Both Question 22 from the second paper of 2014 and the near-identical Question 14 from the first paper of 2010 discuss the principles, indications and complications of plasmapheresis. Question 21 from the first paper of 2013 mentions it in the context of TPP (we are invited to explain the difference between plasmafiltration and plasmapheresis). Question 5 from the first paper of 2005 takes a more reflective "critically evaluate" direction. The college answers to these questions form an excellent concise overview of what is expected from the candidate. Most of the SAQs can be answered after reading Jeffrey L. Winters' 2012 article from Hematology. Additionally, our very own college examiners have put a whole chapter on this technique into Oh's Manual (Chapter 97, pp. 993).
In brief, the principles of plasmapheresis can be summarised as follows:
- Plasmapheresis may refer to a variety of procedures, all involving the therapeutic separation of blood into components.
- Separation of blood into cellular and fluid components offers the opportunity to discard or modify those components.
- This separation is usually accomplished by means of either a centrifuge or by the less frequently used method of porous membrane filtration.
- Unlike dialysis or haemofiltration, there is no size barrier: plasmapheresis removes the whole plasma with molecules of all sizes.
- Some blood components are then reinfused into the patient with or without modification, and the rest may be discarded or stored.
- If plasma is being removed, volume is replaced with 4% albumin or FFP
- For plasmapheresis to be justified, the disease process must have a circulating factor involved in pathogenesis, which can be expected to persist in the circulation without sufficiently rapid endogenous clearance.
The Greek word “ἀφαίρεσις” means “to take away” or “to separate", and crudely that is exactly what the plasmapheresis devices do. It is also a term in linguistics which describes the phonetic loss of a sound, for example the missing "k" in "knife". Plasmapheresis might refer to several possible procedures, depending on which blood components are separated.
There are 2 techniques:
In the centrifuge method, blood separates into layers within a rapidly rotating cylinder, with the plasma staying in the centre of the cylinder, and cellular components moving to the periphery. The components of the blood separate into layers on the basis of density. The red cells end up being the densest and are at the extreme periphery of the blood column; next outermost are granulocytes, then the relatively lighter lymphocytes and finally the platelets. In the centre, plasma remains.
This central plasma compartment is the siphoned off. Depending on what you want to dump and what you want to keep, the plasma may be discarded and the remaining cells mixed with a replacement fluid (albumin or FFP) and returned to the patient, or vice versa. There is some mixing of components at the interface between spinning layers (understandably they are not neatly arranged, as spinning fluid is prone to misbehaviour) so for example the plasma might contain some platelets. In short this means
One measures the "dose" of plasma exchange by volume of replacement fluid as compared to the total plasma volume of the patient. If a patient has about 4L of plasma and 4L of replacement fluid is used in the course of plasma exchange, it is said to be a 1 plasma volume exchange.
The rate of removal is incomplete - one cannot remove the entire fluid volume by spinning the cells down to their "dry" weight. Thus, for every 1-1.5 plasma volumes exchanged, about 60-70% of the plasma substances are removed. For every subsequent volume exchanged, the absolute amount of the substances decreases but the proportion remains the same (60-70%). Thus, with increasing doses of plasma exchange, the efficiency diminishes. Generally speaking only 1-1.5 plasma volumes are exchanged in each session.
Having your plasma sucked out of your body, replaced with a foreign fluid and reinfused back into you is clearly a rather invasive process not without its adverse effects. It is therefore engaged upon only in such circumstances where there is no option other than to remove some component of the blood. There are two main characteristics for a disease process which make plasmapheresis a sensible decision:
Taking these two principles, one can imagine the sort of disease-causing blood components one might want to extract. These might include the following undesirables:
The process of exchange is non-selective, and removes all plasma contents, good and bad. FFP replaces some of these, but not all. Albumin obviously only replaces albumin. Desirable molecules removed by plasma exchange and not replaced with standard replacement fluids include the following:
Russi and Marson have a 2011 article which presents a table with a few indications for urgent plasma exchange. The gospel list of all indications can be found in the ASFA guidelines statement from 2010. Their list is massive, spanning three pages of their ninety-five page statement. Highlights from this list (those indications which received Grade I recommendation) include the following:
Urgent plasma exchange:
Less urgent plasma exchange:
One should note that in their list of indications, the college noted some Grade II, III and IV recommendations, such as:
The question specifically asks about complications unrelated to catheterisation. Catheterisation is classically something which produces an access point the characteristics of which are identical to what you might use for dialysis. Generally a standard vas cath is all that's required.
Non-catheter-related complications include the following: