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Haemofiltration
Continuous Veno venous Haemofiltration ( CVVHF) refers to a pumped method, which uses a dedicated, double lumen cannula sited into a large vein. Continuous haemofiltration in the critical care setting is increasingly the choice of treatment for acute renal failure or fluid overload as opposed to dialysis which is seen as long term renal support.
Continuous haemofiltration is a process where by an ultrafiltrate containing H2O and dissolved substances can be extracted from blood flowing in a continuous circuit through a semi permeable membrane.

Principles
Haemofiltration is a convective process in which there is mass movement of plasma water and solutes across a high permeable membrane.

The blood on one side of the membrane exerts a hydrostatic pressure from the presence of a pump. This pressure allows plasma water and solutes to move across the membrane by hydrostatic ultrafiltration to become the filtrate.

The constant draining of the filtrate compartment ensues a negative pressure on the other side of the membrane, thus maintaining a pressure gradient. Much of the cellular constituents of blood as well as protein are prohibited from moving across the membrane due to their molecular size.

The process is not selective and the removal of waste products can only be achieved by the removal of an accompanying load of water and other solutes.

The volume of filtrate is usually a 2litre/hour cycle I know this is what we currently do but should we not be following the 3.5ml/kg exchange that I believe there is evidence for particularly in the very sick (rather than the night time CVVHF waiting for a bed on ward and dialysis?). However this is variable depending on patient weight and effective clearance. To maintain cardiovascular stability, fluid must be replaced concurrently up to the required fluid balance.

The fluid used as replacement should be Isotonic and should always aim to replace the solutes lost as filtrate, that would otherwise be selectively reabsorbed by the normal kidney.

Advantages
Haemofiltration allows continuous control of electrolyte abnormalities and fluid balance, which promotes fluid stability.

It can be performed in the ITU taking away the need of the critically ill patient to be removed to a specialist renal centre.

Disadvantages
Haemofiltration restricts patient’s mobility and necessitates constant patient centred activity, which can disrupt rest and sleep patterns.

The patient has to be anticoagulated with the occasional exception of mechanical valve patients whose INR allows effective running of the pump.

Fluid balance is open to potential error.


Anticoagulation for Haemofiltration

Heparin
• Heparin 5000 IU/50ml (Dose: 200-600 IU/hr) before the filterInfusion runs at 2mls/hour.
• Daily coagulation screen. ACT not routinely done on this unit.
• If the lines are clotting then prime the ‘Vascath’ with heparin 1000 IU/ml (Venous end 1.2ml Arterial end 1ml) . Bolus 2000 IU of heparin IV when starting the filter, and infuse 400-800 IU/hr upstream of the filter)
• KCCT has to be checked daily. Ensure a ratio < 2.

Prostacyclin
• If the patients platelets are less than 100 109 /L it may be necessary to change to Epoprostenol (Flolan).
• Please refer to Consultant Anaesthetist to confirm this for each patient. Flolan is infused systemically (to the patient) NOT to the filter. It needs to be diluted in N/saline if it is to be infused peripherally. It is made up as 500mcg in 50ml of buffer solution
• The dose is 2.5-5ng/kg/min Please refer to data sheet.
• Flolan is a potent vasodilator. Blood pressure and heart rate should be monitored during administration of Flolan. Tachycardia, bradycardia and hypotension may occur during infusion of Flolan. If excessive hypotension occurs during administration of Flolan, the dose should be reduced or the infusion discontinued.
• Prostacyclin infusion is started 30min before attaching the patient to the filter to give a systemic anti- platelet effect. It is combined with a lower dose heparin infusion: 200 units /hr into the filter.


The following guidelines are to be used in conjunction with the Cardiothoracic Intensive Care daily prescription chart.

The Anaesthetist/ Cardiac Surgeon covering CITU will complete the medical prescription. This will indicate whether CITU guidelines are to be followed in the following areas.
• Fluid turnover rate
• Use of anticoagulants
• Choice of substitute fluid
• Buffer and rate
• Potassium & Phosphate replacement
• Wash back to discontinue CVVHF 250mls N/Saline needs to be prescribed
Need to add the CICU protocol for adding phosphate and K to the bags and the type of replacement fluids that we are using

Haemofiltration trouble shooting

Low access & venous pressures

• Check patient position
• The lines could be kinked
• The lines could be disconnected is the patient covered in blood
• Check the patency of the vascath there may be clots in the vascath it may be necessary to swap the ports around (blue to red)
• Is the machine set up properly
• The vascath could be wrong silicone less collapsible at high flows
• Check patient is well hydrated
• High flow haemofiltration

High return pressures

• Check patient position
• The lines could be kinked
• Check the patency of the vascath there may be clots in the vascath it may be necessary to swap the ports around (blue to red)
• Is the machine set up properly
• The vascath could be wrong, silicone less collapsible at high flows
• Check position of vascath it may be sucking on vessel wall
• There could be clots in the air detector and return lumen

Balance alarm

• Change bags
• Check scales may have been knocked when moving patient
• Are the bags still on the floor and not hanging

High pre-filter pressure
• Check the lines for kinks
• Are the lines still clamped
• Check the vascath's patency

Air detected
• Is the air detector correctly inserted
• Is there blood in the chamber
• Check connections on all the ports e.g. filter, vascath, filter bags to ensure no air is getting into the system

Pump doors
• Are the doors closed

blood leak
• check position of the blood leak chamber
• if this alarm persists a universal container can be filled with water and used as an temporary alternative

no fluid chamber
• check it is inserted correctly

increased trans-membrane pressure (TMP)
• filter may be reaching the end of its life
• to temporarily prolong the filter life reduce pump speed though not below 100mls/hr
• increase pre dilution and decrease post dilution
• do not administer colloid through the filter as this can reduce filter life

check dome connections
• check domes are clicked into position
• check dome membranes for cracks etc(do not remove)
• if this does not work you may need a new giving set

high / low temperature

• increase or decrease temperature on filter
• put foil on lines to ensure the patient is receiving warmed blood
• blanket therapy as required

syringe removed
• check the syringe is inserted correctly
• you will need to select syringe volume, and again set the infusion rate as it will usually zero itself

NB if lines are clamped after changing bags, lines start to 'kick'. The air detector will start to collapse and access and return pressures will increase