Box 83 Causes of acute renal failure Prerenal causes

• Reduced cardiac output (dysfunction or hypovolaema)

• Redistribution of blood flow (sepsis, liver failure)

Renal causes

• Acute tubular necrosis (ATN) - from any prolonged prerenal cause

• ATN from toxins (drugs, contrast media)

• Vasculitis, myeloma, emboli, glomerulonephritis

Post-renal causes

• Obstruction (stones, prostate, tumours, ureteric damage during surgery)

Regardless of the cause of acute renal failure, the reduction in renal blood flow represents a common pathological pathway leading to reduced glomerular filtration. Causes of acute renal failure are traditionally divided into three groups (Box 8.3).

During the period of reduced renal blood flow, the kidneys are particularly vulnerable to further insults. At risk patients are those with diabetes, vascular disease, known renal impairment, with a recent major upset such as myocardial infarction, major surgery or GI bleed, and on nephrotoxic medication (in particular the combination of angiotensin converting enzyme [ACE] inhibitors and non-steroidal anti-inflammatory drugs [NSAIDs]) and also drugs such as aminoglycosides, allopurinol, and digoxin. Up to 25% hospital patients receiving therapeutic aminoglycosides sustain some degree of renal impairment.

Recovery from acute renal failure does not just depend only on restoration of renal blood flow, although early restoration leads to improved outcome. In intrinsic renal failure, once renal blood flow is restored, the remaining functional nephrons increase their filtration and hypertrophy. Recovery is dependent on the number of remaining functional nephrons. If this is below a critical value, continued hyperfiltration results in progressive glomerular sclerosis, which eventually leads to nephron loss. Continued nephron loss causes more hyperfiltration until renal failure results. This has been termed the hyperfiltration theory of renal failure, and it explains why progressive renal failure is sometimes observed after apparent recovery from acute renal failure.

Table 8.1 Measurements which may point towards the cause of acute renal failure

Parameters

Prerenal

Renal

Table 8.1 Measurements which may point towards the cause of acute renal failure

Parameters

Prerenal

Renal

Urine osmolality (mmol litre-1)

> 500

< 350

Fractional excretion of Na (%)*

<1

>4

Urine Na (mmol litre-1)

< 20

> 40

Urine:plasma creatinine ratio

> 40

< 10

*Urine Na/serum Na Urine creatinine/serum creatinine x 100

*Urine Na/serum Na Urine creatinine/serum creatinine x 100

There are various urine tests that can help point towards renal or prerenal causes (Table 8.1). This is based on the fact that in prerenal failure the kidney avidly reabsorbs salt and water, but in renal failure where tubular function is disrupted, the kidney loses sodium in the urine. In practice these are not much help in patients already in hospital where there is often a clear precipitating cause. Diuretics also increase the urinary excretion of sodium and this makes urinary sodium values difficult to interpret.

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