CVVH

SLED

IHD

Mechanism of solute

clearance

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

membrane.

Solute clearance by

diffusion

Advantages

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

time,

anticoagulation

often not required,

cost savings by

using online water

Disadvantages

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

management,

highest risk of

disequilibrium

syndrome. Possible

greater

haemodynamic

instability

CVVH

• 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

IHD

• 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

SLED

• 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