Diuretics: Types and Uses
Diuretics increase renal sodium and water excretion by acting at different nephron segments. Thiazides, loop diuretics, and potassium-sparing agents each have distinct pharmacological profiles and clinical roles.
## Nephron Segments and Diuretic Targets
Each class of diuretic acts at a specific nephron segment, determining its potency, electrolyte effects, and clinical utility:
- **Proximal tubule** -- Carbonic anhydrase inhibitors (acetazolamide). Weak diuretics. Used for glaucoma and altitude sickness.
- **Thick ascending limb of Henle** -- Loop diuretics (furosemide, bumetanide, torsemide). Most potent class.
- **Distal convoluted tubule** -- Thiazides (hydrochlorothiazide, chlorthalidone, indapamide). Moderate potency.
- **Collecting duct** -- Potassium-sparing agents: mineralocorticoid receptor antagonists (spironolactone, eplerenone) and epithelial sodium channel blockers (amiloride, triamterene). Weakest diuretic effect.
## Thiazide Diuretics
Thiazides block the NCC (Na-Cl cotransporter) in the distal convoluted tubule. They are first-line for hypertension due to strong outcome data (ALLHAT trial). Chlorthalidone has a longer half-life and greater 24-hour blood pressure reduction than hydrochlorothiazide and is increasingly preferred.
**Metabolic effects:** Hypokalemia, hyponatremia, hyperuricemia (can precipitate gout), hyperglycemia, and hypercalcemia (useful in preventing calcium kidney stones). The hypocalciuric effect distinguishes thiazides from loop diuretics.
## Loop Diuretics
Loop diuretics inhibit the NKCC2 (Na-K-2Cl cotransporter) in the thick ascending limb, blocking the generation of the medullary concentration gradient. This produces the most potent diuresis, making them essential for acute decompensated heart failure, pulmonary edema, and refractory edema states.
**Furosemide** is the most commonly used but has variable oral bioavailability (10-100%). Torsemide has more reliable absorption and a longer duration of action, potentially reducing heart failure rehospitalizations.
**Key side effects:** Hypokalemia, hypomagnesemia, ototoxicity (especially with rapid IV administration or concurrent aminoglycosides), hypocalcemia (opposite to thiazides).
## Potassium-Sparing Diuretics
**Spironolactone** blocks aldosterone receptors in the collecting duct, reducing sodium reabsorption and potassium excretion. In HFrEF, spironolactone reduced mortality by 30% (RALES trial). Eplerenone is more selective with fewer antiandrogenic side effects (gynecomastia, menstrual irregularities).
**Amiloride** and **triamterene** directly block ENaC channels. Used as adjuncts to prevent thiazide-induced hypokalemia.
**Critical risk:** Hyperkalemia, especially when combined with ACE inhibitors, ARBs, or potassium supplements. Monitor potassium closely.
## Clinical Applications
| Indication | Preferred Diuretic |
|------------|-------------------|
| Hypertension | Chlorthalidone or indapamide |
| Acute heart failure | IV furosemide or bumetanide |
| Chronic HFrEF | Spironolactone + loop diuretic |
| Nephrotic syndrome edema | Loop diuretic + albumin |
| Calcium stones | Thiazide (reduces calcium excretion) |
| Ascites (cirrhosis) | Spironolactone + furosemide (100:40 ratio) |
## Key Takeaways
- Thiazides are first-line for hypertension; chlorthalidone is preferred over HCTZ for sustained control.
- Loop diuretics are the workhorse for volume overload in heart failure and renal disease.
- Spironolactone reduces mortality in HFrEF -- it is not just a diuretic.
- Always monitor electrolytes, especially potassium, when initiating or adjusting diuretic therapy.