Prerenal Acute Kidney Injury

Prerenal acute kidney injury is a rapid decline in glomerular filtration due to reduced renal perfusion, with structurally intact nephrons; it reverses promptly when perfusion is restored.

• Definition of acute kidney injury: creatinine rise of 26.5 micromol/L (0.3 mg/dL) or more within 48 hours, or 1.5 times baseline within 7 days, or urine output below 0.5 mL/kg/h for 6 hours.
• Causes: true hypovolaemia (haemorrhage, vomiting, diarrhoea, burns, polyuria, diuretics, poor intake), reduced effective volume (heart failure, cirrhosis with ascites, nephrotic syndrome, sepsis, anaphylaxis, hypoalbuminaemia), renal vasoconstriction (NSAIDs, ACE inhibitors, angiotensin receptor blockers, calcineurin inhibitors, hepatorenal syndrome, hypercalcaemia, contrast), and renal artery stenosis.
• It is the most common form of acute kidney injury (about 60 percent in the community).

Thirst, dizziness, oliguria, weakness, weight loss, dry mucosa, reduced skin turgor, low jugular venous pressure, postural hypotension, tachycardia, cool peripheries.

• In heart failure and cirrhosis: oedema, ascites and raised or normal venous pressure with low effective arterial volume.
• Differential diagnosis: acute tubular necrosis, postrenal obstruction, acute interstitial nephritis, glomerulonephritis, vascular occlusion, rhabdomyolysis, chronic kidney disease with acute decline.
• Complications: progression to acute tubular necrosis if hypoperfusion persists (usually after 24 to 72 hours), hyperkalaemia, metabolic acidosis, fluid overload with over-resuscitation, chronic kidney disease, and death in severe cases.

Creatinine and urea (urea-to-creatinine ratio above 100:1 when both are in micromol/L, or above 20:1 in mg/dL), electrolytes, bicarbonate, lactate, full blood count, glucose, urinalysis (bland sediment, hyaline casts, little protein).

• Urine indices: sodium below 20 mmol/L, fractional excretion of sodium below 1 percent (below 35 percent for urea when diuretics are used), urine osmolality above 500 mOsm/kg, urine-to-plasma creatinine ratio above 40.
• Response to fluid challenge or removal of the cause with creatinine fall within 24 to 72 hours confirms the diagnosis.
• Ultrasound to exclude obstruction and assess kidney size; echocardiogram, ECG, liver profile as indicated.

Supportive: treat the cause; assess volume status; stop NSAIDs, ACE inhibitors, angiotensin receptor blockers, diuretics, metformin (if eGFR is below 30) and other nephrotoxins; monitor urine output with a catheter in the sick; hourly vital signs.

Pharmacological

• Hypovolaemia: crystalloid (Ringer lactate or 0.9 percent sodium chloride) 500 mL over 15 to 30 minutes, reassess, repeat to a mean arterial pressure above 65 mmHg and urine output above 0.5 mL/kg/h (250 mL boluses in heart failure or elderly); avoid more than 30 mL/kg without reassessment; avoid hydroxyethyl starch.
• Haemorrhage: red cell transfusion (haemoglobin below 7 g/dL), control the source.
• Septic shock: antibiotics within the first hour and noradrenaline 0.05 to 0.5 microgram/kg/min to keep mean arterial pressure above 65 mmHg.
• Cirrhosis with hepatorenal syndrome: stop diuretics, albumin 1 g/kg (maximum 100 g) on day 1 then 20 to 40 g daily with terlipressin 0.85 mg IV every 6 hours (increase to 1.7 mg every 6 hours after 3 days if creatinine has not fallen by 30 percent) or noradrenaline infusion.
• Heart failure: diuretics (furosemide 40 to 80 mg IV), inotropes (dobutamine 2.5 to 10 microgram/kg/min) in low output state.
• Hyperkalaemia: calcium gluconate 10 percent 10 mL IV, insulin 10 units with 25 g glucose, salbutamol 10 to 20 mg nebulised, and bicarbonate if acidotic.
• Diarrhoea: oral rehydration solution 50 to 100 mL/kg over 4 hours.

Procedures: renal replacement therapy for refractory hyperkalaemia, acidosis, pulmonary oedema or uraemic complications; angioplasty or revascularisation for bilateral renal artery stenosis in selected cases.

Special groups: elderly (avoid over-treatment with fluids), pregnancy (hyperemesis, pre-eclampsia, haemorrhage), children (gastroenteritis; calculate fluids by weight), patients on diuretics (restart cautiously).

Red flags: shock, oliguria for more than 12 hours despite fluids, potassium above 6.5 mmol/L, pH below 7.2, fluid overload.

Follow-up: creatinine daily until it falls then at 7 days and 3 months; restart ACE inhibitors cautiously once stable; review at 3 months for chronic kidney disease.