5. RRT Flashcards
Indications for Renal Replacement Therapy (RRT)
acute kidney injury accompanied by a metabolic acidosis, hyperkalaemia or uraemia.
Isolated uraemia is a problem usually only when the urea concentration is high
enough to cause clinical symptoms such as vomiting, diarrhoea, pruritus or mental
disturbance.
manage volume overload and to clear some drugs and poisons from the circulation.
severe hypothermia; veno–venous systems using countercurrent
blood warmers can raise core temperatures by up to 2 C per hour
Principles of Haemofiltration (HF)
Normal Kidney
filters used in HF are in effect literal renal substitutes.
the normal kidney the glomerulus filters water, ions, negatively charged particles
of molecular weight of less than 15,000 and neutral substances of molecular weight
up to about 40,000
normal glomerular filtration rate (GFR) is 125 ml min
Tubular reabsorption reduces the filtrate of 180 l day to about l.0 l day
process salvages many of the filtered ions and other particles
Tubular secretion is the means whereby larger molecules and
protein-bound substances (such as drugs and toxins) are eliminated.
How does HF work practically
In the HF system, arterial pressure
(usually assisted by a peristaltic roller pump)
delivers a flow, if necessary, of more than 200 ml min
to the semi-permeable membrane in the filter.
Water and low-molecular-weight substances (up to 20,000)
cross the membrane (which is acting as the ‘glomerulus’).
This process creates a pressure gradient rather than a concentration gradient,
with the bulk flow of water taking solute with it
(by convective mass transfer).
Whats cleared with HF
Urea and creatinine will be removed,
as will electrolytes and some drugs and toxins.
Plasma proteins and all formed blood components
remain within the circulation.
HF is an efficient means of treating fluid overload, but in comparison with the kidney
is very inefficient at removing solute. Very high volumes of ultrafiltrate (upwards of
15 l day1) are required to remove urea, creatinine and other products of
metabolism.
How is fluid balance managed
Tubular reabsorption is mimicked by the direct infusion of balanced electrolyte
solution, with concentrations adjusted as necessary.
The volume infused will depend on the clinical situation.
If the patient is not volume-overloaded, then infusion will be
at the same rate as the filtration rate, plus a component for maintenance fluid.
If fluid removal is indicated, then negative balance is easily achieved by decreasing the
infusion rate.
The ‘dose’ of renal replacement therapy
This can be specifically defined as the rate of clearance of a marker solute,
indexed to body weight,
but in clinical practice the ‘dose’ of continuous RRT (CRRT)
is essentially the flow rate of the effluent
A number of studies have investigated
the effect of dose on mortality without showing any consistent benefit of higherintensity
regimens. In practice the standard dose rate is around 20–30 ml kg/h
Pre Dilution
Pre- or post-dilution.
The fluid reinfusion to restore volaemic status can be instituted
before or after the filter.
If before (pre-dilution), the dilution decreases the concentration
of solutes to be filtered and so it is less efficient, but the risk of filter failure is
lowered.
During HF, protein particles accumulate on the inner surface of the hollow
fibre filtration tubes leading to impairment of function within 24 hours. A more
dilute solute slows this process
Post Dilution
Post-dilution clears solute more efficiently and a smaller volume of replacement fluid is required
The limiting factor in the rate of post-dilution ultrafiltration is
blood flow; if the rate is too great, then end-filtrate haematocrit will also be too high
HDF
Haemodiafiltration is much more efficient at removing solute.
A dialysis solution is passed across the filter in a counter-current fashion
so that solute can be removed both by convection
(as in HF alone) and by diffusion down the concentration gradient.
CRRT in sepsis
CRRT in sepsis:
the inflammatory mediators and cytokines that are produced in
sepsis have a range of molecular weights from around 5,000 to 50,000,
and so their effective removal is limited by the standard size
of filtration membranes which allow the passage of compounds
of molecular weight up to 20,000.
Experimental high cut-off membranes have shown promise in animal models and may in due course be available for clinical use.
Complications of RRT
- Fluid mismanagement:
- Metabolic Derangement
- Coagulation problems
- Cannulation Complications
- Air embolus
- Air embolus
- Heat
- Disconnection
- Filter Failure
Fluid mismanagement
very large volumes are both filtered and infused,
and the potential scope for error is high.
Fluid overload is possible, but more common is
hypotension secondary to the transient hypovolaemia that may occur with
ultrafiltration.
This may be compounded by any impairment in myocardial reserve.
Metabolic derangement
Metabolic derangement:
high-intensity CRRT can contribute to
hypophosphatemia and hypomagnesaemia,
together with loss of some vitamins and amino acids.
Coagulation problems
Coagulation problems:
Blood clots in extracorporeal circulations and produces diffuse
thrombi on the artificial surfaces unless the system is anticoagulated,
usually with heparins or prostacyclin.
Inadequate coagulation leads to problems with the
circuit as the filter clots,
but this is not immediately dangerous to the patient.
An iatrogenic coagulopathy may be much more hazardous.
Cannulation complication
Cannulation complications:
These are generic,
but apply to the insertion of any wide-bore line into a central vein.
Renal replacement therapy requires a large bore
double lumen catheter inserted into the jugular of femoral veins.
The subclavian vein is usually avoided because of the risk of thrombosis
which would compromise arteriovenous fistula formation should acute kidney injury progress to chronic renal failure.
