Renal Replacement Therapy on the ICU
Dr John Griffiths DICM MRCP FRCA MA
Focus on acute renal failure on the ICU
Undoubtedly, prevention of acute renal failure (ARF) is better than cure. Potentially nephrotoxic drugs should be avoided, and the haemodynamic status of the patient should be optimised to ensure adequate renal perfusion. This initially involves targeted fluid resuscitation. Inotropic support may then be required. Many therapies and treatments have been studied to try to prevent incipient renal failure from developing in the face of multi-organ dysfunction. These have included dopamine, frusemide, n-acetyl-cysteine and mannitol. Unfortunately, there is no current evidence that any of these treatments alter outcome, and the magic bullet for ARF remains elusive. If a patient remains in oliguric renal failure despite optimum management, then some form of renal replacement therapy (RRT) is indicated. It is better to start RRT early rather than wait for complications of ARF to develop.
Focus on vascular access for renal replacement therapies
Vascular access for RRT may be arteriovenous or venovenous. Venovenous is the most common technique applied. A double lumen vascular catheter is placed in a central vein. The subclavian route is usually avoided because the incidence of subclavian vein stenosis following cannulation is high. Blood is drawn from the proximal lumen and then artificially pumped through the extracorporeal circuit and back to the patient through the distal lumen of the catheter.
Focus on principles of renal replacement therapies
All forms of RRT rely on the principle of allowing solute and water clearance through a semi-permeable membrane and then discarding the waste products. Solutes are removed from the blood by diffusion or convection. Diffusion is the movement of solutes down a concentration gradient across a semi-permeable membrane. If a pressure gradient is set up across the dialysis filter, water will be pushed across the membrane carrying dissolved solutes with it. This process is called ultrafiltration, and the process by which solutes move across the membrane is convection.
Focus on modes of renal replacement therapy
In an ICU, renal replacement therapies can be categorised as continuous or intermittent, and a variety of techniques are now available (Table 1). In the UK, continuous therapies prevail and the predominant modes of RRT in use are continuous venovenous haemofiltration (CVVH) and continuous haemodialysis. Both of these techniques require some form of anticoagulation and have the potential for thrombocytopenia, electrolyte dysequilibrium, hypothermia and haemodynamic instability. Continuous RRT offers a number of theoretical advantages, including better fluid management, less haemodynamic disturbance and more stable metabolic and fluid volume control. Common indications for RRT are given in Table 2.
Table 1. Renal replacement therapies
- Slow continuous ultrafiltration
- Peritoneal dialysis
- Slow low efficiency daily dialysis
Table 2. Indications for renal replacement therapies
- Uraemia or uraemic symptoms
- Volume overload
- Severe metabolic acidosis
- Drug overdose
Focus on CVVH
CVVH is the most commonly used RRT on a modern ICU. It is a convective haemodialysis procedure with a high ultrafiltration rate. Blood is usually pumped through the dialysis circuit at flow rates of 100–200 ml/minute and a specialist replacement fluid substitutes the ultrafiltrate fluid removed. There is no consensus on how long or how fast haemofiltration should be undertaken, but in practice about 2 litres of ultrafiltrate are removed each hour. Either all or part of the ultrafiltrate is replaced, depending on the desired overall fluid balance. CVVH is often run for 24–72 hours at a time. It is stopped when there is evidence of a return of renal function.
In contrast to haemodialysis, which excels at removing low molecular weight substances (i.e. <500 Daltons), haemofiltration filters the blood of compounds with molecular weights as high as 30,000 Daltons. Many of the septic mediators (e.g. cytokines, complement) lie within this group. This has raised the possibility that haemofiltration might benefit patients with severe sepsis by filtering the blood of these potentially harmful inflammatory mediators. There is some evidence that high volume ultrafiltration of up to 6 litres/hour may be associated with significantly lower mortality rates in septic patients.
Friedrich JO, Adhikari N, Herridge MS, Beyene J.
Meta-analysis: low-dose dopamine increases urine output but does not prevent renal dysfunction or death.
Ann Intern Med 2005; 142: 510-524
Ho KM, Sheridan DJ.
Meta-analysis of frusemide to prevent or treat acute renal failure.
BMJ 2006; 333: 420-425.
d’Avila DO, Poli de Figueiredo CE.
Critically ill patients and acute renal failure.
JAMA 2006; 295: 624
Bellomo R, Ronco C, Kellum JA et al. Acute Dialysis Quality Initiative workgroup.
Acute renal failure - definition, outcome measures, animal models, fluid therapy and information technology needs: the Second International Consensus Conference of the Acute Dialysis Quality Initiative (ADQI) Group.
Crit Care 2004; 8: R204-R212.
Ronco C, Bellomo R, Homel P et al.
Effects of different doses in continuous veno-venous haemofiltration on outcomes of acute renal failure: a prospective randomized trial.
Lancet 2000; 356: 26–30.