Strategies to Optimise Solute Clearance
The question about optimising solute clearance usually presents the candidate with a clinical scenario where a patient is full of some sort of noxious solute; the candidate is then invited to assist that solute in its timely exit from the patient's body fluids.
Such questions include:
- Question 16 from the second paper of 2011
- Question 12 from the first paper of 2010
- Question 5.4 from the first paper of 2009
- Question 2c from the second paper of 2001
- Question 2d from the second paper of 2001
Surprisingly, it is very hard to find a good single resource for this online.
- Increase filter lifespan with anticoagulation and predilution
- Rationalise planned interruptions to CRRT (eg. scans and procedures)
- Improve vascular access to minimise interruptions to CRRT
- Increase the blood flow rate
- Increase the dose of dialysis
- Increase the dialysate flow rate
- Increase the ultrafiltration rate
- Increase the replacement fluid rate
- Use of pre-dilution
- Adjustment of dialysate to modify concentration gradients
- Increasing the surface are of the filter
Solute removal by dialysis and haemofiltration
Increase filter lifespan
This is cheating - the efficiency of per-hour solute removal is unaffected, but the circuit lasts longer, allowing a larger amount of solute to be removed per session.
Strategies to improve circuit lifespan are discussed elsewhere.
- Protect the circuit with anticoagulation
- Use pre-dilution replacement fluid
- Ensure an adequate blood flow rate
Rationalise interruptions to CRRT
Again, this does nothing to improve the hourly solute removal rate, but it increases the total solute removal over the long term. Is that CT scan really that urgent?
If the patient's vas cath is in some profoundly stupid position, their dialysis will run poorly. The machine will alarm constantly and need to be fiddled with; interruptions and poor flow rates will result in degraded performance.
- Ensure the vas cath is not kinked
- Ensure it is in a comfortable position which allows good flow rates
- Ensure the patient is not fidgeting or constantly changing their position in a disruptive manner (one may consider using some sort of gentle sedation).
Increasing the blood flow rate
- Older studies using older membranes did not demonstrate much benefit from high blood flow rates; diffusive solute clearance plateaued at blood flow rates of 100-150ml/min.
- Modern membranes may benefit from higher blood flow rates. Empirically, small solute clearace seems to be optimal with blood flow rates around 130-150 ml/min. Another study has demonstrated that increasing the blood flow rate from 300ml/min to 500ml/min leads to a 40% improvement in urea clearance.
- A good blood flow rate also ensures the circuit remains clot-free (there is less stasis); sluggish flows have been associated with decreased circuit lifespans.
Increasing the dose of dialysis
It would be slightly silly to define the dose of dialysis in the classical terms of marker solute clearance; it would be as if one says "to increase the clearance of solute, prescribe an increased rate of solute clearance". Fortunately, dialysis dose is usually prescribed in terms of effluent rate, or ultrafiltrate+dialysate rate in CVVHDF. This consists of several components, each of which contribute slightly differently to the clearance of different solutes:
Increasing the dialysate flow rate
- Increased flow rates of the dialysate fluid and effluent will result in greater solute removal by diffusion.
- This strategy favours the clearance of small molecules
Increasing the ultrafiltration rate
- Increased transmembrane pressures will result in increased ultrafiltration rates and thus greater removal of solute by convection.
- This strategy favours the removal of middle molecules (though, strictly speaking, the smaller molecules will also be removed).
Increasing the replacement fluid rate
- An increased rate of ultrafiltration permits the use of large volumes of replacement fluid.
- This has the effect of diluting the bloodstream with an electrolyte solution of known composition
- Thus, noxious solutes are diluted post-filter
- This has the effect of decreasing their concentration in the bloodstream
- Both small and middle molecules benefit equally from this strategy
Use of pre-dilution
- Use of pre-dilution replacement tends to elute more urea out of red cells.
- Though this actually decreases the rate of solute removal, it may improve the efficiency of the overall strategy by allowing a longer circuit lifespan.
- Post-dilution can enhance solute removal by up to 40%, but the circuits die too quickly, clogged with debris; overall this seems to be a poor strategy.
Adjustment of dialysate to modify concentration gradients
One can influence the concentration of the dialysate to improve the countercurrent removal of some solute of interest. This applies to electrolytes. For instance, one would not use a dialysate or replacement fluid with 5mmol/L of potassium in a patient with a 7.5mmo/L hyperkalemia - the concentration gradient would only be 2.5mmol/L. With less potassium in the dialysate, a greater concentration gradient will favour the removal of potassium from the blood; and potassium-poor replacement fluid will contribute to the clearance effect.
Increasing the surface are of the filter
- Change to a filter with a greater surface area
- This uses the Fick principle: the greater the surface area of the membrane, the greater the rate of diffusion
- In practice, not all of the filter's surface area is equally exposed to dialysate, and therefore increasing the filter surface area results in a merely modest increase in solute clearance.