TREATMENT FORACUTE KIDNEY FAILURE

• Kidney replacement therapy

Mandatory indications for immediate instigation of kidney replacement therapy are:

1. Refractory hyperkalaemia;
2. Intractable fluid overload;
3. Acidosis producing circulatory compromise;
4. Overt uraemia manifesting as encephalopathy, pericarditis, or uraemic bleeding.

These indications will be present in some patients on their admission to hospital. However, in most cases kidney function will be seen to decline over a period of days or a few weeks despite optimal medical therapy. In this situation here is no hard and fast rule as to when kidney replacement therapy should be initiated. There is no level of nitrogenous waste at which the patient suddenly becomes susceptible to overt uraemic sequelae.

Nevertheless, it is clearly not sensible to wait until an obvious uraemic complication (which might be fatal) arises. Modern practice is (whenever possible) to begin kidney replacement therapy when the blood urea reaches 25 to 30 mmol/l and the serum creatinine 500 to 700 µmol/l, unless there is clear evidence that spontaneous recovery is occurring. There are three basic options for kidney replacement therapy: peritoneal dialysis, haemodialysis, and haemofiltration.

Peritoneal dialysis

Peritoneal dialysis is technically the simplest form of kidney replacement therapy and is commonly used worldwide, although remarkably little has been published recently about its use in those with acute kidney failure. The principle is the same as that described for the long-term treatment of patients with chronic kidney failure , the major differences being: (1) that catheters are used which can be inserted percutaneously using a metal stylet (although some use the same type of catheter as that used for continuous ambulatory treatment); and (2) that smaller volume exchanges with shorter dwell-times are the norm. The technique requires an intact peritoneum and is therefore precluded in the many patients whose kidney failure is associated with abdominal surgery. Other problems include difficulties in maintaining dialysate flow, leakage, peritoneal infection, protein losses, and restricted ability to clear fluid and uraemic wastes. These limitations mean that, particularly in the hypercatabolic patient, peritoneal dialysis is frequently unable to provide good dialysis of the patient with acute kidney failure as judged by modern standards. It is fair to say that peritoneal dialysis is virtually never the first choice modality for renal replacement therapy in an adult with acute kidney failure in those centres that have a range of techniques at their disposal.
 

Haemodialysis

Traditional haemodialysis, which is usually performed on alternate days but may be associated with better outcome when applied daily, can provide good control of uraemia in patients with acute kidney failure who do not have severe haemodynamic compromise. The major disadvantage and limitation of the technique (apart from cost) arises from the fact that it is intermittent: in each 4-h treatment at least 2 to 3 litres of fluid must typically be removed to make 'space' either for the infusion of drugs/parenteral fluids or for oral fluid intake during the 24- to 48-h period before the next dialysis. This imposes a substantial haemodynamic stress, which often cannot be tolerated by those who are cardiovascularly unstable, and is the main reason why continuous haemofiltration techniques have largely replaced haemodialysis in intensive care units.

Haemofiltration

The standard haemofiltration technique works as follows: a mechanical pump (but sometimes the patient's own arterial pressure) drives blood through a haemofilter of high hydraulic conductivity. An ultrafiltrate of plasma is removed, usually at a rate of between 1 and 2 litres per hour. This is replaced, minus the volume of other fluid inputs and the amount of 'negative balance' required, using (most commonly) a lactate/acetate-based substitution fluid. The process is tolerated well, even by patients who are very ill, and the continuous nature of the technique permits continuous fine tuning of the intravascular volume. A large number of technical variations are possible—for example, combination of filtration and dialysis elements (haemodiafiltration), use of differing replacement fluids—but there is nothing to suggest that any one of these is better than another, excepting in those who are unable to metabolize lactate, when bicarbonate-based substitution fluid is essential.

In the same way that there is no evidence on which to make firm recommendations as to when to start kidney replacement therapy in those with acute kidney failure whose chemistry is gradually 'going off', there is also little information on which to base targets for the clearance of metabolic wastes that should be achieved by treatment. One recent study compared the outcome of patients treated with different doses of venovenous haemofiltration: those randomly assigned to ultrafiltration at a rate of 20 ml/h per kg did less well than those receiving 35 ml/h per kg or 45 ml/h per kg, there being no significant difference between the latter two groups.