Renal, Fluid, Electrolyte, and Acid-Base Disorders
Acute kidney injury
KDIGO stages by creatinine rise (≥0.3 mg/dL or 1.5–1.9× = stage 1; 2.0–2.9× = stage 2; ≥3× or ≥4.0 mg/dL or dialysis = stage 3) and urine output. Prerenal indices (FeNa <1%, FeUrea <35%) lose accuracy with diuretics, CKD, and contrast. Urgent dialysis indications: refractory acidosis, electrolytes (potassium), intoxication, overload, uremia.
Sodium
Correct chronic hyponatremia by ≤8–10 mmol/L per 24 hours to avoid osmotic demyelination; for seizing patients give a 100–150 mL bolus of 3% saline targeting a 4–6 mmol/L rise. In hypernatremia, replace free water gradually and account for ongoing losses.
Acid-base
Anion gap = Na − (Cl + HCO3), normal 8–12; add 2.5 to the expected gap for every 1 g/dL fall in albumin below 4. Delta-delta = (AG − 12)/(24 − HCO3): 1–2 pure high-gap acidosis, <1 coexisting non-gap acidosis, >2 coexisting metabolic alkalosis or chronic respiratory acidosis. Large-volume 0.9% saline causes hyperchloremic non-gap acidosis; balanced solutions reduce this.
Traps
- Giving bicarbonate for every acidosis instead of treating the cause.
- Overcorrecting sodium in a malnourished or alcoholic patient.
- Reading FeNa in a patient on furosemide.
- Missing a second acid-base disorder because the pH looks normal.
- Forgetting potassium repletion before insulin in DKA.
Rule of thumb
In hyperkalemia with ECG changes, give calcium first; then insulin/dextrose, beta-agonist, and definitive removal.