What is the recommended oral dose of Lasix (furosemide) for diuresis in an elderly patient with acute hypertension when only oral formulation is available?

IV to PO Furosemide Conversion for Diuresis

Yes, you should increase the PO dose to 80 mg when converting from 40 mg IV furosemide, as oral bioavailability is approximately 50% compared to IV administration.

Bioavailability and Dose Conversion

• Oral furosemide has significantly reduced bioavailability compared to IV administration, requiring dose adjustment when switching routes 1
• The standard conversion is to double the IV dose when switching to oral administration - meaning 40 mg IV should become 80 mg PO to achieve equivalent diuretic effect 1
• The FDA label confirms that oral furosemide can be safely titrated up to 600 mg/day in severe edematous states, making 80 mg a conservative and safe starting dose 2

Practical Dosing Algorithm

For your specific situation:

• Start with furosemide 80 mg PO as a single dose (equivalent to your intended 40 mg IV) 2
• If inadequate diuresis occurs within 6-8 hours, you can give an additional 80 mg PO dose 2
• Monitor urine output hourly during the first 6 hours - target >100-150 mL/hour for adequate response 1
• Check daily weight with goal of 0.5-1.0 kg loss per day 3

Critical Monitoring Requirements

• Verify systolic blood pressure ≥90-100 mmHg before administration - furosemide will worsen hypotension and should not be given to hypotensive patients 1
• Check for contraindications: marked hypovolemia, severe hyponatremia (<120-125 mmol/L), or anuria 1
• Monitor electrolytes (potassium, sodium) within 3-7 days, especially at doses exceeding 80 mg/day 1
• Assess renal function and watch for signs of hypovolemia (decreased skin turgor, hypotension, tachycardia) 1

Important Clinical Caveats

• In acute hypertension with pulmonary edema, high-dose IV nitrates are superior to high-dose furosemide alone - if the patient has severe hypertension with flash pulmonary edema, prioritize IV nitroglycerin alongside the diuretic 4, 1
• Gut wall edema in heart failure reduces oral bioavailability even further, making IV route more reliable in acute decompensation 1
• For elderly patients, the same 2:1 conversion ratio applies, though starting at the lower end of dosing ranges is generally recommended 2

When Higher Doses Are Needed

• If 80 mg PO produces inadequate diuresis after 6-8 hours, increase by 20-40 mg increments 2
• Maximum single dose can reach 200 mg PO in severe volume overload with prior diuretic exposure 1
• Consider adding thiazide diuretic (hydrochlorothiazide 25 mg) or aldosterone antagonist (spironolactone 25-50 mg) if maximum loop diuretic doses fail, rather than escalating furosemide alone 1, 3

Common Pitfall to Avoid

Do not give the same 40 mg dose PO expecting equivalent effect to 40 mg IV - this will result in inadequate diuresis due to reduced oral bioavailability, potentially leading to treatment failure and prolonged volume overload 1, 3