Haemofiltration and dialysis

Indications:

• Correction of water overload
• To remove larger quantities of water from the body than the kidney is able to achieve in order to enable the administration of therapeutic fluids such as parenteral nutrition
• To remove excess electrolytes
• Correction of disorders of acid/base homeostasis, including inborn errors of metabolism, particularly metabolic acidosis
• Liver failure (but it does not substitute liver function!)
• Removal of urea and other waste products of metabolism in cases of renal failure or hypercatabolic state
• Removal of ingested poisons, drugs or toxins in sepsis

Most common used: continuous veno-venous haemofiltration (CVVH or CVVHF) or continuos veno-venous haemodiafiltration (CVVHDF).

In CVVHF the filtrate depends on blood flow rate (aim for 3-5 ml/kg/min), the transmembrane pressure – TMP (change in oncotic pressure along the filter), prefilter dilution (decreases urea-/creatinine clearance) and the sieving coefficient (ratio between filtrate concentration and plasma concentration for a given molecule, e.g. urea=1, albumin=0).

The filtrate is replaced by a glucose/electrolyte solution (replacement fluid). In CVVHDF the clearance of small and middle sized molecules is enhanced by counter-current dialysate flow (diffusion).

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Anticoagulation:

• UFH/anticoagulation, aim ACT 160-180 secs
• UFH/protamine: 1 mg protamine post-filter for every 100 U Heparin administered pre-filter
• Citrate anticoagulation: 1ml citrate per 30 ml blood flow, aiming for pre-filter Ca⁺⁺<0.4 mmol/l and replace post-filter with Ca⁺⁺>1.2 mmol/l (Mg⁺⁺, citrate accumulation → acidosis)

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Replacement fluid:

• For non-citrate anticoagulation: Na⁺ 140 mmol/l, Ca⁺⁺ 2 mmol/l, Mg⁺⁺ 0.5 mmol/l, Cl^– 110 mmol/l, HCO3^– 32 mmol/l, Lactate 3 mmol/l
• For non-citrate anticoagulation and lactate free: Na⁺ 140 mmol/l, Ca⁺⁺ 1.75 mmol/l, Mg⁺⁺ 0.5 mmol/l, Cl^– 113.5 mmol/l, HCO3^– 35 mmol/l, K⁺ 4 mmol/l, glucose 5 mmol/l
• For citrate anticoagulation: Na⁺ 136 mmol/l, Cl^– 106 mmol/l, citrate 10 mmol/l, citric acid 2 mmol/l

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Catheter/blood Flow/filter:

Always aim blood flow/filtrate flow ratio > 5:1

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Patient monitoring:

• Electrolytes (glucose, Na⁺, K⁺, Cl^–, HCO3^–, Ca⁺⁺) every 4 hrs, hourly for 1st 4 hours if they were abnormal
• Magnesium and phosphate twice daily
• Fluid balance per hour = IV fluids in per hour + enteral feeds per hour – urine – insensible losses – drain losses – patient Fluid removed per hour

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Mode	Clinical Use	QDF (Diffusion)	UFR (Convection)	QRF	Total Clearance
SCUF	Water removal	Nil	=Filtrate flow	Nil	UFR
CVVH	Clearance depends on TMP, QBF, Sieving	Nil	=Filtrate flow	= QRF	UFR
CVVHD	Clearance depends on QBF, QDF	QDF	=QDF + UFR (small)	Nil	QD + UFR (small)
CVVHDF	Improved clearance of small and middle size	QDF	=QDF + UFR	= QRF	QDF + UFR

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References:

[1] Pediatr Nephrol 2012 Feb28: Sutherland et al: Continuous renal replacement therapy

[2] Curr Opin Pediatr 2011 Apr;23(2)181-5: Goldstein: Continuos renal replacement therapy: mechanism of clearance, fluid removal, indications and outcome

[3] Crit Care 2011 Jan 24;15(1)202: Oudemans-van-Straaten et al: Clinical review: anitocagulation for continuous renal replacement therapy – heparin or citrate?

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