How should I prepare and dose a continuous furosemide (Lasix) infusion in a 50 mL syringe for an adult with acute decompensated heart failure (~70 kg), using a starting rate of 0.5 mg·kg⁻¹·h⁻¹?

Furosemide Continuous Infusion Preparation and Dosing for Acute Decompensated Heart Failure

Preparation of 50 mL Syringe

For a 70 kg patient with acute decompensated heart failure, prepare a 50 mL syringe containing furosemide 250 mg (5 mg/mL concentration) by drawing up 25 mL of furosemide 10 mg/mL solution and diluting with 25 mL of normal saline. 1

Standard Concentration Formula

• Target concentration: 5 mg/mL (250 mg in 50 mL total volume) 1
• Mix 25 mL furosemide injection (10 mg/mL = 250 mg) + 25 mL normal saline 1
• This concentration allows precise titration at typical infusion rates of 5-10 mg/hour 1

Initial Dosing Algorithm

Step 1: Loading Bolus (Required Before Infusion)

Administer 40 mg IV push over 1-2 minutes before starting the continuous infusion. 1

• If patient is already on chronic oral furosemide >40 mg/day, use a loading dose equivalent to their home dose (e.g., 80 mg IV if taking 80 mg PO daily) 1
• For diuretic-naïve patients or those on ≤40 mg/day, use 40 mg loading dose 1

Step 2: Starting Infusion Rate

Begin continuous infusion at 5 mg/hour (1 mL/hour of the 5 mg/mL solution) immediately after the loading bolus. 1, 2

• For a 70 kg patient at 0.5 mg/kg/hour: 70 kg × 0.5 mg/kg/hour = 35 mg/hour starting rate 1
• However, start conservatively at 5-10 mg/hour and titrate upward based on response 1, 2

Step 3: Titration Protocol

Double the infusion rate every 2 hours until achieving target urine output >0.5 mL/kg/hour (>35 mL/hour for 70 kg patient). 1

• Hour 0-2: 5 mg/hour (1 mL/hour)
• Hour 2-4: 10 mg/hour (2 mL/hour) if urine output <35 mL/hour
• Hour 4-6: 20 mg/hour (4 mL/hour) if inadequate response continues
• Maximum infusion rate: 4 mg/min = 240 mg/hour (to prevent ototoxicity) 1
• Practical maximum: 10-40 mg/hour for most patients 1

Critical Pre-Administration Requirements

Verify ALL of the following before starting furosemide infusion: 1

• Systolic blood pressure ≥90-100 mmHg (preferably ≥100 mmHg) 1
• Serum sodium >125 mmol/L 1
• Serum potassium >3.0 mmol/L 1
• Absence of anuria 1
• Evidence of volume overload (pulmonary edema, peripheral edema, elevated JVP) 1

Common pitfall: Never give furosemide to hypotensive patients expecting hemodynamic improvement—it will worsen tissue perfusion and precipitate cardiogenic shock. 1

Mandatory Monitoring Protocol

Hourly Monitoring (First 6-12 Hours)

• Urine output via bladder catheter (target >0.5 mL/kg/hour = >35 mL/hour for 70 kg) 1
• Blood pressure every 15-30 minutes for first 2 hours, then hourly 1
• Signs of hypovolemia: tachycardia, hypotension, decreased skin turgor 1

Laboratory Monitoring

• Electrolytes (Na, K) and creatinine within 6-24 hours of starting infusion 1
• Repeat electrolytes every 1-2 days during active titration 1
• Daily weights at same time each day (target loss 0.5-1.0 kg/day) 1

Spot Urine Sodium Check (Optional but Helpful)

• Measure urine sodium 2 hours after starting infusion 1
• Urine sodium <50-70 mEq/L indicates inadequate diuretic effect—increase infusion rate 1

Absolute Contraindications to Continue Infusion

Stop furosemide immediately if ANY of the following develop: 1

• Systolic blood pressure drops <90 mmHg 1
• Serum sodium <120-125 mmol/L 1
• Serum potassium <3.0 mmol/L 1
• Anuria develops 1
• Creatinine rises >0.3 mg/dL from baseline 1
• Signs of marked hypovolemia (hypotension, tachycardia, poor perfusion) 1

Managing Diuretic Resistance

If urine output remains <0.5 mL/kg/hour after 24-48 hours despite reaching 20 mg/hour infusion rate, ADD a second diuretic class rather than escalating furosemide beyond 40 mg/hour. 1

Sequential Nephron Blockade Options

• Hydrochlorothiazide 25 mg PO once daily 1
• Spironolactone 25-50 mg PO once daily 1
• Metolazone 2.5-5 mg PO once daily 1

Rationale: Continuous infusion overcomes diuretic resistance better than boluses by maintaining stable tubular drug concentrations and avoiding rebound sodium reabsorption between doses. 3, 4 However, exceeding 160 mg total daily dose without combination therapy is futile. 1

Dose Limits and Safety Thresholds

24-Hour Dose Limits

• First 6 hours: maximum 100 mg total 1
• First 24 hours: maximum 240 mg total (guideline recommendation) 1
• Practical maximum with close monitoring: up to 600 mg/24 hours in refractory cases 1

Infusion Rate Limits

• Never exceed 4 mg/min (240 mg/hour) to prevent ototoxicity 1
• Doses ≥250 mg must be given as infusion over ≥4 hours 1

Concurrent Therapy for Acute Pulmonary Edema

Furosemide should NOT be used as monotherapy in acute pulmonary edema. 1

• Start IV nitroglycerin concurrently (5-10 mcg/min, titrate to highest tolerable dose) 1
• High-dose nitrates + low-dose furosemide is superior to high-dose furosemide alone (reduced intubation 13% vs 40%, P<0.005) 1
• Consider non-invasive positive pressure ventilation (CPAP/BiPAP) if respiratory rate >20 and SBP >85 mmHg 1

Evidence Supporting Continuous Infusion Over Boluses

Continuous infusion produces greater diuresis than intermittent boluses without increasing adverse effects. 3, 4

• Continuous infusion increased total urine output by 12-26% and sodium excretion by 11-33% compared to equivalent bolus doses 3
• Meta-analysis of 8 trials (n=669) showed continuous infusion achieved greater weight loss (mean difference 0.70 kg, P=0.02) and urine output (mean difference 461.5 mL/24h, P<0.01) without increasing mortality, length of stay, or electrolyte disturbances 4
• Low-dose continuous infusion (mean 5.1 mg/hour) increased mean hourly urine output from 116 to 150 mL/hour (P<0.001) without worsening renal function 2

Practical Example for 70 kg Patient

Day 1 Protocol:

1. Verify SBP ≥100 mmHg, Na >125 mmol/L, K >3.0 mmol/L
2. Place bladder catheter
3. Give 40 mg IV push loading dose over 1-2 minutes
4. Start infusion at 5 mg/hour (1 mL/hour of 5 mg/mL solution)
5. Check urine output hourly—target >35 mL/hour
6. If urine output <35 mL/hour at 2 hours, increase to 10 mg/hour
7. Continue doubling every 2 hours until target achieved (max 40 mg/hour practical limit)
8. Check electrolytes and creatinine at 6-12 hours
9. Daily weight in morning (target loss 0.5-1.0 kg/day)

Common pitfall: The first dose produces maximal effect; subsequent doses at same concentration are up to 25% less effective due to compensatory sodium retention. 1 This is why continuous infusion is superior—it maintains steady tubular drug levels.