100 Cases in Surgery
ANSWER 97 Urine production should be greater than 0.5 mL/kg/h. The aetiology of acute renal failure can be thought of in three main categories:
• pre-renal: the glomerular filtration is reduced because of poor renal perfusion. This is usually caused by hypovolaemia as a result of acute blood loss, fluid depletion or hypotension. The patient’s tubular and glomerular function are normal, so renal function should be restored with appropriate fluid replacement • renal: this is the result of damage directly to the glomerulus or tubule. The use of drugs such as NSAIDs, contrast agents or aminoglycosides, all have direct nephrotoxic effects. Acute tubular necrosis can occur as a result of prolonged hypoperfusion, either perioperatively or postoperatively. Pre-existing renal disease such as diabetic nephropathy or glomerulonephritis makes patients more susceptible to further renal injury • post-renal: this can be simply the result of a blocked catheter. This should always be checked as a cause for complete anuria in a previously fit patient. Calculi, blood clots, ureteric ligation and prostatic hypertrophy can also all lead to obstruction of urinary flow. This patient is likely to be dehydrated as a result of his poor oral intake since his operation. Firstly, check the catheter by flushing it and palpate the abdomen for a distended bladder. Then calculate his fluid balance since the operation. Check for any evidence of sepsis. With his current blood pressure, his antihypertensive medication does not need to be restarted. It is important to maintain a good blood pressure to ensure adequate renal perfusion. The NSAIDs should be stopped as these have a direct nephrotoxic effect which may worsen his renal function. Examine the patient for any evidence of fluid overload and check his history for previous renal problems or cardiovascular disease. Initially, the patient should be given a fluid challenge. A bolus infusion of 250 mL should give an improvement in urine output if the cause is pre-renal. If after two attempts no improvement is seen, the patient should be considered for transfer to a high-dependency unit and central-venous-pressure monitoring.
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Biochemical changes in acute renal failure
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Hyponatraemia Hyperkalaemia Hypocalcaemia Metabolic acidosis
KEY POINT
• Urine production should be greater than 0.5 mL/kg/h.
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