Acute Kidney Injury and Critical Care Nephrology
Recognition and classification
Know the KDIGO-style criteria: a rise in serum creatinine over baseline within a defined interval and/or reduced urine output over hours. Classify by cause (prerenal, intrinsic including tubular, interstitial, glomerular, vascular, and postrenal obstruction) and by stage, because stage drives monitoring intensity. Review the urinalysis and microscopy, fractional excretion of sodium or urea when interpretation is valid, and bladder scanning or imaging when obstruction is suspected.
Critical care management
Prioritize hemodynamics: restore perfusion, treat sepsis, stop nephrotoxins, relieve obstruction, and adjust medications. Recognize indications for renal replacement therapy in the critically ill: refractory hyperkalemia, severe acidosis, volume overload unresponsive to diuretics, uremic complications, and certain poisonings. Understand continuous modalities (slow continuous ultrafiltration, continuous venovenous hemofiltration and hemodiafiltration) versus intermittent hemodialysis: continuous therapy offers gentler fluid removal and hemodynamic stability but requires continuous anticoagulation and immobilization.
Traps
Do not assume all oliguria requires diuretics; a fluid challenge may be appropriate in hypovolemia but harmful in volume overload. Do not treat a single creatinine value as baseline if prior values exist. Remember that creatinine lags injury, and that low muscle mass masks severity. Also distinguish contrast-associated injury from other causes when multiple insults coexist.
Worked example and rule of thumb
A septic patient on norepinephrine has rising creatinine, potassium 6.4 mmol/L, pH 7.19, and worsening oxygenation with diffuse infiltrates. Refractory hyperkalemia and acidosis plus volume overload point to renal replacement therapy. Rule of thumb: in critical illness, match modality to hemodynamic tolerance-unstable patients often need continuous therapy; stable patients can use intermittent treatments.