Diuretics: remove sodium and water safely—treat congestion without causing collapse
Diuretics increase renal sodium excretion (natriuresis); water follows sodium, producing diuresis. They rapidly relieve oedema and pulmonary congestion, but they do not replace disease-modifying heart-failure therapy. Correct class selection, fluid assessment and electrolyte monitoring are what make them safe.
1. The renal map: where each class acts
| Nephron site | Drug class | Target | Key effect |
|---|---|---|---|
| Proximal tubule | Carbonic anhydrase inhibitor | Carbonic anhydrase | Bicarbonate-rich urine; weak diuresis |
| Thick ascending limb | Loop diuretic | Na-K-2Cl cotransporter (NKCC2) | Most powerful natriuresis; calcium and magnesium loss |
| Early distal tubule | Thiazide/thiazide-like | Na-Cl cotransporter (NCC) | Moderate diuresis; calcium retention |
| Collecting duct | Potassium-sparing | Aldosterone receptor or ENaC | Mild diuresis; potassium retention |
| Whole nephron | Osmotic | Filtered osmoles | Water loss; expands extracellular fluid first |
Core principle: where sodium goes, water follows. A diuretic is therefore mainly a sodium-management drug, not simply a “urine-making” drug.
2. Classification and quick selection
Loop diuretics
Furosemide, bumetanide, torsemide. Best for acute pulmonary oedema, significant heart-failure congestion, severe oedema, and low eGFR states.
Thiazides
Hydrochlorothiazide, chlorthalidone, indapamide. Important for hypertension; add-on to loops for resistant oedema.
Mineralocorticoid receptor antagonists
Spironolactone, eplerenone. Potassium-sparing and disease-modifying in eligible HFrEF—not merely “weak diuretics.”
Other potassium-sparing
Amiloride and triamterene block ENaC; useful with potassium-wasting drugs and selected specific conditions.
3. Loop diuretics: furosemide is the acute-congestion workhorse
Mechanism: blockade of NKCC2 in the thick ascending limb prevents sodium, potassium and chloride reabsorption. Loops are “high-ceiling”: larger doses can create greater natriuresis, but safety limits remain.
| Common use | Typical adult approach* | Expected problems | Bedside priority |
|---|---|---|---|
| Acute pulmonary oedema/acute HF congestion | Furosemide 20–40 mg IV in diuretic-naïve adults; dose/route must be individualised | Hypotension, AKI/azotaemia, low K/Mg/Na, alkalosis, gout, ototoxicity | Monitor response early: breathlessness, urine output, weight, BP, renal function and electrolytes |
| Chronic oedema | Oral furosemide, bumetanide or torsemide; lowest dose that maintains euvolaemia | Over-diuresis, adherence problems, interactions | Teach daily weights, sick-day review and when to seek help |
*Educational examples only; use local protocols, product information and prescriber review. Excessive furosemide can cause profound water/electrolyte depletion. Rapid IV administration, high doses and renal impairment increase the risk of tinnitus/hearing injury.
Important: a rise in creatinine during effective decongestion requires clinical interpretation. Look at BP, perfusion, congestion, urine output and trend—do not reflexively stop a lifesaving decongestion plan or ignore hypotension/true volume depletion.
4. Thiazide and thiazide-like diuretics
Mechanism: inhibit NCC in the early distal convoluted tubule. They lower blood pressure initially through sodium/water loss and later through reduced peripheral resistance. Examples are hydrochlorothiazide, chlorthalidone and indapamide.
- Best uses: hypertension, mild oedema, nephrogenic diabetes insipidus, prevention of calcium renal stones in hypercalciuria, and add-on “sequential nephron blockade” when loop response is poor.
- Adverse effects: hypokalaemia, hyponatraemia, hypomagnesaemia, dehydration, hyperuricaemia/gout, hyperglycaemia, photosensitivity and erectile dysfunction.
- Calcium: thiazides reduce urinary calcium, unlike loops. This helps distinguish the classes.
- Safety: severe hyponatraemia can present with confusion, seizure or collapse—check sodium urgently in an unwell patient.
5. Potassium-sparing diuretics: do not overlook hyperkalaemia
| Subgroup | Examples | Mechanism | Major caution |
|---|---|---|---|
| Aldosterone antagonist (MRA) | Spironolactone, eplerenone | Blocks mineralocorticoid receptor | Hyperkalaemia, renal impairment; spironolactone can cause gynaecomastia |
| ENaC blocker | Amiloride, triamterene | Blocks epithelial sodium channel | Hyperkalaemia; avoid unsafe combinations |
MRAs improve outcomes in appropriately selected HFrEF patients. Before starting or increasing, check potassium and renal function; repeat testing after initiation and dose changes. Combining an MRA with ACE inhibitor/ARB/ARNI or potassium supplements may be appropriate only with deliberate monitoring—not by accident.
6. Less common but examinable classes
- Acetazolamide: carbonic anhydrase inhibitor; causes bicarbonate loss and metabolic acidosis. Uses include glaucoma, acute mountain sickness, selected metabolic alkalosis and adjunctive decongestion. Can cause paraesthesia, renal stones and sulfonamide-related reactions.
- Mannitol: osmotic diuretic used to reduce raised intracranial/intraocular pressure in selected patients. It initially expands intravascular volume and may worsen pulmonary oedema; avoid in anuria, severe dehydration or uncontrolled HF/pulmonary congestion.
- Vaptans: vasopressin antagonists produce aquaresis (water loss more than sodium loss); specialist use in selected hyponatraemia settings.
7. Diuretics in heart failure: a safe practical sequence
- Confirm congestion: dyspnoea, orthopnoea, raised JVP, oedema, crackles, weight gain, hepatomegaly or pulmonary oedema.
- Check perfusion and BP. Cold/hypotensive patients may have cardiogenic shock and need urgent escalation—not just escalating diuretic doses.
- Give loop diuretic for fluid overload, reassess urine output and symptoms, and monitor fluid balance/weight.
- If response is inadequate, review dose, adherence, absorption, renal function, salt intake, NSAIDs and diagnosis. Sequential nephron blockade may be prescribed by an experienced clinician, with closer electrolyte checks.
- Once euvolaemic, use the lowest maintenance loop dose and optimise disease-modifying HFrEF therapy where indicated.
Diuretics mainly improve symptoms and prevent congestion-related admission; they do not by themselves reverse the underlying cardiomyopathy.
8. Nursing monitoring and patient education
| Monitor | Why it matters | Escalate urgently if |
|---|---|---|
| BP, pulse, postural symptoms | Over-diuresis causes hypotension/falls | Syncope, shock, symptomatic hypotension |
| Input/output, daily weight, oedema, lungs | Measures decongestion better than one urine measurement | Worsening breathlessness/pulmonary oedema or very low urine output |
| Na, K, Mg, creatinine/eGFR, bicarbonate | Detects electrolyte and renal injury | Severe K/Na disturbance, arrhythmia, confusion or seizure |
| Hearing (high-dose/rapid IV loop) | Early ototoxicity detection | Tinnitus or sudden hearing change |
- Give oral doses early in the day when possible; explain nocturia if a late dose is unavoidable.
- Daily weight is more useful than guessing fluid retention. Agree a local action plan for rapid gain, worsening oedema or breathlessness.
- Avoid self-medicating with NSAIDs: they can blunt diuretic effect and worsen renal function.
- Do not advise potassium supplements or salt substitutes containing potassium without prescriber direction.
9. High-yield comparisons and cases
Loop vs thiazide
Loop: strongest, loses calcium, preferred for major HF congestion. Thiazide: excellent antihypertensive, retains calcium, more likely to cause hyponatraemia.
Case: spironolactone
HFrEF patient on ACE inhibitor develops weakness and K 6.2 mmol/L. Answer: urgent assessment for hyperkalaemia/ECG changes; withhold relevant drugs as directed and treat using local emergency protocol.
Case: furosemide
After IV furosemide a patient becomes dizzy, hypotensive and dry with low K. Answer: suspect over-diuresis; assess perfusion, hold/reduce under prescriber direction and correct fluid/electrolytes.
References
- Supplied class reference: Diuretics (SlideShare).
- 2022 AHA/ACC/HFSA Heart Failure Guideline; 2021 ESC Heart Failure Guideline.
- DailyMed: furosemide and hydrochlorothiazide prescribing information.
Educational resource; use local protocols and patient-specific prescribing review.
