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