An idiots

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Renal Replacement Therapy in Critical Care

• An idiots guide

2

• Theres nothing more dangerous than a
  resourceful idiot
• Scot Adams, American Cartoonist

3
Removing badness from the blood

• First used in the Korean War
• Dialysis, cf Greek to pass through

4
Indications

• Oliguria (urine output lt200 mL/12 h)
• Anuria/extreme oliguria (urine output lt50 mL/12
  h)
• Hyperkalaemia (K gt6.5 mEq/L)
• Severe acidaemia (pH lt7.1)
• Azotemia (urea gt30 mg/dL)
• Pulmonary oedema
• Uraemic encephalopathy
• Uraemic pericarditis
• Uraemic neuropathy/myopathy
• Severe dysnatraemia (Na lt115 or gt160 mEq/L)
• Hyperthermia
• Drug overdose with dialyzable toxin
• to clear poison or fluid

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Dialysis (....diffusion)

• Solutes flow down an electrochemical gradient,
  across a membrane.
• Solute removal is proportional to dialysate flow
  rate
• Dialysate flows counter-current to blood

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Ultrafiltration (..... convection)

• Water moves along a hydrostatic gradient across a
  filter
• Solute moves by solvent drag

7

• There are no stupid questions, but there are a
  lot of inquisitive idiots
• Larry Kersten, American Sociologist

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How can I dialyse a patient?

• Intermittent haemodialysis
• Peritonal Dialysis
• Rarely used in UK ICUs as labour intensive and
  risks infection, but nb CAPD
• Continuous Haemodiafiltration
• Arterio-venous (pts own BP drives blood through
  the filter)
• Veno-venous (blood is pumped doesnt rely on
  BP)
• Plasma exchange, esp in immune disorders, eg GBS
• Plasma is removed / exchanged by filtration or
  centrifugation

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Intermittent haemodialysis

• Gold standard though patient must be
  haemodynamically stable
• Dialysate is typically deionised water
• Blood flow typically 200-400mL/min, dialysate
  flow 500mL/min, filtration rate of 300-2000mL/hr
  and urea clearance of 150/250mL/min
• Complications mostly due to fluid and osmitic
  shifts

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Problems with IHD

• Removal of intravascular volume quicker than it
  can be replaced from the extravascular space can
  cause cardiovascular collapse particularly if
  intravascularly deplete.
• Hypotension can cause ischaemic injury,
  particulary in AKI or head injury.
• Intermittent by nature, so ICU patients may
  develop overload in-between sessions (nb reduced
  venous capacitance)

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Dialysis Disequilibrium syndrome

• Self-limiting syndrome typically after first
  dialysis of very uraemic patients.
• Characterised by nausea, vomiting, headache,
  seizures and coma.
• Syndrome is triggered by rapid reduction in
  plasma osmolality causing cerebral (cellular)
  oedema.
• Treatment
• Supportive
• Hypertonic saline / manitol

12

• Watson, you idiot. Somebody stole our
  tent.....
• Sherlock Holmes, Sleuth

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Continuous veno-venous heamofiltration (CVVH)

• Convective dialysis
• Filtration rate is high
• Electrolyte replacement
• solution is required
• Removes a lot of middle
• molecules, e.g. cytokines
• Slow continuous ultra-filtration
• (SCUF) is slower and doesnt
• use a replacement fluid
• (i.e. removes volume
• only)

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Continuous veno-venous haemodialysis (CVVHD)

• Continuous diffusive dialysis
• Mostly small molecules
• are removed

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Continuous veno-venous haematodiafiltration
(CVVHDF)

• Diffusive and convective
• dialysis
• Small and middle
• molecules removed
• Requires dialysate and
• replacement fluid
• Most popular mode

16

• He is a dreamer, a thinker, a speculative
  philosopher... or, as I like to put it, an idiot
• Christina Hallsworth, my wife

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Advantages of CRRT

• Suitable for use in haemodynamically unstable
  patients.
• Precise volume control, which is immediately
  adaptable to changing circumstances.
• Very effective control of uraemia,
  hypophosphataemia and hyperkalaemia.
• Rapid control of metabolic acidosis
• Available 24 hours a day with minimal training.
• Safer for patients with brain injuries and
  cardiovascular disorders (particularly diuretic
  resistant CCF).
• May have an effect as an adjuvant therapy in
  sepsis.
• Probable advantage in terms of renal recovery
• Makes space for TPN in anuria

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Disadvantages of CRRT

• Expense probably the same as IHD.
• Anticoagulation to prevent extracorporeal
  circuit from clotting.
• Complications of line insertion and sepsis.
• Risk of line disconnection.
• Hypothermia.
• Theraputic drugs doses need adjusting nb
  vasoactive drugs
• Severe depletion of electrolytes particularly
  K and PO4, where care is not taken.

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Using CVVHDF

• CVVHDF is similar to IHD but in slow motion
• Requires a 12F double lumen catheter (VasCath) in
  a big vein
• Typically
• Blood flow 100-200mL/min
• Filtration rate 10-20mL/min
• Urea clearance 10-20mL/min
• To increase the urea clearance, you can increase
  the blood flow rate, dialysate flow rate, or
  both.
• Membrane is usually a hollow fibre
  polyacrilonitrile, polyamide or polysulphone with
  a surface area of 0.6-1m2

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Using CVVHDF

• Anticoagulation
• Classically heparin, but
• Risks bleeding
• Requires antithrombin 3
• Causes HIT
• PGI2 (prostacyclin short t½ ),
• Citrate (binds Ca, metabolised to bicarbonate in
  liver)
• LMWH
• Aprotonin

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Using CVVHDF

• Typically the dialysate and replacement fluid are
  similar to what you want the blood to be, i.e.
  Hartmanns
• Watch potassium, calcium and phosphate levels
  closely
• There is often no bicarbonate in the dialysate,
  and bicarb in the blood is replaced with lactate
  from the dialysate.
• This can be a problem in liver failure best to
  use a lactate free dialysate

22

• When you left home you deprived the village of
  its idiot
• Chris Hallsworth, my father