What is the recommended concentration and dosing regimen for an intravenous aminophylline infusion in a 70‑kg adult, including loading dose, maintenance rate, and adjustments for smokers or patients over 60 years or with hepatic or cardiac disease?

Aminophylline Intravenous Infusion: Concentration and Dosing

For a 70-kg adult requiring aminophylline for acute bronchospasm, administer a loading dose of 5.7 mg/kg (approximately 400 mg) as aminophylline infused over 20–30 minutes, followed by a maintenance infusion of 0.5 mg/kg/hr (35 mg/hr), prepared as 500 mg aminophylline in 250 mL D5W (concentration 2 mg/mL), targeting serum theophylline levels of 10–20 mcg/mL. 1, 2, 3

Standard Concentration Preparation

• Prepare aminophylline as 500 mg in 250 mL of 5% dextrose (D5W), yielding a concentration of 2 mg/mL 4
• This standardized concentration simplifies flow-rate calculations and reduces medication errors 4
• Alternative concentrations of 400 mg theophylline (equivalent to 500 mg aminophylline) in 250 mL are also acceptable 4

Loading Dose Administration

• Administer 5–6 mg/kg aminophylline (or 4.6 mg/kg theophylline equivalent) intravenously over 20–30 minutes 1, 2, 3
• For a 70-kg adult: 350–420 mg aminophylline (approximately 400 mg is practical) 2, 3
• Never administer as a rapid IV push—the 20–30 minute infusion time is essential to prevent toxicity including arrhythmias, hypotension, and seizures 2
• This loading dose achieves a target serum theophylline concentration of approximately 10 mcg/mL (range 6–16 mcg/mL) 3

Critical Caveat for Patients Already on Oral Theophylline

• If the patient is currently taking oral theophylline, measure serum theophylline concentration before administering IV aminophylline 5, 6
• In one study, 68% of patients on maintenance theophylline already had therapeutic levels (>10 mcg/mL) and 9% had toxic levels (>20 mcg/mL) when presenting to the emergency room 6
• Omit or reduce the loading dose if serum theophylline is already therapeutic to avoid toxicity 5, 6

Maintenance Infusion Dosing

Standard Adult (Non-Smoker, Age <60, No Hepatic/Cardiac Disease)

• Infuse 0.5 mg/kg/hr aminophylline (0.4 mg/kg/hr theophylline equivalent) 2, 3
• For a 70-kg adult: 35 mg/hr aminophylline 3
• Using the standard 2 mg/mL concentration: infuse at 17.5 mL/hr 4
• This achieves steady-state serum concentrations averaging 10 mcg/mL (range 7–26 mcg/mL) 3

Dose Adjustments for Special Populations

Smokers (Tobacco or Marijuana):

• Increase maintenance rate by approximately 50% due to enhanced clearance 3
• For a 70-kg smoker: 0.75 mg/kg/hr (approximately 52.5 mg/hr aminophylline) 3

Elderly Patients (>60 Years):

• Reduce maintenance rate to 0.4 mg/kg/hr aminophylline 2, 3
• For a 70-kg elderly patient: 28 mg/hr aminophylline 2
• Theophylline clearance decreases by approximately 30% in elderly adults 3

Hepatic Insufficiency (Cirrhosis, Acute Hepatitis, Cholestasis):

• Reduce maintenance rate by 50% or more to 0.25 mg/kg/hr or less 3
• For a 70-kg patient with liver disease: 17.5 mg/hr or less 3
• Frequent monitoring of serum levels is mandatory 3

Congestive Heart Failure:

• Reduce maintenance rate by 50% or more to 0.25 mg/kg/hr or less 3
• The reduction correlates directly with CHF severity 3
• For a 70-kg patient with CHF: 17.5 mg/hr or less 3

Monitoring Requirements

• Measure serum theophylline concentration within 5.5 hours after the loading dose 1
• Check serum levels at 12–24 hour intervals during maintenance infusion 2
• Target therapeutic range: 10–20 mcg/mL 1, 2, 3
• Continuous cardiac monitoring is essential during infusion 2
• Monitor glucose and potassium levels throughout treatment 1
• Continuous pulse oximetry is recommended 1

Clinical Decision Algorithm

1. Confirm indication: Aminophylline should only be used for severe bronchospasm resistant to nebulized beta-agonists and systemic corticosteroids after 15–30 minutes of optimal first-line therapy 2

2. Assess current theophylline use: If patient is on oral theophylline, measure serum level before IV administration 5, 6

3. Identify risk factors for altered clearance:

   • Age >60 years → reduce dose 2, 3
   • Smoker → increase dose 3
   • Hepatic disease → reduce dose by ≥50% 3
   • CHF → reduce dose by ≥50% 3

4. Administer loading dose: 5–6 mg/kg over 20–30 minutes (adjust or omit if already on theophylline) 1, 2, 3

5. Start maintenance infusion: Adjust rate based on patient factors (see above) 2, 3

6. Monitor closely: Check serum levels, cardiac rhythm, and metabolic parameters 1, 2

Important Safety Considerations

• Aminophylline exhibits nonlinear elimination kinetics, meaning small dose increases can produce disproportionately large increases in serum concentration 3
• Make dose adjustments in small increments 3
• Contraindicated in patients with uncontrolled seizures—aminophylline lowers seizure threshold 1
• Avoid in patients with active tachyarrhythmias 1
• Do not mix aminophylline with other drugs in the same IV line, particularly epinephrine, isoproterenol, or norepinephrine 2