Renal Replacement Therapy

What is it?

Renal replacement therapy is what it sounds like, a replacement of (some) of the functions of the kidney. It’s broken into 2 functions

  • Dialysis: works via diffusion. A dialysate fluid is run counter-current to the patient’s blood and the solute in the blood and dialysate equilibrate across a semi-permeable membrane to a degree dependent on Fick’s law of diffusion
  • Filtration: work via solute drag. A negative pressure is exerted on the patient’s blood in the circuit and solvent and solutes are dragged across a semi-permeable membrane. This is how fluid is “taken off” during renal replacement therapy.


The indications for renal replacement therapy in the ICU include AEIOU.

  • A: Acid base disturbance.
    • Oligo/anuric AKI and pH <7.2 (due to metabolic acidosis)
  • E: Electrolyte derangement.
    • K+>5.5 – 6.0 refractory to medical management (diuresis, insulin/glucose).
  • I: Intoxication
    • Dialyzable toxic substances like metformin overdose. Will require dialysis for the most part.
  • O: Overload
    • Pulmonary oedema not responding to diuresis
  • U: Uraemia
    • Urea >30 with oligo/anuric AKI? Consider dialysis, if uraemic encephalopathy or pericarditis (pericarditis is rare) then they require dialysis. Aim to decrease their urea by <50% to avoid disequilibrium syndrome.

Troubleshooting common problems for patients on RRT


  • Extracorporeal circuits tend to be inflammatory, although as they get better and better over time new generations of circuits are less so. Simply having their blood in a RRT circuit can result in vasodilation due to this inflammation and an increase in noradrenaline requirement.
  • In addition the circuit contains 200mls of blood and when this is initially taken from a patient into the circuit the decrease in preload may result in hypotension.
  • If fluid is being taken off during renal replacement therapy the patient may be unable to haemodynamically compensate for this decreased preload. Concentrated albumin (20% albumex) is a reasonable way to maintain preload during RRT and pull fluid from tissues to the vascular space where it may be removed during dialysis, a bottle or two should do.
  • These haemodynamic changes can be extreme, causing patients to “crash” when going on the filter. Patients at higher risk are those with significant valvular pathologies (think HOCM with systolic anterior mitral excursion or SAM) or severe aortic stenosis. Other patients at risk are those with pre-existing significant cardiomyopathy, right ventricular dysfunction or those already very shocked on high doses of vasopressors and inotropes.

“Clagging” filters

The nurses may inform you that they’re getting “high pressures” during RRT. This can be a problem with the patient, the dialysis access catheter (VasCath/Permacath) or the dialysis machine.

Patient problems

  • If the patient is hypercoagulable, for example DIC, it will be very difficult to prevent them from clotting of the dialysis filters
    • If a patient is not at high risk of bleeding often a heparin infusion will be run during renal replacement therapy. The heparin works to decrease the viscosity of the patient’s blood, 500units/hr is reasonable, dose is often according to consultant preference.

Problems with the dialysis access catheter

  • Vascath/permacath (a big central line that sits in the IVC/distal SVC if it’s a vascath or in the right atrium if it’s a permacath)
    • If the vascath hasn’t been put in in a particularly straight vessel or is in a smaller vessel then resistance will be higher, resulting in higher pressures.
    • If the above phenomenon is prohibitive to the patient receiving renal replacement therapy, it may need to be replaced.

Problems with the dialysis machine or circuit

  • Although it seems obvious ensure that no point of the circuit is kinked. Most of the time the nurses will have checked for this.
  • Clotting or “clagging” of the filter happens often, if pressures are rising then it may be prudent to return the blood to the patient and change the circuit if ongoing RRT is required (if it clots then the blood in the circuit will have to be discarded). It might be that the patient does not need to be recommenced immediately on RRT at that point.
  • Anticoagulation of the RRT circuit is often a site specific protocol. In addition to heparin, LMWH, citrate anticoagulation, and prostaglandins can be used. Each of these have their own monitoring protocols.

Further Reading

British Journal of Anaesthesia Education: Renal replacement therapy in critical care

Intensive Care Network: Continuous Renal Replacement Therapy

Author: Nick Ryan

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