Furosemide (Lasix®)
| Furosemide |  |
|---|---|
| Brand Name | Lasix® |
| Drug Class | Loop Diuretic |
| Primary Indication | Edema, Heart Failure |
| Site of Action | Thick Ascending Limb |
| Mechanism | Na⁺/K⁺/2Cl⁻ Cotransporter Inhibition |
| Potassium Effect | ↓ (Hypokalemia risk) |
| Calcium Effect | ↓ Reabsorption |
| Bioavailability | Variable (~50%) |
| Half-Life | ~1–2 hours |
| FDA Approval | 1966 |
Overview
Furosemide is a loop diuretic that inhibits sodium reabsorption in the thick ascending limb of the loop of Henle.
It is a potent natriuretic agent used primarily for volume overload states including heart failure, cirrhosis, and renal disease.
Loop diuretics remain the cornerstone of acute decompensated heart failure management.
Mechanism of Action
Site of Action
- Thick ascending limb of the loop of Henle
Transporter Blocked
- Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2)
Physiologic Effects
- ↑ Sodium and water excretion
- ↑ Potassium excretion
- ↑ Calcium excretion
- ↑ Magnesium excretion
- Disrupts medullary concentration gradient
Net effect:
- Powerful diuresis
- Rapid reduction in preload
Indications
- Acute decompensated heart failure
- Chronic heart failure
- Pulmonary edema
- Cirrhosis with ascites
- Nephrotic syndrome
- Severe hypertension (adjunct)
- Hypercalcemia (with IV fluids)
Effective even when eGFR < 30.
Contraindications
Absolute:
- Anuria
- Severe electrolyte depletion
Relative / Caution:
- Hypotension
- Dehydration
- Gout
- Concomitant nephrotoxic drugs
Dosing
Oral:
- 20–80 mg once or twice daily
IV (acute HF):
- 20–40 mg initial dose
- Titrate based on response
Higher doses required in renal impairment.
Dose-response curve is steep.
Pharmacokinetics
Absorption:
- Oral and IV
Bioavailability:
- ~50% (variable)
Half-life:
- ~1–2 hours
Duration:
- 6–8 hours
Elimination:
- Renal
Short duration often requires twice-daily dosing.
Adverse Effects
Electrolyte:
- Hypokalemia
- Hyponatremia
- Hypomagnesemia
- Hypocalcemia
Metabolic:
- Hyperuricemia
- Metabolic alkalosis
Other:
- Ototoxicity (high doses or rapid IV infusion)
- Hypotension
- Volume depletion
Drug Interactions
Aminoglycosides:
- ↑ Ototoxicity risk
Lithium:
- ↑ Lithium levels
RAAS inhibitors:
- Additive hypotension
Other diuretics:
- Additive electrolyte abnormalities
Monitoring
- Weight
- Urine output
- Electrolytes (Na⁺, K⁺, Mg²⁺)
- Renal function
- Blood pressure
Clinical Pearls
- Most commonly used loop diuretic.
- Effective in renal impairment.
- Causes hypokalemia and metabolic alkalosis.
- IV administration acts within minutes.
- Ototoxicity risk with high-dose or rapid infusion.
- Often combined with thiazides for diuretic synergy.
Comparison Within Class
Compared to Chlorthalidone:
- More potent
- Effective in renal failure
- Shorter duration
Compared to Torsemide:
- Shorter half-life
- More variable bioavailability
Compared to Bumetanide:
- Less potent per mg
- More commonly used