Fluid, Electrolyte, and Acid-Base Disorders
Fluid, Electrolyte, and Acid-Base Disorders
This domain tests stepwise interpretation of sodium, potassium, and acid-base disorders, including mixed states and the urine studies that distinguish them.
Core rules
- Hyponatremia: confirm hypo-osmolality, assess volume status, then check urine osmolality and urine sodium. SIADH is euvolemic with urine osmolality >100 mOsm/kg and urine Na >40 mmol/L after excluding thyroid and adrenal disease.
- Hyperkalemia: ECG changes include peaked T waves and widened QRS; stabilize with calcium, shift with insulin/glucose and beta-agonists, then remove potassium.
- Metabolic acidosis: calculate anion gap (Na − [Cl + HCO3]); if elevated, check delta gap. Winter's formula gives expected PaCO2 = 1.5 × HCO3 + 8 ± 2.
- Metabolic alkalosis: assess saline responsiveness using urine chloride; <20 mmol/L suggests saline-responsive.
- Type 1 RTA: distal, urine pH >5.5, hypokalemia, nephrocalcinosis. Type 2: proximal, bicarbonate wasting. Type 4: hyperkalemia, low aldosterone effect.
Worked example
A 65-year-old with pneumonia has Na 118 mmol/L, serum osmolality 250 mOsm/kg, urine osmolality 520 mOsm/kg, urine Na 45 mmol/L, and no edema. These findings fit SIADH after excluding hypothyroidism and adrenal insufficiency. Treat with fluid restriction and treat the pneumonia; correct sodium slowly.
Traps
- Correcting chronic hyponatremia faster than 8–10 mmol/L in 24 hours, risking osmotic demyelination.
- Using urine sodium in patients on diuretics.
- Missing a normal-anion-gap acidosis from diarrhea or RTA.
- Forgetting mixed acid-base disorders when compensation is off.