Amiodarone vs Adenosine: Key Differences in Cardiac Arrhythmia Management
Primary Distinction
Adenosine is the first-line drug for acute termination of supraventricular tachycardia (SVT), while amiodarone is a broad-spectrum antiarrhythmic reserved for ventricular arrhythmias and refractory cases where other agents have failed. 1
Mechanism of Action
Adenosine:
- Naturally occurring purine nucleotide that selectively blocks AV nodal conduction 1
- Works by transiently interrupting reentrant circuits dependent on the AV node 1
- Has an extremely short half-life of seconds 1
Amiodarone:
- Complex multi-class antiarrhythmic with properties of all four Vaughan Williams classes 1, 2
- Blocks sodium channels (Class I), has beta-blocking effects (Class II), prolongs repolarization via potassium channel blockade (Class III), and calcium channel blocking properties (Class IV) 1, 3
- Inhibits automaticity and terminates reentry through multiple mechanisms 1
Clinical Indications
Adenosine:
- Drug of choice for terminating paroxysmal supraventricular tachycardia (PSVT) that depends on AV nodal conduction 1
- Diagnostically useful to transiently slow ventricular rate in atrial fibrillation or flutter, allowing rhythm identification 1
- Has no effect on ventricular tachycardia, which helps differentiate SVT from VT 1
- Contraindicated in asthma due to risk of bronchospasm 1
Amiodarone:
- Indicated for life-threatening ventricular arrhythmias including ventricular tachycardia and ventricular fibrillation 1
- Can be used for both ventricular and supraventricular arrhythmias when other agents fail 1
- Safe in patients with left ventricular dysfunction, heart failure, and post-myocardial infarction—settings where other antiarrhythmics are contraindicated 1, 2
- Not first-line unless the patient is clinically stable with refractory arrhythmia 1
Onset of Action and Administration
Adenosine:
- Immediate onset within 1-2 minutes 1
- Given as rapid IV bolus: 6 mg initially, then 12 mg if no response, followed by saline flush 1
- Must be administered through large peripheral vein (antecubital preferred) 1
- Requires monitored environment due to risk of transient complete heart block 1
Amiodarone:
- Antiarrhythmic effect takes 20-30 minutes when given IV 1
- Standard IV dose: 5 mg/kg (300 mg) over one hour 1
- In cardiac arrest, can be given over 15 minutes and repeated after one hour 1
- Oral therapy has extremely long half-life (58-100 days) requiring loading doses 1, 4
Safety Profile and Side Effects
Adenosine:
- Transient side effects lasting less than 60 seconds: flushing, dyspnea, chest discomfort 1
- Can cause transient complete heart block (expected and brief) 1
- Safe in pregnancy 1
- Minimal cardiovascular depression 1
- Contraindicated in asthma due to bronchospasm risk 1
Amiodarone:
- Serious long-term toxicity affecting multiple organ systems 1, 4
- Pulmonary toxicity (most serious): interstitial pneumonitis, occurs at 0.6% annually 1, 4
- Thyroid dysfunction in 2-24% of patients (both hypo- and hyperthyroidism) 1, 4
- Liver toxicity with transaminase elevation 1, 4
- Corneal microdeposits in nearly all patients (rarely symptomatic) 4, 5
- Photosensitivity, skin discoloration, neurologic effects (tremor, ataxia, peripheral neuropathy) 1, 4
- QT prolongation common but torsades de pointes rare (<1% annually) 1, 4
- IV administration causes hypotension in 16% of patients 4
Drug Interactions
Adenosine:
- Effect enhanced by dipyridamole (reduce initial dose to 3 mg) 1
- Effect reduced by theophylline and caffeine (may require higher doses) 1
- Can be given with beta-blockers without significant interaction 1
Amiodarone:
- Extensive drug interactions via cytochrome P450 inhibition 1, 4
- Doubles digoxin levels—reduce digoxin dose by 50% when starting amiodarone 1, 6
- Increases warfarin effect—reduce warfarin dose by 50%, monitor INR weekly for 6 weeks as peak effect occurs at 7 weeks 1, 6, 4
- Increases risk of myopathy with statins—limit simvastatin to 20 mg daily 6, 4
- Grapefruit juice inhibits metabolism and should be avoided 6, 4
Monitoring Requirements
Adenosine:
- Must be given in monitored setting (emergency department, critical care unit) 1
- Continuous ECG monitoring during administration 1
- No long-term monitoring required 1
Amiodarone:
- Extensive baseline and ongoing monitoring mandatory 1, 6, 4
- Baseline: liver function tests, thyroid function (TSH), chest radiograph, pulmonary function tests including DLCO, eye examination if visual impairment 6, 4
- Every 6 months: TSH, liver transaminases, directed history and physical for adverse effects 1, 6, 4
- Weekly heart rate monitoring initially during maintenance phase 6
- Digoxin levels every 6 months if on concurrent therapy 6
Clinical Decision Algorithm
Use Adenosine when:
- Patient has stable narrow-complex tachycardia consistent with PSVT 1
- Vagal maneuvers have failed 1
- No history of asthma 1
- Immediate rhythm conversion is needed 1
- Diagnostic clarification of wide-complex tachycardia is required 1
Use Amiodarone when:
- Patient has life-threatening ventricular tachycardia or ventricular fibrillation 1
- SVT is refractory to adenosine and other first-line agents 1
- Patient has structural heart disease, heart failure, or reduced ejection fraction where other antiarrhythmics are contraindicated 1, 2
- Long-term rhythm control is needed for atrial fibrillation after other agents have failed 1
- Patient has stable monomorphic VT and immediate cardioversion is not required 1
Critical Pitfalls to Avoid
With Adenosine:
- Do not use in patients with known Wolff-Parkinson-White syndrome and atrial fibrillation, as it may precipitate ventricular fibrillation by allowing rapid conduction through accessory pathway 1
- Do not give slowly—must be rapid bolus followed immediately by saline flush 1
- Do not use in asthmatics 1
- Warn patients about transient but intense chest discomfort and flushing 1
With Amiodarone:
- Do not use as first-line therapy for SVT when adenosine is appropriate 1
- Do not overlook the 7-week delay to peak drug interaction effects with warfarin and digoxin 1, 6, 4
- Do not continue if liver enzymes exceed three times normal unless treating life-threatening arrhythmia 1, 4
- Do not ignore new respiratory symptoms—pulmonary toxicity is the most serious adverse effect and requires immediate evaluation 1, 4
- Do not use maintenance doses above 200-400 mg daily to minimize toxicity 1, 6