Furosemide

Lasix (named for LAst SIX) is only effective for 6 hours
Optimal dosing is given at least twice daily (patient otherwise is Sodium avid 18 hours/day)

Oral
1. Onset: 1 hour
2. Half-Life: 1 to 1.5 hours
3. Duration: 6 to 8 hours
4. Highly variable Bioavailability on oral ingestion (typically 50%, varies between 10 and 90%)
  1. Increased Bioavailability when taken with meals
  2. Gastrointestinal absorption is markedly decreased in severe edema (e.g. CHF exacerbation)
  3. Contrast with Torsemide which has excellent and consistent Bioavailability
Intravenous
1. Onset: 5 minutes
2. Duration: 2 hours
Protein-Binding: 95%
Eliminated unchanged in urine
Higher doses (2-3x) required for Nephrotic Syndrome
Renal dysfunction results in decreased response and increased Half-Life
1. Only 15-20% of Furosemide dose is delivered to the renal tubule in stage 5 CKD

Start: 20 to 40 mg orally daily to twice daily
1. Twice daily dosing is recommended due to short duration of activity
Maximum: 600 mg/day (rare to exceed 400 mg/day)
Peak effect at 1 to 1.5 hours after oral dose
Duration: 6-8 hours

Intravenous dose is typically one half of oral dose
1. In exacerbations, the oral dose is often used IV
Administer slowly over 1-2 minutes
1. Doses higher than 80 mg should infuse slowly to avoid Ototoxicity
Bolus
1. Dose 20 to 40 mg IV (0.5-1.0 mg/kg, max 2 mg/kg)
2. In CHF exacerbations, 60 mg IV is often given
3. In Renal Insufficiency, consider starting dose = 40 * sCr
  1. Where sCr = Serum Creatinine
  2. Reflects the diminishing effect of Furosemide as Serum Creatinine increases (and GFR drops)
Pharmacokinetics
1. Onset: Diuresis starts within 10 minutes
2. Peak effect in 10-30 minutes
3. Duration: 6 hours
Peak Diuretic effect of repeat dosing is 25% of the first dose
1. Compensatory Sodium retention may be overcome by frequent IV doses or continuous infusion
Maximum effective dose (ceiling dose)
1. Chronic Kidney Disease or Nephrotic Syndrome: 80 to 200 mg
2. Congestive Heart Failure or Cirrhosis: 40-80 mg

Background
1. More effective at maintaining a constant increased Urine Output with less adverse effects (e.g. Ototoxicity)
2. Maximum diuresis at 3 hours after continuous infusion started
Precautions
1. Very high dose Furosemide infusions (4 mg/min) risk Ototoxicity
Loading dose: 40-200 mg
1. Loading doses higher than 80 mg should infuse slowly to avoid Ototoxicity
Infusion dose
1. Start: 10-20 mg/hour (0.25 to 0.75 mg/kg/hour) IV
2. Maximum: 40 mg/hour IV

IV/IM/PO
1. Start: 0.5 to 2 mg/kg/dose IV/IM/PO every 6 to 12 hours
2. Max: 6 mg/kg/dose
Intravenous Infusion
1. Start: 0.05 mg/kg/hour and titrate

See Loop Diuretic
Ototoxocity
1. See Loop Diuretic
2. Most common with high doses or rapid infusion rates (>4 mg/min)
Decreased Thyroid Hormone levels
1. Associated with Furosemide doses >80 mg/day

Warfarin
1. Furosemide displaces Warfarin from Protein binding and increases Warfarin levels and INR
Cyclosporine
1. Cyclosporine decreases Uric Acid excretion, and increased gout risk when used with Furosemide
Lithium
1. Avoid use with Furosemide
Agents that decrease Furosemide Diuretic effect
1. Indomethacin
2. Probenacid