Describe the PHYSICAL PRINCIPLES that are involved in the flow of blood through a dialysis circuit, and, in the movement of solutes across a dialysis membrane.
This question required candidates to describe the physical principles of blood flow
through a dialysis circuit and the movement of solute across a dialysis membrane.
While most candidates were able to allude to important factors contributing to the
flow of a fluid through a hollow tube, few did so in a systematic way and only some
provided relevant formulae showing the relationship between pressure, fluid viscosity
and tube resistance. A short discussion proceeding to flesh out the factors that
determine blood viscosity, circuit pressures and practical examples was expected.
Some candidates discussed convective processes extensively, which was not
required in this question focussed on dialysis. Most candidates were able to describe
the physical chemistry involved in diffusion across a semipermeable membrane in
basic terms, however few provided sufficient details of these important principles.
Very few candidates went on to properly discuss electrochemical forces affecting
solute and water movement across a membrane or the factors that influence the
performance of dialytic therapies in practical application.
Syllabus: A2c, R2e, D1, 2b,c
Recommended sources: Basic Physics and Measurement in anaesthesia, Davis and
Kenny, various sections. Also Review of Medical Physiology, Ganong, chp 2, 32
Without appearing to be a complete wanker, it is impossible to politely point out that we already know "this question required candidates to describe the physical principles of blood flow
through a dialysis circuit and the movement of solute across a dialysis membrane" because the question clearly asked us to "describe the physical principles that are involved in the flow of blood through a dialysis circuit, and, in the movement of solutes across a dialysis membrane". The examiner's comments which follow this statement are in equal parts informative and horrifying. The candidates were expected to produce:
This would have to be written with zero notice and over ten minutes. Moreover, it is not clear what "the factors that influence the performance of dialytic therapies in practical application" was supposed look like in quickly scribbled point form, or indeed what exactly was meant by this statement. Did they want causes of access pressure alarms, problems with circuit lifespan which affect the efficiency of continuous treatment, pre and post-dilution replacement fluid choices, or something simple like dialysate electrolyte prescription?
In short, reader, it was difficult to construct a model answer for this question on the basis of the examiner comments. It is encouraging to see that the pass rate was only 17%, as from this one can surmise that most people also found this question challenging. What follows is an earnest attempt to answer the question as it is written without becoming too distracted by the examiner comments or the sense of existential despair they naturally generate.