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:

 

 

Surprisingly, it is very hard to find a good single resource for this online.

In brief:

 

 

 

  • 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.

Briefly:

  • 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?

 

Improve access

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

 

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

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.

References

Chapter (pp. 540) 48   Renal  replacement  therapy, also by Rinaldo  Bellomo

 

John, Stefan, and Kai-Uwe Eckardt. "Renal replacement strategies in the ICU."CHEST Journal 132.4 (2007): 1379-1388.

 

Clark, William R., and Claudio Ronco. "CRRT efficiency and efficacy in relation to solute size." Kidney International 56 (1999): S3-S7.

 

Bellomo, R., and C. Ronco. "Renal replacement therapy in the intensive care unit." Intensive Care Med (1999) 25: 781±789

 

Ricci, Zaccaria, et al. "Solute removal during continuous renal replacement therapy in critically ill patients: convection versus diffusion." Critical Care 10.2 (2006): R67.

 

Wizemann, V., et al. "Efficacy of haemodiafiltration." Nephrology Dialysis Transplantation 16.suppl 4 (2001): 27-30.

 

Brunet, Sylvain, et al. "Diffusive and convective solute clearances during continuous renal replacement therapy at various dialysate and ultrafiltration flow rates." American journal of kidney diseases 34.3 (1999): 486-492.

 

Relton, Sudhakar, ARTHUR GREENBERC, and Paul M. Palevsky. "Dialysate and blood flow dependence of diffusive solute clearance during CVVHD."ASAIO journal 38.3 (1992): M691-M696.

 

Baldwin, Ian, Rinaldo Bellomo, and Bill Koch. "Blood flow reductions during continuous renal replacement therapy and circuit life." Intensive care medicine30.11 (2004): 2074-2079.

 

Gilbert, Roger W. "Blood flow rate effects in continuous venovenous hemodiafiltration on blood urea nitrogen and creatinine reduction." Nephrology nursing journal: journal of the American Nephrology Nurses' Association 27.5 (2000): 503-6.