Question 21

Compare Continuous Venovenous Haemofiltration (CVVHF), Sustained Low Efficiency Dialysis (SLED) and Intermittent Hemodialysis (IHD) with respect to:

a)    Mechanism of solute clearance

b)    Advantages

c)    Disadvantages

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College Answer




Mechanism of solute


Solvent removal occurs as a consequence of a pressure gradient across a semi permeable  membrane. Solute removal occurs only by convection (solvent drag).

Solute removal occurs

predominantly by

diffusion down a

concentration gradient

created by dialysate fluid

on the other side of the

semi permeable


Solute clearance by



Achieves better clearance

of middle molecules (< 15

Kd) than CVVHD/IHD, fluid

management easier and

flexible, lesser

hemodynamic instability as

compared to IHD.

Can be done at night so

patient can be mobilized

during the day. Period of

anticoagulation reduced.

Possible cost savings by

using online water and

ability for one machine

to deliver 2 treatment

episodes per day.

Shortest treatment



often not required,

cost savings by

using online water


Patient immobilized, need

for continuous

anticoagulation, higher

nursing requirement

Inferior clearance of

middle molecules,

reduced fluid

management flexibility.

Higher risk of

disequilibrium syndrome

Least clearance of

middle molecules,

least flexible fluid


highest risk of


syndrome. Possible




Additional Examiners’ Comments:

Candidates’ knowledge and understanding of a core topic was overall poor. Some candidates were not able to describe the mechanisms involved. Many described CVVHDF rather than CVVHF


The college answer is of a surprisingly high quality. Little can be done to improve on it, except to furnish the points with references. And to rearrange the table, to make it look like a different table.

A Comparison of Renal Replacement Therapies
Modality and mechanism Advantages Disadvantages  


  • Solute clearance is convective
  • A negative pressure is generated by the effluent pump, pulling plasma water through a semipermeable membrane to produce an ultrafiltrate; the volume is then restored with replacement fluid.
  • More efficient clearance of all solutes*; but especially of larger molecules (because their size is a barrier to diffusion)
  • Haemodynamically better tolerated, because of more gradual fluid shifts
  • There is greater flexibility in the fluid management ***
  • Large volumes of fluid removal can be accomplished (over time)
  • Able to remove substances with a high volume of distribution (over time)
  • ICU-specific: does not require external staff (eg. dialysis unit nurses)
  • Expensive consumables: relies on pre-made sterile bags of dialysate and replacement fluid
  • Labour-intensive 
  • Requires special training
  • Increased removal of solutes is purely a function of time, i.e. the modality is only superior in this parameter because the circuit runs 24/7.
  • Requires anticoagulation
  • Continuous modality: impossible to mobilise the patient; treatment must be interrupted for procedures or investigations


  • Solute clearance is mainly by diffusion**
  • Ultrafiltration occurs to achieve fluid balance, and as a consequence some clearance also occurs by convection
  • Rapid removal of small solutes: this may be of critical importance if the solute is a toxin or a lethally high potassium level
  • Does not require special training (i.e. only standard dialysis training is required)
  • May be possible to borrow staff from the dialysis unit
  • It is usually possible to run IHD without anticoagulation
  • Does not need expensive pre-made bags of dialysate
  • Offers prolong breaks between sessions, eg. to mobilise the patient or perform investigations
  • Haemodynamic instability: hypotension is seen in 20-30% of IHD treatments in maintenance haemodialysis, to say nothing of criticially ill patients
  • Large volumes of fluid removal are impossible (there is only so much you can safely remove in a 4-hour session)
  • Requires the availability of ultrapure water for dialysate
  • Reliance on non-ICU dialysis staff may delay the availability of urgent RRT


  • Solute clearance can be purely by diffusion if the filter is run in HD mode, or also by convection if any fluid removal is prescribed. The latter is usually designated as something other than SLED, eg. something like SLED-f
  • Does not require special training (i.e. only standard dialysis training is required)
  • Lower nursing workload
  • Does not require pre-made sterile dialysate bags
  • It may be possible to run SLED without anticoagulation
  • Relatively ill ICU patients will still tolerate this haemodynamically
  • Usually it is possible to use minimal or no anticoagulation
  • Offers significant breaks between treatments
  • The most unstable ICU patients will not tolerate this modality.
  • Requires the availability of ultrapure water for dialysate
  • A CRRT machine usually cannot do SLED, i.e. additional equipment needs to be purchased

Some footnotes of interest:

* The college answer makes much of the improved clearance of middle molecules by CVVH, but in actual fact  all molecules  are cleared better by this modality. An excellent comparison, almost purpose-made for this SAQ, comes from Liao et al (2003). The authors found that clearance of small solutes by CVVH  was 8% better than by SLED, and 60% better than by IHD. This can be illustrated using graphs which were stolen shamelessly from the Liao paper. For large and middle molecules, CVVH is even better - twice as effective as SLED and four times as effective as IHD. This is mainly because the therapy is sustained continuously.

small solute clearance in different RRT modalities

** Though the college answer presents intermittent haemodialysis as a technique purely dependent on diffusion, in practice this is not the case, even though it's called "haemodialysis".   Some ultrafiltration needs to occur in order to control the fluid balance, and this is the norm for maintenance dialysis - that's why people discuss the "dry weight" of chronic IHD patients. In case this baremetal fact needs to be varnished with peer-reviewed literature, one could pull out just about anything from the Am J Kid Dis  or Nephrology and Transplantation. Literally anything. Here's a 2016 study by Assimon et al correlating ultrafiltration rate to mortality, for example. For interests' sake, its worth noting that in that study the average ultrafiltration rate was about 6.6ml/hr/kr, or about 1.8L total fluid removal after a 4hr session for an average 70kg patient. Since probably the late 1980s straight HD has been replaced by HDF (haemodiafiltration), because the latter was a significant improvement. Canaud et al (1989) described it as "the new standard"

*** The college suggests that fluid management is inherently more "flexible" in CVVH. This is an interesting statement. An article by Murugan (2016) also suggests that CVVH and CRRT in general allows more precision in the control of fluid balance. Practically, it is difficult to determine how the same 100ml/hr fluid removal by SLED or IHD is any less easy or less flexible. Arguably, it is actually easier to achieve a high negative fluid balance with SLED or IHD because the nurses do not need to laboriously exchange full bags of effluent. The main "flexibility"is really the ability to run the circuit continuously, which means you remove as much fluid as you want - you just need to keep it running for longer.


Assimon, Magdalene M., et al. "Ultrafiltration rate and mortality in maintenance hemodialysis patients." American Journal of Kidney Diseases 68.6 (2016): 911-922.

Jean, Guillaume, et al. "Online-haemodiafiltration vs. conventional haemodialysis: a cross-over study." BMC nephrology 16.1 (2015): 70.

Canaud, B., et al. "Hemodiafiltration with on-line production of bicarbonate infusate." Improvements in Dialysis Therapy. Vol. 74. Karger Publishers, 1989. 91-100.

Liao, Zhijie, et al. "Kinetic comparison of different acute dialysis therapies." Artificial organs 27.9 (2003): 802-807.

Murugan, Raghavan, et al. "Precision fluid management in continuous renal replacement therapy." Blood purification 42.3 (2016): 266-278.

O'Reilly, Philip, and Ashita Tolwani. "Renal replacement therapy iii: IHD, CRRT, SLED." Critical care clinics 21.2 (2005): 367-378.