Potassium-Sparing Diuretics

Loop diuretics waste potassium. Potassium-sparing diuretics do the opposite — but that protective effect becomes dangerous the moment you stop paying attention to K+ levels.

Potassium-sparing diuretics act on the distal convoluted tubule and collecting duct to promote sodium and water excretion while retaining potassium. There are two subclasses with different mechanisms. Aldosterone antagonists (spironolactone, eplerenone) block aldosterone receptors, preventing sodium reabsorption and potassium secretion. Epithelial sodium channel (ENaC) blockers (amiloride, triamterene) directly block sodium channels, reducing the electrochemical gradient that drives potassium loss. These are weak diuretics on their own, so they're usually paired with thiazides or loop diuretics to counterbalance potassium wasting. Spironolactone has a unique secondary role: it reduces mortality in heart failure with reduced ejection fraction (HFrEF) by blocking aldosterone's fibrotic effects on the heart. The critical nursing concern is hyperkalemia — serum K+ above 5.0 mEq/L. Risk escalates when the client is also taking ACE inhibitors, ARBs, or potassium supplements. You monitor potassium levels at baseline and regularly during therapy, watch for signs of hyperkalemia (muscle weakness, peaked T waves, bradycardia), and teach the client to avoid salt substitutes, which contain potassium chloride.

Key Distinctions

Don't confuse spironolactone's antiandrogenic side effects (gynecomastia, menstrual irregularities) with eplerenone, which is selective and avoids these effects. Students often pair potassium-sparing diuretics with potassium supplements — this combination is contraindicated. Unlike loop diuretics that cause hypokalemia, potassium-sparing diuretics cause hyperkalemia — the electrolyte risk runs in the opposite direction.

Clinical Pearl

Salt substitutes are secret potassium bombs. A client on spironolactone who switches to 'low-sodium' salt at home is self-supplementing potassium without knowing it.