Renal, Fluid, Electrolyte, and Acid-Base Disorders
Core concepts
Interpret acid-base disorders systematically: calculate expected compensation, check the anion gap, and use the delta gap to detect mixed disorders. Know causes of high-anion-gap acidosis (MUDPILES) and normal-gap acidosis. Understand acute kidney injury staging (KDIGO), prerenal versus intrinsic versus postrenal physiology, and indications for dialysis (AEIOU: acidosis, electrolytes, intoxication, overload, uremia). Master sodium disorders: assess volume status and osmolality, and correct hyponatremia slowly to avoid osmotic demyelination. Know potassium management in hyperkalemia (calcium for membrane stabilization, insulin/glucose and beta-agonists for shift, removal with diuretics or dialysis).
Traps
Correcting hyponatremia too fast causes osmotic demyelination; correcting hypernatremia too fast causes cerebral edema. A normal anion gap does not exclude a mixed disorder - always compute the delta gap. In hyperkalemia, calcium stabilizes the myocardium but does not lower potassium. Distinguish dilutional hyponatremia from true hypotonic hyponatremia with serum osmolality.
Worked example
A patient has pH 7.25, PaCO2 30, and HCO3 13 with an anion gap of 24. The gap is elevated, confirming high-anion-gap metabolic acidosis; the delta gap (24−12=12) versus bicarbonate drop (24−13=11) is close, so no additional disorder. Rule of thumb: correction limit for chronic hyponatremia is about 8–10 mEq/L in 24 hours.