🫀 Chapter 86: Acute Kidney Injury (AKI)

pRIFLE · KDIGO criteria · Biomarkers (NGAL, cystatin C, KIM-1) · Renal angina index · FENa vs FEUrea · Prevention · Fluid overload · Contrast-induced AKI · Sepsis-associated AKI · RRT indications

🔍 Core Concepts: Acute Kidney Injury

📌 Definition & Classification
AKI = abrupt cessation of kidney function. pRIFLE: Risk (eCCL ↓25%), Injury (↓50%), Failure (↓75% or eCCL <35). KDIGO: SCr ↑≥0.3 mg/dL in 48h or 1.5-1.9x baseline, urine output <0.5 mL/kg/h for 6-12h.
🔬 Biomarkers (Early Detection)
NGAL (serum >150, urine >50 ng/mL), cystatin C, KIM-1, L-FABP, IL-18. Rise before SCr. NGAL 90% sensitive/specific for AKI.
📊 Renal Angina Index (RAI)
RAI = risk level × injury level (fluid overload % + eCCL decline). Score ≥8 predicts AKI. Guides biomarker testing.
⚖️ FENa vs FEUrea
FENa <1% → prerenal; >2% → intrinsic (ATN). FEUrea <35% prerenal, >50% intrinsic (better if on diuretics).
💊 Prevention Strategies
Isotonic crystalloids (avoid HES). Avoid nephrotoxins (NSAIDs, aminoglycosides, contrast). Early vasopressors. Single daily aminoglycoside. Lipid amphotericin B.
🩸 RRT Indications (AEIOU)
Acidosis (severe metabolic), Electrolytes (K >6.5-7), Ingestion (toxins), Overload (fluid with pulmonary oedema), Uraemia (BUN >150, pericarditis, encephalopathy).

🩺 Stepwise Approach: Acute Kidney Injury

1
Identify & stage AKI (pRIFLE/KDIGO)
Calculate eCCL, assess urine output, baseline SCr. Determine risk/injury/failure. Check for prerenal vs intrinsic vs postrenal causes.
2
Optimise haemodynamics & volume status
Fluid resuscitation with isotonic crystalloids (avoid HES). For vasodilatory shock, add vasopressors. Avoid nephrotoxins (NSAIDs, aminoglycosides, contrast).
3
Evaluate with urinalysis & biomarkers
FENa (<1% prerenal, >2% intrinsic). FEUrea (better if on diuretics). Urine sediment: muddy brown casts = ATN; RBC casts = GN. NGAL, cystatin C for early detection.
4
Fluid & electrolyte management
Daily fluid = insensible (400 mL/m²) + UOP + losses. Replace UOP with 0.45% NS + D5W. Avoid K+. Treat hyperkalaemia (calcium, insulin+glucose, kayexalate). Monitor daily weight.
5
Renal replacement therapy (RRT) indications
AEIOU: Acidosis (pH <7.15), Electrolytes (K >6.5-7), Ingestion (toxins), Overload (pulmonary oedema, FO >15-20%), Uraemia (BUN >150, pericarditis, encephalopathy).
6
Specific therapies & prevention
Aminoglycosides: single daily dose. Contrast-induced AKI: IV fluids, NAC. Sepsis: early antibiotics, fluids, vasopressors. Avoid low-dose dopamine (no benefit).