Principles

all forms of renal replacement therapy rely on principle of allowing solute and water clearance through a semi-permeable membrane and then discarding the waste products
water removed by ultrafiltration. Requires driving force to overcome oncotic pressure and drive fluid across membrance. Achieved by:
- applying negative pressure to dialysate side of membrane (haemodialysis)
- using a hyperosmolar solution (peritoneal dialysis)
- using patient's BP or pump-dependent pressure and a highly permeable membrane (haemofiltration or haemodiafiltration)
if poorly permeable membranes used (eg standard HD) convection of solutes associated with ultrafiltration will result in removal of only small amounts of electrolytes and other molecules
effective solute removal can only be achieved if an electrochemical gradient is generated by rapid countercurrent flow of a dialysis fluid or by use of highly porous membrane as in haemofiltration or peritoneal dialysis
dialysis fluid must have appropriate concentration of electrolytes and none of the waste solutes
rate of diffusion depends on blood flow rate, dialysate flow rate, duration of dialytic procedure, concentration gradient across membrane and size of membrane
diffusion poor for molecules of MW >500 daltons. Can only be extracted using highly porous membrane

Mode of therapy

A variety of techniques are now available:

Continuous techniques

haemofiltration
- venovenous (CVVH)
- arteriovenous (CAVH)
haemodiafiltration (CVVHDF)
slow continuous ultrafiltration (SCUF)
peritoneal dialysis (PD)

Intermittent

haemodialysis (HD)
slow low efficiency daily dialysis

Peritoneal dialysis and arteriovenous haemofiltration are rarely used in ICU except where there are tecnological constraints or the patient is already receiving continuous ambulatory peritoneal dialysis (CAPD).

Currently the predominant modes of renal replacement therapy are CVVH (or CVVHDF) and HD. There is no consensus on the best technique. Two meta-analyses have failed to show a difference in outcome, although one analyses showed an improved outcome for patients treated with continuous renal replacement therapy after adjustment for study quality and severity of illness

Technique

CVVH/CVVHDF

Haemodialysis

Advantages

Suitable for haemodynamically unstable patients – slower rate of fluid removal

Continuous therapy avoids periods of dehydration and/or hypotension that may be associated with intermittent haemodialysis and which may worsen acute renal failure

Precise fluid volume control - prevents overload, allows space for nutrition

Efficient control of urea & electrolytes even in severely catabolic patients

Probable advantages in terms of renal recovery

Biocompatible membrane

Cytokine removal may be an advantage

Precise volumetric ultrafiltration control

Online bicarbonate production

High solute clearances  short dialysis times  increased access to patient for other procedures

Cheaper (but not if patient is dialyzed daily)

Disadvantages

Cost – approximately 2-3 x cost of haemodialysis, but the cost disadvantage disappears if compared to daily haemodialysis. Urea kinetic modelling suggests that daily dialysis is required

Low efficiency - patient has to receive therapy for large part of each day. Restricts timing of other procedures (eg CT)

Anticoagulation may be required

Hypothermia

Electrolyte depletion - especially phosphate. May be depletion of other electrolytes, depending on the contents of the replacement fluid

?loss of amino acids

Haemodynamic instability - swings in hydration

Removal of excess fluid over short periods: alternate between relative dehydration and relative fluid overload

?delays renal recovery- episodic renal ischaemia and loss of pressure-flow autoregulation

Episodic nature of small solute removal - electrolyte dysequilibrium, seizures, ?worsens outcome in cerebral oedema

Limited ability to control middle molecules ?toxins

Episodic acid-base control

?Low grade inflammatory response with some membranes – though outcomes may be similar with cellulose filters

Needs water purification system

©Charles Gomersall, June, 2013 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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