Nephrology, Fluid, Electrolyte, and Acid-Base Disorders
Interpret acid-base disorders systematically: check pH, then PaCO2, then bicarbonate, and calculate the anion gap and delta-delta to detect mixed disorders. Know the causes of high-anion-gap metabolic acidosis (MUDPILES/KULT) and normal-gap acidosis (GI losses, renal tubular acidosis). Understand sodium disorders by volume status and tonicity, and the safe correction rates (hyponatremia ≤8-10 mEq/L per 24 hours to avoid osmotic demyelination; correct hypernatremia slowly).
Traps
Questions test whether you recognize mixed disorders: a normal pH does not exclude a mixed acidosis and alkalosis. Watch for the patient with a high anion gap and a normal bicarbonate - the gap can be masked by concurrent metabolic alkalosis or hypoalbuminemia. Correct the anion gap for albumin (add 2.5 to the gap for every 1 g/dL drop in albumin below 4). In hyponatremia, do not correct too fast, and recognize that patients with chronic hyponatremia are at highest risk for osmotic demyelination.
Worked example
A patient with pH 7.30, PaCO2 30, HCO3 15, Na 130, Cl 95, albumin 2.0. The measured gap is 20; correcting for albumin adds ~5, giving a gap of 25 - a high-anion-gap acidosis with respiratory compensation.