Fluid, Electrolyte, and Acid-Base Disorders
Core concepts
Use systematic acid-base analysis: pH, PaCO2, HCO3, then calculate anion gap (Na − [Cl + HCO3]), check delta-delta, and assess compensation. High-AG acidosis: MUDPILES (methanol, uremia, DKA, propylene glycol, isoniazid, lactate, ethylene glycol, salicylates). Normal-AG: diarrhea, RTA, acetazolamide, saline. RTA types: type 1 (distal, urine pH >5.5, stones), type 2 (proximal, bicarbonaturia), type 4 (hyperkalemia, hypoaldosteronism).
Hyponatremia: assess volume status and serum osmolality; correct slowly (≤8–10 mEq/L/day) to avoid osmotic demyelination. Hyperkalemia: ECG changes, calcium first if unstable, then insulin/glucose, beta-agonist, bicarbonate, and removal (diuretics, binders, dialysis). Hypokalemia: check magnesium. Hypercalcemia and hypocalcemia have classic ECG and symptom patterns.
Traps
Compensation formulas are estimates-if not matching, suspect a second disorder. Saline-responsive metabolic alkalosis vs saline-resistant (mineralocorticoid excess). Anion gap must be corrected for hypoalbuminemia. Pseudohyponatremia with hyperlipidemia/hyperproteinemia.
Rule of thumb
For any acid-base disorder, calculate the anion gap and delta-delta; for any dysnatremia, check osmolality and volume status first.
Worked example
A patient with DKA: pH 7.15, HCO3 8, Na 135, Cl 100. AG = 27. Delta-delta = (27−12)/(24−8) ≈ 0.94, consistent with pure high-AG acidosis; treat with fluids, insulin, and potassium replacement.