Viva 3

What is this device?

Image result for fresenius filter

What characteristics make it suitable for its purpose?

(Originally, this was a question on the "Management of post-operative renal failure", but all will probably agree that this would have made a boring viva.)

Anyway: that's a dialysis filter. Ideally, you will have a filter to hand to the trainee, so they can fiddle with it.

Cross section of a dialysis filter

The characteristics of an ideal membrane must be:

  • Cheap - according to Sakai (2000) over 70 million m2 of membrane are used per year, worldwide
  • As thin as possible
  • As strong as possible, including when it is wet
  • Chemically, or at least biologically inert
  • Non-thrombogenic
  • Selectively permeable, allowing diffusion only for specific undesirable molecules

Most membranes these days are synthetic, highly biocompatible and approximately 100-200 μm thick  with an internal diameter of 200-500 100-200 μm. The typical filter surface area is usually about 1.5-2.0 m2

What are the different modalities of therapy which this device could be used for?
  • SLED
What is the difference between dialysis and haemofiltration?
  • Hemodialysis (HD): An extracorporeal, primarily diffusive therapy, where solute and water are transported across a semi-permeable membrane into dialysate.
  • Rate of diffusion (the solute diffusive flux, Jd) is influenced by several factors:
    • Concentration gradient (dc, probably the most important factor)
    • Filter membrane characteristics (eg. thickness, porosity)
    • Available surface area
    • Temperature of the solution
    • Diffusivity coefficient, which is turn influenced by:
      • the gas constant
      • the viscosity of the solvent
      • size of the solute particles.
  • Haemofiltration
How does haemofilttration achieve the clearance of solutes?
  • Hemofiltration (HF): An extracorporeal, primarily convective therapy, where solute and water are transferred across a semi-permeable membrane. Replacement fluid is used to achieve fluid balance.
  • Convection is bulk-flow of solute across a semi-permeable membrane together with a solvent in a manner that is dependent on transmembrane pressure and membrane characteristics.
  • This is how small and middle molecules are cleared during haemofiltration.
  • Replacement fluid is administered post-filter, which dilutes the returning blood volume
How  would you define a "dose" of dialysis?


  • Definition of "dose" in CRRT is volume of blood "purified."
  • Measure of "dose" in CRRT: clearance rate of a representative marker solute.


  • Dialysis dose is equivalent to the effluent rate in ml/kg/hour.
  • Effluent rate is the ultrafiltration rate for haemofiltration (CVVH), or the sum of ultrafiltration rate and dialysis rate for CVVHDF
What is meant by the term "transmembrane presure"?
  • Transmembrane pressure is the hydrostatic pressure gradient across the membrane. This is the driving force that causes ultrafiltration.

Transmembrane pressure (TMP) = (Filter pressure + Return pressure) / 2 – (Effluent pressure)

TMP is the effluent pressure subtracted from the average of the pressures in the blood side of the circuit (which are the filter pressure and the return pressure).

What is replacement fluid?

Replacement fluid  is the fluid which is used to restore volume, buffer base and a normal haematocrit to the post-filter blood in haemofiltration and haemodiafiltration. The use of replacement fluid is what defines haemofiltration as a distinct modality from SCUF. The difference between ultrafiltration rate and replacement fluid rate is the total fluid removal rate. ADQI define replacement fluid as follows: 

"Replacement (substitution) fluid: A solution of variable composition, often physiologic, used to replace large volumes of ultrafiltrate during hemofiltration or hemodiafiltration. Replacement fluid may be given as predilution or postdilution."

What are the advantages and disadvantages of pre and post-dilution?


  • Clearance of middle molecules (those dependent on convection) is proportional to ultrafiltration rate
  • Clearance of small molecules (those dependent on diffusion) is greater because of the higher concentration gradient.
  • Less replacement fluid is required, which affects the cost of treatment
  • Filter lifespan might be shorter, espeically if you don't want to use anticoagulation
  • Maximum dose of dialysis will be limited by blood flow rate


  • Clearance of solutes might be slower
  • Filter lifespan might be longer
  • Theoretical maximum dose of dialysis is much higher
What is dialysate?

Dialysate: A solution of variable composition designed to facilitate diffusion of solutes into the ultrafiltrate-dialysate compartment of the hemofilter or hemodialyzer.

What is the normal composition of dialysate?

A few basic characteristics are expected of the product:

  • It should contain a physiological concentration of electrolytes, i.e  resembling  the concentration of electrolytes in the body fluids of a healthy human
  • It should contain buffer, so that it may act as a source of buffer for an acidotic patient
  • It should contain some physiologic amount of glucose
  • It should be sterile
  • It should be warmed to near-normal body temperatures.  Dialysis with a room-temperature dialysate tends to result in a patient core body temperature drop of about 2°C on average. 

Generally, modern dialysate contains:

  • Sodium 138-140 mmol/L
  • Buffer 35-45 mmol/L
  • Calcium 1.5-2.0 mmol/L
  • Magnesium 1.0mmol/L
  • Glucose 6.0 mmol/L
  • Zero potassium
  • Zero phosphate
What are the complications of using these therapies, other than the complications of vascular access?
  • Haemolytic complications
    • All RRT filters tend to eat red cells. This is a complication of forcing blood to rub against a cheesegrater-like porous membrane.
  • Inflammatory reponse
    • The dialyser membrane is a proinflammatory surface. Modern membranes are a massive improvement, but some inflammatory reaction (particularly complement activation) is to be expected. Additionally, one's bloodstream becomes showered with the shredded remains of red blood cells, which exerts its own proinflammatory effect.
  • Blood loss due to circuit loss
    • If a filter clots, the whole thing is discarded, together with whatever blood is in the circuit. This could be a little or a lot, depending on the filter and circuit. Usually, the amount of blood lost is no greater than 200-300ml, equivalent to a drop of 10g/L of haemoglobin.
  • Hypothermia
    • Because a large volume of blood (roughly 5-10% of the blood volume) spends every minute outside the body, it is exposed to the ambient temeperature, which in the ICU is typically rather chilly. The returning blood is usually cool. The patient may become hypothermic as a result. This phenomenon may obscure the presence of a fever, or it may result in a clinically significant drop in the core body temperature.
  • Electrolyte disturbance
    • Unintelligently prescribed dialysis can lead to electrolyte disturbance. If you have prescribed a dialysate or replacement fluid which is completely free of potassium, you should not be surprised that the patient becomes dramatically hypokalemic.
  • Hypocapnea
    • The dialyser membrane is no obstacle for the highly water-soluble CO2; some CO2 will diffuse through the membrane and into the dialysate.
  • Hypoxia
    • Activation of complement and the inflammatory mechanisms leads to an increase in the activity of nitric oxide synthase, which countracts the normal mechanisms of hypoxic pulmonary vasoconstriction. Increased shunt develops; therefore hypoxia ensues.
  • Malnutrition due to dialytic nutrient loss
    • The bloodstream is a necessary destination for all the absorbed nutrients, as well as for TPN. Dialysis removes many of the useful nutrient molecules. Specific easily cleared nutrients are amino acids (all highly water soluble small molecules) and water-soluble vitamins. Depending on one's ultrafiltration volume, the total amino acid loss may be around 10-20g/day. If on TPN, up to 10% of infused protein content may end up in the effluent bags.
  • Delayed renal recovery
  • Haemodynamic instability
  • Dialysis disequilibrium syndrome
  • Complications related to anticoagulation

Disclaimer: the viva stem above may be an original CICM stem, acquired from their publicly available past papers. Or, perhaps it is a slightly altered version of the original CICM stem. Or, it is a completely original viva stem, concocted by the monstrously amoral author of Deranged Physiology for nothing more than his own personal amusement. In either case, because the college do not make the main viva text or marking criteria available, almost everything here has been confabulated. It might sound like a plausible viva and it could be used for the purpose of practice, but all should be aware that it does not represent the "true" canonical CICM viva station.