Acute Kidney Injury and Critical Care Nephrology
Core concepts
Acute kidney injury is defined by KDIGO criteria: a rise in serum creatinine of at least 0.3 mg/dL within 48 hours, a rise to at least 1.5 times baseline within 7 days, or urine output below 0.5 mL/kg/h for 6 hours. Classify prerenal, intrinsic renal, and postrenal causes. Prerenal azotemia improves with volume; intrinsic disease includes acute tubular necrosis, glomerulonephritis, interstitial nephritis, and vascular events; postrenal obstruction requires imaging and decompression. In critically ill children, recognize sepsis, nephrotoxins, rhabdomyolysis, tumor lysis syndrome, and fluid overload. Indications for renal replacement therapy include refractory acidosis, electrolyte imbalance, intoxication, fluid overload, and uremic complications.
Traps
Creatinine lags behind injury and may be normal with low muscle mass or diluted by fluid overload. FENa below 1% suggests a prerenal state, but diuretics, CKD, and contrast can make it unreliable; FEUrea may help. Do not give potassium or nephrotoxins while awaiting improvement. Differentiate oliguria from urinary retention and obstruction. In rhabdomyolysis, watch for hyperkalemia, hypocalcemia, hyperphosphatemia, and compartment syndrome; aggressive fluids are key. In tumor lysis, prevent and treat hyperuricemia and hyperkalemia.
Worked example / rule of thumb
A child with severe diarrhea, oliguria, BUN-to-creatinine ratio above 20, FENa 0.5%, and urine sodium 10 mEq/L has prerenal AKI. Give an isotonic fluid bolus and reassess perfusion, urine output, and creatinine. Rule: FENa = (urine Na x plasma Cr)/(plasma Na x urine Cr) x 100; use with caution after diuretics.