The candidates have been asked to compare different CRRT modalities in numerous past paper questions:

  • Question 21 from the second paper of 2015 - IHD vs SLED vs CVVHDF
  • Question 10 from the first paper of 2011 -CVVHD vs IHD vs SCUF
  • Question 19 from the second paper of 2008 - CVVHF vs SLED vs IHD
  • Question 2c from the second paper of 2001 - "List the available dialytic therapies and their associated advantages/disadvantages"

Typically, the college has expected the answers to be presented in a tabulated format. The table below makes an attempt to combine all these tables into a single reference. It has had to be split into two, so as not to induce severe strain among the eyesight-impaired readers.

Modality SCUF PD CAVH
Access Vas cath PD abdominal catheter Arterial catheter
Flow rate Low flow rate;
50-200ml/min
nil; rather, fluid dwell time is the important feature Arterial flow rate
Anticoagulation Continuous None Intermittent boluses may be required
Fluid removal Medium Slow Slow
Electrolyte removal Slow;
by convection
Very slow;
by diffusion alone
Slow;
by convection (mainly)
Efficiency of solute clearance Very low (minimal, really)
- but it is not meant for solute clearance
Poor efficiency of fluid and electrolyte clearance Low
However, good solute clearance is ultimately achieved over a prolonged course
Hemodynamic impact Very well tolerated Tolerated by most patients Unsuitable for hemodynamically unstable patients - arterial flow rate may be too low
Cost Expensive Cheap Cheap
Advantages Achieves good fluid removal
Well tolerated unless very unstable
Does not require anticoagulation.
Patient's blood is not exposed to the circuit
Intermittent, thus less labour intensive;
Allows periods of mobility for the patient
Well tolerated unless very unstable
Anticoagulation may not be required;
Pump may not be required
Disadvantages      
Poor solute clearance
Slow and inefficient
Poor solute clearance
Requires abdominal access Potential for peritonitis
Requires arterial access
Dependent on arterial flow rates without a pump Poor solute clearance
Modality CVVHD CVVHF CVVHDF IHD
Access Vas cath Vas cath Vas cath Vas cath or fistula
Flow rate Low flow rate ; 50-200ml/min High flow rate
up to 500ml/min
Anticoagulation Continuous Intermittent boluses or saline flushes
Fluid removal Slow Rapid
Electrolyte removal Slow;
mainly diffusion
Slow;
mainly convection
Slow;
Convection and diffusion
Rapid;
by convection and diffusion
Efficiency of solute clearance Low
However, good solute clearance is ultimately achieved over a prolonged course
High efficiency;
however the short couse of treatment and the intermittent nature of the treatment results in less solute clearance than CVVHDF
Hemodynamic impact Well tolerated Unsuitable for hemodynamically unstable patients
Cost Expensive Cheaper
Advantages Well tolerated hemodynamically Rapid
Rarely requires anticoagulation
Allows mobilization
May access fistula
Good clearance of small molecules

Good control over electrolyte and acid-base
Good clearance of middle molecules
Good control over fluid removal and solute exchange
Good clearance of middle molecules
Good control over fluid removal and solute exchange
Good control over acid-base balance
Disadvantages Expensive
Requires anticoagulation
Prolonged immobilization
Slow and inefficient
 
Electrolyte and acid-base control is better with CVVHDF Electrolyte and acid-base control is faster with SLEDD and IHD Poorly tolerated by hemodynamically unstable patients
Risk of disequilibrium syndrome

And, because it is asked for in Question 21 from the second paper of 2015:

Modality CVVHF (CVVH) IHD SLED
Access Vas cath Vas cath or fistula Vas cath or fistula
Flow rate Low flow rate;
50-200ml/min
High flow rate
up to 500ml/min
Moderate flow rate;
150-200ml/min
Anticoagulation Continuous Intermittent boluses or saline flushes Intermittent boluses may be required
Fluid removal Slow Very rapid Rapid
Electrolyte removal Slow;
by convection
Rapid;
by convection and diffusion
Rapid;
by convection and diffusion
Efficiency of solute clearance Low; however, good solute clearance is ultimately achieved over a prolonged course High efficiency;
however the short couse of treatment and the intermittent nature of the treatment results in less solute clearance than CVVHDF
Lower efficiency than IHD, but higher then CRRT
 
Hemodynamic impact Very well tolerated Unsuitable for haemodynamically unstable patients Tolerated by most patients
Cost Expensive Cheapest Cheap
Advantages

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)

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

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

Disadvantages      

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

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

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

References

D'Intini, Vincent, et al. "Renal replacement therapy in acute renal failure." Best Practice & research clinical anaesthesiology 18.1 (2004): 145-157.

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

Wei, S. S., W. T. Lee, and K. T. Woo. "Slow continuous ultrafiltration (SCUF)--the safe and efficient treatment for patients with cardiac failure and fluid overload." Singapore medical journal 36.3 (1995): 276-277.

Kanno, Yoshihiko, and Hiromichi Suzuki. "Selection of modality in continuous renal replacement therapy." (2010): 167-172. -This seems to be an entire issue of Contributions to Nephrology
(Vol. 166) by Claudio Ronco.