialised into trial findings (i.e. the data does not support the use of "fresh" cells over older ones), but it is still worth knowing about. The changes in PRBC composition during storage, broadly described as "storage lesions" appear more commonly in the Second Part exam (eg. Question 29.1 from the first Fellowship Exam paper of 2010) and there is an entire chapter dealing with storage lesions in the Part Two revision section.
Cellular content of Packed Red Blood Cells
The Australian Red Cross Blood Service claim that their house red contains 40g of haemoglobin per 200ml bag, which would come to a Hb of 200 per litre, at a hematocrit of 0.5 to 0.7. This resembles the full blood count of a polycythaemic person from the foothills of the Himalayas.
Electrolyte content of Packed Red Blood Cells
Surprisingly, nobody seems to have ever picked up a bag of packed cells and done an EUC and CMP on it. I would love to know the results. Whats the osmolality? How much citrate remains in there? Who knows. There are some articles where electrolytes in red cells are measured, and there is the transfusion.com.au website where red cell additive solutions are described (though not the reasons for their addition!) Different companies seem to offer different mixtures of additives. In the figures above the Fresenius constituents were used.
References
R. Sümpelmann , T. Schürholz , E. Thorns , J. Hausdörfer Acid–base, electrolyte and metabolite concentrations in packed red blood cells for major transfusion in infants Pediatric Anesthesia Volume 11, Issue 2, pages 169–173, March 2001