Pathophysiology
Clinical meaning
Acute kidney injury (AKI) and chronic kidney disease (CKD) represent acute and chronic forms of renal dysfunction with distinct diagnostic criteria and management implications. AKI is defined by the KDIGO criteria: Stage 1 = creatinine rise โฅ 0.3 mg/dL within 48 hours OR 1.5-1.9x baseline within 7 days OR urine output < 0.5 mL/kg/hr for 6-12 hours; Stage 2 = creatinine 2.0-2.9x baseline OR UO < 0.5 mL/kg/hr for โฅ 12 hours; Stage 3 = creatinine โฅ 3x baseline OR โฅ 4.0 mg/dL OR initiation of RRT OR UO < 0.3 mL/kg/hr for โฅ 24 hours OR anuria โฅ 12 hours. AKI etiology classification: prerenal (60-70%, from decreased renal perfusion โ dehydration, heart failure, shock; BUN:Cr ratio > 20:1, FENa < 1%), intrinsic renal (25-30%, from tubular necrosis, glomerulonephritis, interstitial nephritis; FENa > 2%, muddy brown casts in ATN), and postrenal (5-10%, from obstruction; hydronephrosis on ultrasound). CKD is staged by GFR using the CKD-EPI equation: Stage 1 = GFR โฅ 90 with kidney damage markers; Stage 2 = 60-89; Stage 3a = 45-59; Stage 3b = 30-44; Stage 4...