Furosemide (Lasix): Dosing, Monitoring, and Contraindications
Start with furosemide 20–40 mg orally once daily for most patients with edema or heart failure, ensuring systolic blood pressure is ≥90–100 mmHg and serum sodium is >125 mmol/L before administration. 1, 2
Initial Dosing Strategy
Standard Oral Dosing
- Edema (heart failure, nephrotic syndrome, cirrhosis): Begin with 20–40 mg orally as a single morning dose 1, 2
- Hypertension: Start with 40 mg twice daily (total 80 mg/day) 2
- Pediatric patients: 2 mg/kg as a single dose, maximum 6 mg/kg/day 1, 2
Intravenous Dosing for Acute Situations
- Acute pulmonary edema or decompensated heart failure: 20–40 mg IV bolus over 1–2 minutes for diuretic-naïve patients 1
- Patients on chronic oral diuretics: Use at least the equivalent of their oral dose IV, or 2–2.5× their home dose for acute decompensation 1
- Maximum single IV bolus: 160–200 mg 1
- First 24-hour limits: Maximum 100 mg in first 6 hours, 240 mg in first 24 hours (higher doses require close monitoring) 1
Dose Escalation Protocol
Oral Titration
- If diuresis is inadequate after 6–8 hours, increase by 20–40 mg increments 2
- Administer subsequent doses no sooner than 6–8 hours after the previous dose 2
- Maximum daily dose: Up to 600 mg/day in severe edematous states, though doses >160 mg/day typically indicate need for combination therapy 1, 2
Intravenous Escalation
- If urine output remains <0.5 mL/kg/hour after 2 hours, double the dose 1
- Increase in 20 mg increments every 2 hours until adequate diuresis is achieved 1
- Continuous infusion option: 5–10 mg/hour (maximum rate 4 mg/min) after initial bolus for severe volume overload 1
Disease-Specific Maximums
- Cirrhosis with ascites: Maximum 160 mg/day; exceeding this indicates diuretic resistance requiring paracentesis 1
- Heart failure: Doses >160 mg/day signal need for combination therapy rather than further furosemide escalation 1
Critical Pre-Administration Requirements
Absolute Contraindications
- Anuria (no urine output) 1, 2
- Severe hyponatremia (serum sodium <120–125 mmol/L) 1
- Marked hypovolemia 1
- Hypotension (systolic BP <90–100 mmHg without circulatory support) 1
Safety Checklist Before Each Dose
- Verify systolic blood pressure ≥90–100 mmHg 1
- Confirm serum sodium >125 mmol/L 1
- Ensure patient is not anuric 1
- Assess for signs of volume depletion (decreased skin turgor, tachycardia, orthostatic hypotension) 1
Essential Monitoring Parameters
Acute Phase (First 24–48 Hours)
- Urine output: Place bladder catheter and monitor hourly; target >0.5 mL/kg/hour 1
- Blood pressure: Every 15–30 minutes in first 2 hours after IV administration 1
- Daily weights: Same time each day; target loss of 0.5 kg/day without peripheral edema or 1.0 kg/day with edema 1
- Electrolytes and renal function: Check within 6–24 hours after starting IV furosemide 1
Ongoing Monitoring
- Electrolytes (sodium, potassium, magnesium): Every 3–7 days during titration, then weekly 1
- Renal function (creatinine, eGFR): Every 3–7 days initially 1
- Clinical assessment: Resolution of peripheral edema, jugular venous distension, pulmonary crackles 1
High-Dose Monitoring (>80 mg/day)
- More frequent electrolyte checks (every 1–2 days initially) 1
- Watch for ototoxicity, especially with doses >250 mg; infuse over 4 hours at maximum rate of 4 mg/min 1
- Monitor for metabolic alkalosis and severe electrolyte disturbances 1
Managing Diuretic Resistance
When Standard Doses Fail (After 24–48 Hours)
Add a second diuretic class rather than escalating furosemide beyond 160 mg/day: 1
- Hydrochlorothiazide 25 mg orally once daily 1
- Spironolactone 25–50 mg orally once daily 1
- Metolazone 2.5–5 mg orally 1
Alternative Strategies
- Switch from intermittent boluses to continuous infusion (5–10 mg/hour) 1
- In cirrhosis exceeding 160 mg/day: Perform large-volume paracentesis instead of further dose escalation 1
Disease-Specific Modifications
Cirrhosis with Ascites
- Start: Furosemide 40 mg + spironolactone 100 mg as single morning dose 1
- Maintain 100:40 ratio (spironolactone:furosemide) during titration 1
- Increase both drugs simultaneously every 3–5 days if weight loss inadequate 1
- Prefer oral route to avoid acute GFR reduction 1
- Stop immediately if: Severe hyponatremia, progressive renal failure, worsening hepatic encephalopathy, or incapacitating muscle cramps 1
Acute Pulmonary Edema
- Concurrent therapy mandatory: Start IV nitroglycerin immediately alongside furosemide 40 mg IV; nitrates are superior to high-dose furosemide alone 1
- Do not use furosemide as monotherapy in acute pulmonary edema 1
- Apply non-invasive positive pressure ventilation (CPAP/BiPAP) if respiratory rate >20 breaths/min and SBP >85 mmHg 1
Nephrotic Syndrome
- Severe edema: 0.5–2 mg/kg per dose IV or orally up to six times daily (maximum 10 mg/kg/day) 1
- With albumin infusions: 0.5–2 mg/kg IV at end of infusion, avoiding marked hypovolemia or hyponatremia 1
- Duration limit: High doses (>6 mg/kg/day) should not exceed 1 week 1
Pediatric Patients
- Initial dose: 2 mg/kg orally as single dose 2
- Escalation: Increase by 1–2 mg/kg no sooner than 6–8 hours after previous dose 2
- Maximum: 6 mg/kg/day 1, 2
- Note: Reduced clearance and prolonged half-life compared to adults 1
Geriatric Patients
- Start low: 20 mg IV or orally 1
- Titrate slowly: 2–3× longer half-life increases risk of orthostatic hypotension 1
- Monitor frequently: Supine and standing blood pressure, renal function, potassium 1
- Prefer morning dosing to reduce nocturia and improve adherence 1
Absolute Indications to Stop Immediately
- Systolic blood pressure drops <90 mmHg 1
- Severe hyponatremia develops (sodium <120–125 mmol/L) 1
- Severe hypokalemia occurs (<3 mmol/L) 1
- Anuria develops 1
- Progressive renal failure with worsening azotemia despite adequate diuresis 1
- In cirrhosis: Worsening hepatic encephalopathy or incapacitating muscle cramps 1
Common Pitfalls to Avoid
- Do not give furosemide to hypotensive patients expecting hemodynamic improvement; it worsens tissue perfusion and can precipitate cardiogenic shock 1
- Do not withhold higher doses out of fear of mild azotemia; transient renal function worsening is acceptable when volume status improves 1
- Do not exceed 160 mg/day without adding a second diuretic class; this represents the ceiling effect for monotherapy 1
- Do not use furosemide to treat or prevent acute kidney injury; it is indicated only for managing volume overload that complicates AKI 1
- Do not combine with NSAIDs, which block diuretic effects and worsen renal function 3
- Do not combine with aminoglycosides at high doses; this dramatically increases ototoxicity risk 1
- Do not give evening doses; they cause nocturia and poor adherence without improving outcomes 1
Pharmacokinetic Considerations
- Onset: IV produces diuresis within minutes; oral requires ~1 hour 1
- Peak effect: 1–1.5 hours after oral administration 1
- Duration of action: 6–8 hours (necessitates twice-daily dosing for 24-hour coverage) 1
- Bioavailability: Highly variable orally; IV route more reliable in acute settings 4
- First-dose effect: Greatest natriuretic effect occurs with first dose; subsequent doses show up to 25% less effect 1
Special Clinical Situations
Diuretic Conversion
- Furosemide to bumetanide ratio: 40:1 (40 mg furosemide = 1 mg bumetanide) 3
- Furosemide to torsemide ratio: 40:10 (40 mg furosemide = 10 mg torsemide) 3
- Monitor renal function and electrolytes within 1–2 weeks after conversion 3
Chronic Hemodialysis Patients
- Eligibility: Urine output ≥100 mL/day 1
- Caution: Diuretic response declines over time as residual renal function worsens 1
- Ototoxicity risk: Higher with furosemide than bumetanide; assess hearing if doses >6 mg/kg/day 1