A client is prescribed furosemide (Lasix) for fluid volume excess. Which electrolyte must the nurse monitor most closely?

FUROSEMIDE (LASIX) is a LOOP DIURETIC — it inhibits sodium and chloride reabsorption in the Loop of Henle, resulting in increased excretion of: water (the desired diuretic effect); sodium; chloride; POTASSIUM (the most clinically important side effect); magnesium; calcium; hydrogen ions. HYPOKALEMIA RISK: Every loop diuretic dose causes some potassium loss; at therapeutic doses for fluid overload (40-120 mg/day for furosemide), hypokalemia is common and can be severe; symptoms: muscle weakness, fatigue, leg cramps, cardiac dysrhythmias (PVCs, atrial fibrillation, life-threatening arrhythmias); ECG changes: U waves, flat T waves; dangerous at levels below 3.0 mEq/L. NURSING INTERVENTIONS: Check potassium before administration; hold and notify provider if K+ below 3.5 mEq/L (per facility policy — some protocols hold at <3.0); supplement potassium per order (oral KCl with food; IV KCl if severe — never rapid IV push); encourage dietary potassium (bananas, oranges, leafy greens, potatoes); monitor for hypokalemia symptoms; educate client to report muscle cramps, weakness, palpitations. ADDITIONAL MONITORING: Daily weight (most sensitive early indicator of fluid balance); I&O; vital signs; skin turgor; lung sounds; electrolytes (sodium, magnesium, BUN, creatinine in addition to potassium); ototoxicity risk with rapid IV administration of large doses.