Acute renal failure
Renal failure is the cessation of kidney function. In acute renal failure (ARF), the kidneys fail over a period of hours or days. Chronic renal failure (CRF) develops over months or years and leads eventually to end-stage renal failure (ESRF). ARF may be reversed and normal renal function regained, whereas CRF is irreversible.
Aetiology
ARF arises from a variety of problems affecting the kidneys and/or their circulation. It usually presents as a sudden deterioration of renal function indicated by rapidly rising serum urea and creatinine concentrations. As acute renal failure is common in the severely ill, sequential monitoring of kidney function is important for early detection in this group of patients.
Usually, urine output falls to less than 400 mL/24 hours, and the patient is said to be oliguric. The patient may pass no urine at all, and be anuric. Occasionally urine flow remains high when tubular dysfunction predominates.
Kidney failure or uraemia can be classified as (Fig 18.1):
Pre-renal: the kidney fails to receive a proper blood supply.
Post-renal: the urinary drainage of the kidneys is impaired because of an obstruction.
Renal: intrinsic damage to the kidney tissue. This may be due to a variety of diseases, or the renal damage may be a consequence of prolonged pre-renal or post-renal problems.
Diagnosis
In nearly all cases the clinical history and presentation will indicate that a patient has, or may develop, ARF. The first step in assessing the patient with ARF is to identify any pre- or post-renal factors that could be readily corrected and allow recovery of renal function. The history and examination of the patient, including the presence of other severe illness, drug history and time course of the onset of the ARF, may well provide important clues. Factors that precipitate pre-renal uraemia are usually associated with a reduced effective ECF volume and include:
Pre-renal factors lead to decreased renal perfusion and reduction in GFR. Both AVP and aldosterone are secreted maximally and a small volume of concentrated urine is produced.
Biochemical findings in pre-renal uraemia include the following:
Serum urea and creatinine are increased. Urea is increased disproportionately more than creatinine because of its reabsorption by the tubular cells, particularly at low urine flow rates. This leads to a relatively higher serum urea concentration than creatinine, which is not so readily reabsorbed.
Metabolic acidosis: because of the inability of the kidney to excrete hydrogen ions.
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