What is the equivalent IV (intravenous) dose of Lasix (Furosemide) for a patient taking Torsemide 40 mg orally (po)?

Torsemide 40 mg PO to IV Furosemide Conversion

For a patient taking torsemide 40 mg orally, convert to IV furosemide 80-100 mg as the initial dose when transitioning to intravenous therapy in acute heart failure.

Dose Equivalence Rationale

The conversion is based on established loop diuretic equivalency ratios and guideline-recommended dosing strategies for acute decompensation:

Standard Equivalency

• Torsemide 40 mg PO is approximately equivalent to furosemide 80-160 mg PO based on the typical 2:1 to 4:1 conversion ratio 1, 2
• Recent mechanistic data from the TRANSFORM trial demonstrated that a 4:1 dose equivalence (furosemide:torsemide) resulted in similar natriuresis, though clinicians commonly use a 2:1 ratio in practice 1
• Older pharmacokinetic studies suggested a 2:1 ratio (furosemide 40 mg = torsemide 20 mg), but this may underestimate the potency difference 2

Acute Heart Failure Dosing Strategy

• When converting to IV therapy for acute heart failure, guidelines recommend starting with at least 2× the home oral diuretic dose 3
• For a patient on torsemide 40 mg PO (roughly equivalent to furosemide 80-160 mg PO), the IV furosemide dose should be at least 80-100 mg IV 3
• The ESC guidelines specify that the initial IV furosemide dose should be "at least equivalent to the oral dose" and that patients on chronic oral therapy require higher doses than diuretic-naive patients 3

Key Pharmacokinetic Considerations

Bioavailability Differences

• Torsemide has superior oral bioavailability (~80-100%) compared to furosemide (~40-60%), which is why lower oral doses of torsemide are effective 4, 2, 5
• Furosemide bioavailability can be as low as 40% in certain disease states, requiring higher oral doses 6
• IV furosemide bypasses first-pass metabolism, providing more predictable drug delivery 7

Duration and Delivery

• Despite theoretical advantages, torsemide showed no superior kidney bioavailability or duration of action compared to furosemide in the TRANSFORM-Mechanism trial 1
• Furosemide actually demonstrated longer duration of kidney drug delivery and natriuresis than torsemide 1

Practical Conversion Algorithm

Step 1: Determine baseline oral diuretic equivalency

• Torsemide 40 mg PO ≈ Furosemide 80-160 mg PO (use 4:1 ratio for more accurate equivalence) 1

Step 2: Apply acute heart failure dosing principle

• Start IV furosemide at minimum 2× the oral furosemide equivalent dose 3
• For torsemide 40 mg PO: Start with furosemide 80-100 mg IV as initial bolus

Step 3: Adjust based on clinical factors

• Higher doses (100-160 mg IV) may be needed if:
  • Patient has significant volume overload 3
  • Previous diuretic resistance 3
  • Worsening renal function (creatinine >1.3 mg/dL) 3
• The DOSE trial showed that 2.5× home oral dose improved fluid loss and dyspnea despite transient creatinine elevation 3

Important Caveats

Monitoring Requirements

• Assess urine output frequently after initial IV dose; bladder catheter placement is recommended for accurate monitoring 3
• Total furosemide dose should remain <100 mg in first 6 hours and <240 mg in first 24 hours unless diuretic resistance is evident 3
• Monitor for hypotension (SBP <90 mmHg), which contraindicates further diuretic escalation 3

Neurohormonal Activation Risk

• Higher diuretic doses activate the renin-angiotensin-aldosterone system, potentially offsetting benefits 1
• The TRANSFORM trial showed that excessive torsemide dosing led to greater neurohormonal activation without improved fluid status 1
• Consider adding thiazide diuretics or aldosterone antagonists for synergistic effect rather than escalating loop diuretic doses indefinitely 3, 7

Contraindications to Diuretics

• Avoid diuretics in patients with hypoperfusion until adequate perfusion is restored 3
• Patients with severe hyponatremia, acidosis, or hypovolemia are unlikely to respond 3