18.6 Diuretics

Types and Mechanisms of Diuretics

Diuretics lower blood pressure by increasing urine output, which reduces overall blood volume. They're among the most commonly prescribed antihypertensives, and different types act on different parts of the nephron. Knowing where each class works helps you predict both its potency and its side effect profile.

Types of Hypertension Diuretics

Thiazide diuretics (hydrochlorothiazide, chlorthalidone, indapamide) inhibit the sodium-chloride symporter in the distal convoluted tubule. They block sodium and chloride reabsorption, so more water follows those electrolytes into the urine. Thiazides are first-line for most patients with uncomplicated hypertension because they're effective at low doses and have strong outcome data for reducing cardiovascular events.

Loop diuretics (furosemide, bumetanide, torsemide) inhibit the sodium-potassium-chloride (Na⁺/K⁺/2Cl⁻) cotransporter in the thick ascending limb of the loop of Henle. This is where the bulk of sodium reabsorption happens, which makes loop diuretics the most potent class. They're typically reserved for patients with significant fluid overload, heart failure, or renal impairment where thiazides aren't effective enough.

Potassium-sparing diuretics work in the collecting duct through two different mechanisms:

• Epithelial sodium channel (ENaC) blockers (amiloride, triamterene) directly block sodium channels, reducing the electrical gradient that drives potassium secretion.
• Aldosterone antagonists (spironolactone, eplerenone) block aldosterone receptors, which prevents aldosterone from stimulating sodium reabsorption and potassium excretion.

These are the weakest diuretics on their own but are often combined with thiazides or loops to counteract potassium loss.

Mechanism and Side Effects

Each class has a predictable side effect profile based on where it acts:

Thiazide side effects:

• Hypokalemia and hyponatremia (increased sodium and potassium loss)
• Hyperglycemia and hyperlipidemia (metabolic effects, especially at higher doses)
• Hyperuricemia (can trigger gout flares)
• Orthostatic hypotension (from volume depletion)

Loop diuretic side effects:

• Hypokalemia, hyponatremia, hypomagnesemia (massive electrolyte wasting)
• Hypochloremic metabolic alkalosis (loss of chloride and hydrogen ions)
• Ototoxicity (especially with rapid IV administration or high doses; risk increases when combined with other ototoxic drugs like aminoglycosides)
• Hyperuricemia

Potassium-sparing side effects:

• Hyperkalemia (the primary concern; risk increases with renal impairment or concurrent use of ACE inhibitors/ARBs)
• Gynecomastia and menstrual irregularities (spironolactone specifically, due to its antiandrogenic effects; eplerenone is more selective and causes this less often)
• GI disturbances (nausea, diarrhea)

Hormonal and Physiological Factors in Diuretic Action

Antidiuretic hormone (ADH) promotes water reabsorption in the collecting duct. Diuretics don't directly block ADH, but by increasing solute excretion they reduce the osmotic gradient that ADH relies on to concentrate urine.

Osmotic diuresis occurs when excess solutes in the tubular fluid (such as glucose in uncontrolled diabetes) pull water into the urine. This is a different mechanism from pharmacologic diuretics but produces a similar result: increased urine output.

Diuretic therapy can also stimulate the renin-angiotensin-aldosterone system (RAAS). When blood volume drops, the kidneys release more renin, which can partially counteract the blood pressure-lowering effect. This is one reason diuretics are often paired with ACE inhibitors or ARBs.

Urine output is a direct indicator of diuretic effectiveness. Tracking daily output helps you assess whether the drug is working and whether the patient's fluid balance is appropriate.

Nursing Considerations and Patient Education

Nursing Considerations for Diuretic Administration

Monitor fluid status closely. Track blood pressure, daily weight (same time, same scale, same clothing), and intake/output. A weight change of 1 kg roughly equals 1 liter of fluid gained or lost. Report rapid weight changes to the provider.

Watch for electrolyte imbalances. Know the signs for each:

• Hypokalemia: muscle weakness, fatigue, leg cramps, arrhythmias (flattened T waves, U waves on ECG)
• Hyponatremia: confusion, lethargy, headache, seizures in severe cases
• Hyperkalemia: muscle weakness, paresthesia, arrhythmias (peaked T waves, widened QRS on ECG)

For patients on thiazides or loop diuretics, the provider may order potassium supplements or recommend potassium-rich foods such as bananas, oranges, potatoes, and spinach. Patients on potassium-sparing diuretics should avoid excess potassium intake and potassium-containing salt substitutes.

Assess renal function. Check BUN, creatinine, and electrolyte panels at baseline and regularly throughout therapy. Dose adjustments may be needed if renal function declines.

Timing matters. Administer diuretics in the morning (or early afternoon for a second dose) to avoid nocturia that disrupts sleep.

Patient Education for Diuretic Use

1. Purpose of therapy: Explain that diuretics help lower blood pressure by removing excess fluid, which reduces strain on the heart and blood vessels over time.
2. Medication adherence: Take the medication exactly as prescribed. Stopping abruptly can cause rebound fluid retention and blood pressure spikes.
3. Blood pressure monitoring: Check blood pressure at home at consistent times each day and keep a written log to bring to appointments.
4. Dietary changes: Follow a low-sodium diet ($<$2,300 mg/day, or lower if directed). Read food labels, limit processed foods, and avoid adding salt at the table.
5. Recognizing side effects: Contact the provider for excessive thirst, muscle cramps or weakness, dizziness, irregular heartbeat, or chest pain. These may signal electrolyte problems or excessive fluid loss.
6. Hydration and electrolytes: Stay adequately hydrated, especially in hot weather or during exercise. Replace electrolytes as directed, but don't take potassium supplements without provider approval.
7. Preventing orthostatic hypotension: Rise slowly from sitting or lying positions. Compression stockings may help if dizziness is a recurring problem. This is especially important in the first few weeks of therapy or after dose increases.
8. Follow-up: Keep all scheduled lab and clinic appointments. Regular monitoring of electrolytes, kidney function, and blood pressure allows the care team to catch problems early and adjust the treatment plan.