Page 6 of 31
PH4.5 | PH4.5 | Diuretics and Antidiuretic Drugs — SDL Guide (Part 3)
Clinical Decision-Making: Choosing the Right Diuretic
Selecting the optimal diuretic requires integrating the patient's primary indication, renal function, electrolyte status, comorbidities, and existing medications. The following clinical scenarios illustrate the decision framework.
Acute pulmonary oedema (cardiac): IV furosemide 40–80 mg immediately — vasodilator effect within 5 minutes provides immediate relief before diuresis; follow with hourly urine output monitoring and daily electrolytes. The oral route fails in pulmonary oedema because gut wall oedema impairs absorption.
Chronic heart failure (HFrEF, LVEF <40%): The standard combination is oral loop diuretic (furosemide, torsemide) for volume control + spironolactone/eplerenone 25–50 mg for neurohormonal blockade and potassium conservation (RALES/EMPHASIS-HF evidence). Torsemide may be preferred over furosemide in stable HF due to better oral bioavailability and more predictable diuresis.
Hypertension (uncomplicated): Thiazide or thiazide-like diuretic — chlorthalidone is preferred over HCTZ in guidelines (longer half-life, better cardiovascular outcome data — SHEP and ALLHAT trials). Indapamide is a good alternative with fewer metabolic effects.
Cirrhotic ascites: Spironolactone 100–200 mg/day first-line (ascites is driven by hyperaldosteronism from splanchnic vasodilation). If insufficient, add furosemide 40 mg/day in a 5:2 ratio (spironolactone:furosemide) — this maintains normokalemia. Avoid over-diuresis: serum Na <120 mEq/L or creatinine >2 mg/dL = stop diuretics.
Hyperkalaemia risk: Monitor K when combining ACEi/ARB + potassium-sparing diuretics + NSAIDs. This triple combination can cause life-threatening hyperkalaemia. The RALES trial saw increased gynaecomastia and hyperkalaemia when spironolactone was used alongside ACEi — use the lowest effective dose and monitor K at 1, 3, and 6 months.
Diuretic resistance: When adequate loop diuretic doses fail to produce diuresis: add a thiazide (sequential nephron blockade — furosemide + metolazone is a potent combination requiring close monitoring for electrolyte depletion); correct hypoalbuminaemia; restrict sodium intake; discontinue NSAIDs.
SELF-CHECK
A patient with hepatic cirrhosis and ascites has serum potassium 5.8 mEq/L while on spironolactone 100 mg and furosemide 40 mg daily. Which change is most appropriate?
A. Increase furosemide to 80 mg and add amiloride
B. Stop furosemide, reduce spironolactone to 50 mg, and monitor K closely
C. Add tolvaptan to increase free-water excretion
D. Switch spironolactone to eplerenone at the same dose
Reveal Answer
Answer: B. Stop furosemide, reduce spironolactone to 50 mg, and monitor K closely
With potassium of 5.8 mEq/L (hyperkalaemia), reducing potassium-retaining agents is the priority. Furosemide causes potassium loss (reduces K); spironolactone retains K. Reducing spironolactone and stopping furosemide will lower K, but the right answer is to reduce or stop the potassium-retaining drug (spironolactone). Adding amiloride would worsen hyperkalaemia. Tolvaptan is for hyponatraemia, not hyperkalaemia. Switching to eplerenone at the same dose changes only the ADR profile (less gynaecomastia) but does not address the K elevation.
Self-Assessment: Diuretic Effects and Prescribing Decisions
Review the following summary tables and self-check questions before the module assessment.
Electrolyte effects of diuretics — summary:
| Diuretic class | K | Na | Ca | Mg | HCO₃ | Urate |
|---|---|---|---|---|---|---|
| Loop (furosemide) | ↓↓ | ↓↓ | ↓ | ↓ | ↓ | ↑ |
| Thiazide (HCTZ) | ↓↓ | ↓ | ↑ | ↓ | ↓ | ↑ |
| K-sparing (spiro/amiloride) | ↑ | ↓ | — | — | ↑ | — |
| CA inhibitor (acetazolamide) | ↓ | ↓ | — | — | ↓↓ | — |
| Osmotic (mannitol) | ↓ | ↓ | — | — | — | — |
Key ADR associations:
- Furosemide → ototoxicity (dose-related; highest risk: ethacrynic acid)
- Spironolactone → gynaecomastia (anti-androgenic; eplerenone is selective MR, avoids this)
- Acetazolamide → metabolic acidosis, renal stones (alkaline urine → Ca-phosphate precipitation)
- All diuretics → volume depletion, postural hypotension
- Thiazides → hyperglycaemia, dyslipidaemia (relevant in metabolic syndrome)
Vasopressin/vaptan quick-reference:
- Desmopressin: V2-selective agonist → CDI, nocturnal enuresis, haemophilia A
- Terlipressin: V1a agonist → variceal bleeding, hepatorenal syndrome
- Tolvaptan: V2 antagonist (vaptan/aquaretic) → SIADH, ADPKD
- Key warning: vaptans → correct Na no faster than 6–8 mEq/L/24h (ODS risk)