IV Furosemide Equivalent of 80 mg Oral Furosemide
The IV equivalent of 80 mg oral furosemide is 40 mg IV furosemide due to the approximately 50% oral bioavailability of furosemide.
Pharmacokinetics of Furosemide
Furosemide is a potent loop diuretic with variable bioavailability when administered orally. The key factors affecting the oral-to-IV conversion include:
- Bioavailability: Oral furosemide has approximately 40-50% bioavailability compared to IV administration 1
- Absorption: Oral furosemide has variable absorption from the gastrointestinal tract, which contributes to its lower bioavailability 2
- Protein binding: Furosemide is highly protein-bound (98.5-99.1%), which affects its volume of distribution 3
Dosing Conversion Guidelines
The 2:1 oral-to-IV conversion ratio is supported by pharmacokinetic studies:
- Studies demonstrate that approximately 30% of an oral dose of furosemide is excreted unchanged in urine, while about 36% of an IV dose is excreted unchanged 1
- This results in an estimated bioavailability of approximately 40-50% for oral furosemide compared to IV administration 3, 1
Clinical Applications
When converting from oral to IV furosemide:
- For patients requiring IV furosemide after being on 80 mg oral furosemide, the appropriate IV dose would be 40 mg
- In clinical practice, this conversion is particularly important in:
- Patients transitioning from outpatient to inpatient care
- Patients who cannot take oral medications
- Situations requiring more rapid diuresis
Special Considerations
Heart Failure
- In acute heart failure, IV furosemide provides more rapid and predictable diuresis than oral administration
- Studies show that even low-dose IV furosemide (20 mg) produces significant diuretic and natriuretic effects in heart failure patients 4
Cirrhosis
- In patients with cirrhosis and ascites, guidelines recommend oral furosemide starting at 40 mg daily, often in combination with spironolactone 5
- When converting to IV, maintain the 2:1 ratio, but be cautious of rapid fluid shifts
Renal Impairment
- In patients with renal impairment, higher doses may be required due to decreased delivery to the site of action 6
- However, the same conversion ratio (2:1) should be maintained
Potential Pitfalls
- Overdiuresis: IV furosemide acts more rapidly than oral, potentially causing more pronounced fluid and electrolyte shifts
- Electrolyte abnormalities: Monitor for hypokalemia, hyponatremia, and hypochloremic alkalosis
- Ototoxicity: High doses of IV furosemide (>6 mg/kg/day) administered rapidly can cause hearing loss; infusions should be administered over 5-30 minutes 5
Monitoring Recommendations
When converting from oral to IV furosemide:
- Monitor urine output
- Check daily weight
- Monitor serum electrolytes (particularly potassium, sodium, and magnesium)
- Assess volume status and hemodynamic parameters
Remember that the bioavailability of furosemide can vary between individuals, so clinical response should guide subsequent dosing adjustments.