Electrolyte and Acid/Base Disorders
Master the anion gap, the delta gap, and compensation formulas. The anion gap (Na − Cl − HCO3) is normally 8–12; elevated gap acidosis arises from ketoacidosis, lactic acidosis, renal failure, and toxins (methanol, ethylene glycol, salicylate). Normal-gap acidosis results from bicarbonate loss (diarrhea, renal tubular acidosis) or acid retention (hyperalimentation).
Respiratory and metabolic compensation: Winter's formula (expected PCO2 = 1.5 × HCO3 + 8 ± 2) for metabolic acidosis; expected changes in metabolic alkalosis and respiratory disorders can be checked against standard rules. Mixed disorders occur when compensation is inappropriate.
Electrolytes: hyponatremia classification by volume status and osmolality; pseudohyponatremia; hypernatremia from water loss. Potassium disorders-transcellular shifts, renal versus extrarenal losses, and the ECG changes of hyperkalemia. Calcium, magnesium, and phosphate relationships; corrected calcium for albumin.
Worked example: pH 7.20, PCO2 24, HCO3 9, Na 140, Cl 100. Anion gap = 31, so high-gap metabolic acidosis. Expected PCO2 = 1.5(9)+8 = 21.5 ± 2, so measured 24 is acceptable compensation; no second disorder.
Trap: forgetting to calculate the anion gap when the bicarbonate is low, or failing to check whether compensation is appropriate.