Setting up and using CRRT

      
       

 

          
       

The machine circuit is set up as follows:

  1. A double lumen catheter.
  2. A line leading to the filter where blood flow is controlled by a series of roller pumps: blood flow is usually set at 120ml/min.
  3. Anticoagulant – to prevent blood clotting on the filter.
  4. Dialysis fluid, which runs in countercurrent to the blood, the standard rate is 1litre per hour. This can be increased to improve clearance.
  5. A bag to collect the ultrafiltrate.
  6. Replacement fluid, to replace the excess ultrafiltrate over and above the required fluid removal.

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PICTURE GALLERY OF MODES OF CRRT

CVVHDF is similar to IHD in slow motion: the blood flow is 100 – 200ml/min, the dialysate flow is 1000ml/hour, the filtration rate is 10-20ml/hour (very efficient), the urea clearance is 10-20ml/hour. As you can see, continuous hemofiltration is as efficient as IHD at fluid removal by ultrafiltration, but not as efficient at dialysis (diffusion), due to the slow fluid flows. If you want to increase the urea/creatinine clearance, you could increase the dialysate flow or the blood flow, or both.

Anticoagulation is necessary to prevent clotting of the filter. This may be a problem in patients who are at risk for bleeding or who have had recent surgery. Classically heparin has been used. This agent has a number of potential drawbacks: 1. The risk of bleeding, as the patient requires systemic anticoagulation. 2. Heparin requires the presence of antithrombin III, which is often deficient in the ICU population. 3. Heparin may cause thrombocytopenia (HIT syndrome). Agents that have been used instead of heparin include: 1. PGE1 and PGI2, which have anti platelet effects. 2. Citrate, which binds calcium and inhibits the coagulation cascade – and is metabolized to bicarbonate in the liver. 3. Low molecular weight heparins. 4. Hirudin. 5. Aprotinin.

By and large the dialysate and replacement solutions should mirror what one wishes the blood chemistry to be – the closest solution is Ringers Lactate (Hartmann’s Solution).

The reason for this is that as time passes, the blood and dialysate levels of electrolytes will equilibrate: this is dissimilar to IHD, whereby one rigorously cleans the blood and ECF for a few hours and then awaits reaccumulation. The continuous nature of CRRT means that any depletion of electrolytes during the process will continue ad infinitum, until the dialysate prescription is changed.

Are there any electrolyte issues that I need to be concerned with?

Be careful of potassium and phosphate loss: standard dialysate solutions contain neither – and levels can drop very low. KPO4 supplementation is often necessary. Note also that there is no NaHCO3 in the dialysate, leading to loss of bicarbonate: compensated for by the passage of lactate (anionic, a base) into the bloodstream. Calcium may also be required, although Ca and HCO3 cannot be given together, because they precipitate. This is usually metabolized into bicarbonate in the liver. In liver failure, it is wiser to use a lactate free dialysate – such as normal saline, add give bicarbonate supplementation.

 

          
                   
       

         
     

       
       

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