What is the appropriate IV amiodarone dosing and infusion rate for a 45‑kg patient?

IV Amiodarone Dosing for a 45-kg Patient

For a 45-kg patient, administer IV amiodarone using weight-based dosing: 150 mg (approximately 3.3 mg/kg) over 10 minutes as the loading dose, followed by a maintenance infusion of 1 mg/min for 6 hours (360 mg), then 0.5 mg/min for the remaining 18 hours (540 mg), for a total of approximately 1050 mg in the first 24 hours. 1

Standard Dosing Protocol

The ACC/AHA/HRS guidelines provide the foundational dosing regimen that applies regardless of patient weight for adults 1:

• Loading dose: 150 mg IV over 10 minutes 1, 2
• Early maintenance: 1 mg/min infusion for 6 hours (360 mg total) 1, 3
• Late maintenance: 0.5 mg/min for the remaining 18 hours (540 mg total) 1, 3
• Maximum 24-hour dose: Do not exceed 2.2 g total 3

For a 45-kg patient, the 150 mg loading dose represents approximately 3.3 mg/kg, which falls within the acceptable range and does not require adjustment 1.

Preparation and Administration Requirements

Dilution is critical: Amiodarone must be diluted in 5% dextrose in water (D5W) only—never use normal saline or lactated Ringer's solution, as these cause precipitation 2. For the 150 mg loading dose, dilute in 100 mL of D5W 4.

Concentration limits: Maintain concentrations ≤2 mg/mL for peripheral IV administration 1, 2. Higher concentrations require central venous access to prevent phlebitis 1, 4, 2.

Use an in-line filter during administration, as recommended for all amiodarone infusions 4.

Critical Monitoring During Infusion

Continuous monitoring is mandatory throughout amiodarone administration 4, 2:

• ECG monitoring: Watch for bradycardia (occurs in 4.9% of patients), AV block, QT prolongation, and changes in QRS duration 1, 4, 2
• Blood pressure: Hypotension develops in 16% of patients and is directly related to infusion rate 1, 4, 2
• Heart rate: If heart rate drops below 50 bpm with symptoms, reduce or discontinue the infusion immediately 4, 3

Contraindications and Precautions for This Patient

Absolute contraindications that must be excluded before administration 1, 4, 2:

• Second- or third-degree AV block without a pacemaker
• Severe sinus node dysfunction without a pacemaker
• Cardiogenic shock
• Marked sinus bradycardia

Relative contraindications requiring extreme caution 4:

• Baseline heart rate <60 bpm (use only if arrhythmia is immediately life-threatening)
• Systolic blood pressure <100 mmHg
• Moderate to severe left ventricular dysfunction
• Decompensated heart failure

Drug interactions to assess 1, 4, 3:

• Concomitant beta-blockers, calcium channel blockers, or digoxin increase bradycardia risk
• If patient is on digoxin, reduce digoxin dose by 50% when starting amiodarone 3
• If on warfarin, reduce warfarin dose by 30-50% and monitor INR weekly for 6 weeks 1, 4, 3

Clinical Context and Timing

Onset of action: The antiarrhythmic effect typically becomes apparent within 20-30 minutes after administration 4, 2. However, conversion to sinus rhythm most commonly occurs after 6-8 hours and usually requires a cumulative dose of at least 1 g 4.

For cardiac arrest (VF/pulseless VT): The dosing differs—give 5 mg/kg (225 mg for a 45-kg patient) as a rapid bolus, which may be repeated up to a total of 15 mg/kg daily 1, 2. This rapid administration is appropriate only after defibrillation and epinephrine have been initiated 2.

For hemodynamically stable arrhythmias: Use the standard 150 mg over 10 minutes protocol described above 1, 2, 3. The loading dose may be repeated after 10-30 minutes if breakthrough arrhythmias persist 4.

Common Pitfalls to Avoid

Do not give rapid bolus in stable patients: The delayed onset of action means rapid administration provides no therapeutic advantage in non-arrest situations while significantly increasing hypotension risk 2. For stable wide-complex tachycardia or atrial fibrillation, infuse over 20-60 minutes rather than 10 minutes 2.

Avoid peripheral administration when possible: Central venous access is strongly preferred because peripheral administration causes phlebitis, especially with concentrations >2 mg/mL 1, 4, 2.

Do not stop abruptly: If the patient is already on beta-blockers or amiodarone, never discontinue suddenly unless absolutely necessary due to risk of rebound myocardial ischemia, infarction, and arrhythmias 3.

Account for the extremely long half-life: Amiodarone has a terminal elimination half-life of 9-36 days (average 58 days, range 15-100 days) 4, 3, 5. Effects persist long after discontinuation, and cardiologist consultation is recommended when considering amiodarone outside cardiac arrest settings 2.

Adjustments for Adverse Effects

If hypotension develops: Slow or temporarily discontinue the infusion 1, 4, 2. Hypotension is rate-dependent and usually resolves with rate reduction 2.

If bradycardia occurs: Reduce the infusion rate by half if heart rate falls below 50 bpm with worsening symptoms 3. Review and consider discontinuing other heart rate-slowing medications before reducing amiodarone 3. Obtain an ECG to exclude heart block 3.

If heart block develops: Discontinue the infusion immediately 1, 4. Second- or third-degree heart block represents an absolute contraindication to continued therapy without pacemaker support 4.

Special Considerations for Low Body Weight

For this 45-kg patient, the standard adult dosing protocol is appropriate and does not require weight-based adjustment 1. The 150 mg loading dose represents 3.3 mg/kg, which is within the therapeutic range. Pediatric dosing (5 mg/kg) is reserved for patients under 18 years of age or in cardiac arrest situations 1, 2.

The maintenance infusion rates (1 mg/min then 0.5 mg/min) are fixed and not weight-adjusted for adults 1, 3, 5. However, monitor this patient more closely for hypotension and bradycardia given the lower body weight 4, 2.