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Diuretics

7 min read

Diuretics

Key Points

  • Four diuretic classes: loop (most potent), thiazide (first-line for HTN), potassium-sparing, and osmotic
  • Each class acts on a different nephron segment to inhibit sodium/water reabsorption
  • Loop diuretics cause hypokalemia (potassium loss); potassium-sparing diuretics risk hyperkalemia (potassium retention)
  • Severe dyskalemia from diuretic therapy can be fatal; ongoing electrolyte surveillance is safety-critical.
  • Monitor daily weights (1 kg ≈ 1 liter fluid), electrolytes, and intake & output
  • Loop diuretics IV rapid push → ototoxicity risk
  • Carbonic anhydrase inhibitors (for example acetazolamide) can reduce CSF production and are used in selected intracranial-hypertension pathways.
  • In pediatric congenital-cardiac pathways, diuretics are used to reduce pulmonary congestion/fluid overload with close kidney-function and electrolyte surveillance.

Drug Class Overview

Diuretics increase urinary output to treat edema, hypertension, heart failure, cirrhosis, and renal disease. All diuretics inhibit sodium and water reabsorption at specific nephron segments. Different classes are often used in combination for complementary effects.

Nephron segment physiology helps explain class differences: the proximal tubule normally returns about 60-70% of filtered sodium/water, the ascending loop of Henle reabsorbs about 20-25% of sodium (loop-diuretic target), the distal tubule is a key thiazide target, and aldosterone/ADH effects in distal nephron-collecting pathways shape potassium and water balance during diuretic therapy.

Diuretic Classes

Loop Diuretics — Most Potent

Mechanism: Inhibit Na-K-2Cl (NKCC2) cotransporters in the thick ascending loop of Henle → loss of sodium, potassium, chloride, calcium, and magnesium → significant diuresis.

Agents: Furosemide (Lasix) — prototype; torsemide; bumetanide.

Key Dosing (Furosemide):

  • Edema: 20–80 mg orally daily; max 600 mg
  • IV/IM: 20–40 mg, may repeat in 2 hours
  • Acute pulmonary edema: 40 mg IV stat
  • Half-life: 1.5–2 hours; bioavailability oral ~50%

Indications: Heart failure, cirrhosis, renal disease, nephrotic syndrome, acute pulmonary edema, hypertension (combined with other agents).

Adverse Effects:

  • Hypokalemia (most common electrolyte imbalance) → cardiac dysrhythmia risk, digoxin toxicity
  • Hyponatremia, hypomagnesemia, hypocalcemia
  • Ototoxicity — especially with rapid IV infusion
  • Postural hypotension, dizziness
  • Hyperglycemia, metabolic alkalosis

Ototoxicity Risk

Furosemide administered as rapid IV push can cause permanent hearing loss. Administer IV furosemide no faster than 4 mg/minute (some sources: no more than 20 mg/min). Report tinnitus or hearing changes immediately.

Thiazide Diuretics — First-Line for Hypertension

Mechanism: Inhibit Na-Cl cotransporters in the distal convoluted tubule → sodium, chloride, and water excretion. Unlike loop diuretics, thiazides increase calcium reabsorption → useful adjunct for osteoporosis prevention, but can cause hypercalcemia.

Agents: Hydrochlorothiazide (HCTZ), chlorothiazide, chlorthalidone (48–72h duration), indapamide, and metolazone.

Key Dosing (Hydrochlorothiazide):

  • Hypertension: 25 mg orally daily; may increase to 50 mg/day
  • Edema: 25–100 mg orally daily

Indications: First-line antihypertensive; edema from heart failure, cirrhosis, renal disease; nephrolithiasis (calcium-containing stones — calcium reabsorption decreases urinary calcium).

Adverse Effects:

  • Hypokalemia → increases risk of digoxin toxicity
  • Hyperuricemia → gout exacerbations
  • Hyperglycemia (impaired insulin release)
  • Hyponatremia, hypercalcemia
  • Photosensitivity reactions, possible skin cancer risk with long-term use
  • Rare severe dermatologic toxicity (for example Stevens-Johnson syndrome) requires immediate escalation.
  • Sodium-wasting pattern can produce hypovolemic/isotonic hyponatremia when sodium and water are both depleted.
  • Hydrochlorothiazide can interact with large licorice intake and alter potassium balance; avoid concentrated licorice products during therapy.

Contraindication: Sulfonamide allergy (cross-reactivity possible).

Potassium-Sparing Diuretics — Weakest, Often Combined

Mechanism: Inhibit sodium reabsorption in collecting tubule while decreasing potassium and hydrogen ion secretion → sodium and water excreted, potassium retained.

Agents:

  • Spironolactone (Aldactone): Also blocks aldosterone (mineralocorticoid) receptors; heart failure; cirrhosis; resistant hypertension. RALES trial: 30% reduction in all-cause mortality in heart failure.
  • Eplerenone: More selective aldosterone antagonist; fewer hormonal side effects than spironolactone.
  • Amiloride, triamterene: Block sodium transporters directly (not aldosterone-dependent).

Key Dosing (Spironolactone):

  • Heart failure: 25–50 mg orally once daily (potassium <5 mEq/L required; creatinine clearance >30 mL/hr)
  • Hypertension: 25–100 mg/day
  • Edema: 100–200 mg/day

Key Dosing (Eplerenone):

  • Heart-failure pathways: 25–50 mg orally daily

Indications: Combined with loop or thiazide diuretics to prevent hypokalemia; heart failure (NYHA II–IV); hyperaldosteronism.

Adverse Effects:

  • Hyperkalemia — most dangerous; fatal if severe
  • Spironolactone: gynecomastia, menstrual irregularities, impotence (androgen blockade side effects)
  • Spironolactone can increase lithium levels and raise lithium-toxicity risk
  • FDA Black Box Warning: Spironolactone may cause tumor development at high doses exceeding recommendations
  • Avoid in pregnancy and in severe kidney-dysfunction/hyperkalemia contexts; spironolactone is contraindicated in Addison’s disease.

Hyperkalemia Risk with Potassium-Sparing Diuretics

Avoid concurrent ACE inhibitors, ARBs, potassium supplements, or NSAID use. Avoid potassium-rich foods (bananas, oranges, avocado, potatoes) and potassium-based salt substitutes.

Osmotic Diuretics

Mechanism: Non-reabsorbable solutes (mannitol) pull water into the tubular lumen via osmosis → increased urinary output.

Indications: Increased intracranial pressure, increased intraocular pressure, acute oliguria, prevention of acute kidney injury, and promotion of excretion for selected toxic substances.

Route: IV infusion only (oral forms not effective for diuresis); use cautiously in clients with heart failure or pulmonary edema.

Preparation/Safety:

  • Inspect mannitol solution for particulate matter, discoloration, or crystallization before and during administration; discard if present.
  • Monitor serum osmolality and volume status closely to reduce AKI, pulmonary-edema, and fluid-shift complications.
  • Contraindication patterns include anuria, severe hypovolemia/dehydration, preexisting pulmonary edema, and active intracranial bleeding (except selected craniotomy contexts).

Carbonic Anhydrase Inhibitors (CAIs)

Mechanism: Inhibit carbonic anhydrase in the proximal tubule, lowering bicarbonate reabsorption and reducing CSF production.

Agent example: Acetazolamide.

Selected uses: Idiopathic intracranial hypertension, glaucoma, altitude-sickness pathways, and selected edema-related contexts.

Adverse effects/cautions:

  • Hypokalemia, hyponatremia, metabolic acidosis, nephrolithiasis, fatigue, and GI upset.
  • Serious toxicity pathways include aplastic-anemia and severe hepatic injury patterns.
  • Avoid or use extreme caution in significant hepatic disease or major electrolyte imbalance.
  • High-dose aspirin coadministration increases severe toxicity risk with acetazolamide and requires urgent escalation if metabolic-deterioration cues appear.

Nursing Assessment

Before Administration:

  • Verify current serum electrolytes (especially potassium — normal 3.5–5.0 mEq/L)
  • Obtain baseline weight; auscultate breath sounds (crackles = fluid overload)
  • Review renal function (BUN, creatinine, creatinine clearance) — impaired renal function reduces diuretic effectiveness
  • Check current medications for interactions: digoxin (loop/thiazide increase toxicity), ACE inhibitors/ARBs (potassium-sparing → hyperkalemia risk), NSAIDs (reduce diuretic effectiveness)
  • In older adults and in clients with hepatic impairment, renal impairment, arrhythmias, or gout, plan closer adverse-effect surveillance.

Ongoing Monitoring:

  • Daily weight at same time, same scale, same clothing — weight change >1–2 kg in 24h is clinically significant
  • Strict intake and output
  • Monitor urine output trend using an adult minimum target around 0.5 mL/kg/hour unless a different goal is ordered.
  • A practical minimum adult urine-output safety floor is about 30 mL/hour (about 600 mL/24 hours) unless a different target is ordered.
  • Serum electrolytes (potassium, sodium, magnesium)
  • Trend BUN, creatinine, and creatinine clearance during ongoing therapy
  • Blood pressure (orthostatic changes with aggressive diuresis)
  • For spironolactone, report urine output <30 mL/hour and monitor for possible lithium toxicity when combined
  • Signs of electrolyte imbalance: muscle cramps (hypokalemia), palpitations, weakness
  • In pediatric CHD/heart-failure use, monitor weight and urine-output trends closely to evaluate preload reduction while avoiding renal hypoperfusion.

Patient Education:

  • Rise slowly from seated/lying position to avoid orthostatic hypotension
  • Loop/thiazide diuretics: increase dietary potassium (bananas, oranges, leafy greens, potatoes)
  • If taking hydrochlorothiazide, avoid large amounts of licorice-containing products
  • Potassium-sparing diuretics: avoid potassium supplements and potassium-rich foods
  • Report muscle cramps, excessive thirst, decreased urination, palpitations
  • Take diuretics early in the morning to avoid nocturia disrupting sleep; take with food when GI upset occurs.
  • Report rapid fluid-shift cues, including weight gain or loss greater than about 2 lb/day or 5 lb/week.

Self-Check

  1. Which diuretic class is most potent? Where in the nephron does it act?
  2. A patient taking furosemide also takes digoxin. What electrolyte imbalance increases digoxin toxicity risk?
  3. Which class of diuretics should NOT be combined with potassium supplements, and why?

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