Furosemide
Lasix
diureticIV · IM · oralOnset: 5 minutes (IV); 30–60 minutes (oral)Duration: 2–4 hours (IV); 6–8 hours (oral)
Mechanism of Action
Loop diuretic. Inhibits Na+/K+/2Cl− cotransporter (NKCC2) in the thick ascending limb of Loop of Henle → blocks reabsorption of Na+, K+, and Cl− → profound diuresis. Also reduces preload via venodilation (acute effect).
Dosing
Adult
Oral: 20–80 mg/day. IV: 20–80 mg bolus; may double q2h to max 200 mg single dose. Continuous infusion: 0.05–0.5 mg/kg/hr after loading dose
Pediatric
1–2 mg/kg IV q6–12h; max 6 mg/kg/day
Neonatal
1–2 mg/kg IV q12–24h; use with caution (nephrocalcinosis risk)
Indications
- Pulmonary edema (acute — cardiogenic)
- CHF with fluid overload
- Hypertension
- Nephrotic syndrome / cirrhosis with ascites
- Hypercalcemia (with saline)
- Fluid overload in mechanically ventilated patients
Contraindications
- Anuria (no urine production — obstruction must be ruled out)
- Hypovolemia/dehydration
- Electrolyte depletion (severe hypokalemia)
- Allergy to sulfonamides (cross-reactivity risk — low but documented)
Side Effects
- Hypokalemia (most common and clinically important)
- Hyponatremia
- Hypomagnesemia
- Hypochloremic metabolic alkalosis
- Ototoxicity (high doses IV — esp. with aminoglycosides)
- Hyperuricemia / gout
- Dehydration and hypovolemia
- Azotemia (pre-renal)
Monitoring Parameters
- Serum electrolytes (K+, Na+, Mg2+, Cl−) — before and after doses
- BUN and creatinine (pre-renal azotemia)
- Urine output
- Blood pressure
- Daily weights
- Hearing (prolonged high-dose IV use)
Drug Interactions
- Aminoglycosides (additive ototoxicity — significant)
- Digoxin (hypokalemia increases toxicity — monitor K+)
- NSAIDs (reduce diuretic effect)
- Lithium (furosemide increases lithium levels)
- Probenecid (reduces furosemide effect)
Clinical Pearls
- IV Lasix has immediate venodilatory effect before diuresis begins — reduces preload quickly
- Administer IV no faster than 4 mg/min (ototoxicity risk at rapid infusion rates)
- Replete potassium and magnesium with aggressive diuresis
- Hypokalemia + digoxin = dangerous combination — monitor closely
- In renal failure: higher doses needed (GFR-dependent efficacy)
- Furosemide in neonates: risk of nephrocalcinosis with chronic use
Mnemonic
💡 LASIX: Lose A Salt In eXcess — K, Na, Mg, Cl all lost with the urine
NBRC High-Yield
- Loop diuretic — inhibits NKCC2 in Loop of Henle
- Most common side effect: hypokalemia
- Monitor K+, Na+, Mg2+, BUN/Cr with use
- Ototoxicity: rapid IV infusion or combination with aminoglycosides
- Digoxin toxicity potentiated by hypokalemia
- IV onset: 5 minutes (venodilation before diuresis)