Prerenal AKI

Prerenal AKI results from reduced renal perfusion — the kidney itself is intact but is not receiving adequate blood flow. It is the commonest cause of AKI in critically ill patients. Causes include hypovolaemia (haemorrhage, dehydration, third-space fluid losses), reduced cardiac output (cardiogenic shock, cardiac tamponade), and vasodilation (septic shock, hepatorenal syndrome). Prerenal AKI is characterised by a disproportionate rise in urea relative to creatinine (urea:creatinine ratio >100:1 in μmol/L), concentrated urine (osmolality >500 mOsm/kg, urine sodium <20 mmol/L), and a good response to fluid resuscitation or haemodynamic improvement. If treated promptly, prerenal AKI is reversible.

Intrinsic Renal AKI

Intrinsic AKI involves structural damage to the kidney itself — most commonly acute tubular necrosis (ATN), which results from either ischaemia (prolonged prerenal injury that progresses to cause tubular cell death) or direct nephrotoxin exposure. Common nephrotoxins in ICU include contrast media (contrast-induced nephropathy), aminoglycoside antibiotics, NSAIDs, amphotericin, vancomycin at high doses, and myoglobin (rhabdomyolysis). Less commonly, intrinsic AKI results from glomerulonephritis, interstitial nephritis (drug-induced or autoimmune), or vascular disease.

ATN is characterised by urinary casts (granular or muddy-brown casts), urine sodium >40 mmol/L, urine osmolality close to plasma (isosthenuria), and a poor response to fluid resuscitation alone. Recovery typically occurs over 1–3 weeks as tubular cells regenerate, though dialysis may be required to bridge this period.

Postrenal AKI

Postrenal AKI results from obstruction to urine outflow. It must always be considered and excluded because it is potentially rapidly reversible with urological intervention. In ICU, causes include urethral obstruction (blocked catheter — always exclude this first), ureteric obstruction (calculi, tumour, or iatrogenic), and bladder outlet obstruction (prostate disease, malignancy). Bedside ultrasound can identify hydronephrosis (dilated renal pelvis and ureters) rapidly and should be performed in any unexplained AKI.