What should be monitored in patients on amiodarone (antiarrhythmic medication)?

Monitoring Requirements for Patients on Amiodarone

Patients on amiodarone require thyroid function tests (TSH) and liver transaminases every 6 months, with baseline assessment including chest radiograph, pulmonary function tests with DLCO, ECG, and consideration of ophthalmologic examination. 1

Baseline Monitoring (Before Starting Amiodarone)

Essential baseline tests include:

• Thyroid function tests (TSH) to establish reference values for detecting both hypothyroidism and hyperthyroidism during therapy 1, 2
• Liver function tests including ALT, AST, and GGT to identify baseline hepatic enzyme elevations, which occur in approximately 54% of patients 1, 3
• Chest radiograph and pulmonary function tests including diffusing capacity of lung for carbon monoxide (DLCO) to establish baseline pulmonary status 1
• ECG to document baseline QTc interval, PR interval, and QRS duration before initiating therapy 1
• Complete history and physical examination focusing specifically on heart failure, existing arrhythmias, and all concomitant medications 1

Additional baseline considerations:

• Ophthalmologic examination for patients with preexisting visual impairment, as amiodarone can cause corneal microdeposits and rarely optic neuropathy 1, 4
• Digoxin level if patient is taking digoxin, as amiodarone typically doubles digoxin levels and requires 50% dose reduction 1

Regular Monitoring Schedule (Every 6 Months)

The following tests must be repeated every 6 months throughout amiodarone therapy:

• Thyroid function tests (TSH) to detect both hypothyroidism (occurring in 2-10% of patients) and hyperthyroidism, with hypothyroidism being 2-4 times more common 1, 4, 2
• Liver transaminase levels to detect hepatotoxicity, which occurs at a rate of 0.6% annually with long-term therapy 1, 4
• Directed history and physical examination specifically assessing for pulmonary symptoms (cough, dyspnea), visual changes, neurologic symptoms (ataxia, paresthesias, tremor), dermatologic effects (photosensitivity, bluish discoloration), and cardiovascular effects (bradycardia, heart block) 1, 4
• Digoxin levels if patient is on concurrent digoxin therapy 1

Cardiac Monitoring

During initiation and maintenance:

• Weekly heart rate monitoring by pulse check, event recorder, or office ECG to detect excessive bradycardia, particularly in the first 6 weeks 1
• ECG assessment after each dose change monitoring PR interval, QRS duration, and QT interval 1
• QTc monitoring during infusion for intravenous amiodarone, as QTc prolongation to ≥500 ms increases risk of torsade de pointes 3

Bradycardia occurred in 4.9% of patients in clinical trials and is more common in women, potentially requiring permanent pacemaker implantation. 1, 3

Drug Interaction Monitoring

Critical medication interactions requiring specific monitoring:

• Warfarin: Monitor prothrombin time and INR at least weekly during the first 6 weeks of concomitant therapy, with peak interaction effects occurring at approximately 7 weeks; reduce warfarin dose by 50% when initiating amiodarone 1
• Simvastatin: Limit to maximum 20 mg daily due to increased myopathy risk 1
• Rate-control medications: Reduce doses of beta-blockers, calcium channel blockers, or other rate-control agents approximately 6 weeks after starting amiodarone; discontinue entirely if excessive bradycardia develops 1

Symptom-Triggered Monitoring

If specific symptoms develop, perform additional testing:

• Pulmonary symptoms (cough, dyspnea, fever, wheezing): Obtain chest radiograph and repeat pulmonary function tests immediately, as acute pulmonary toxicity can progress to ARDS or respiratory failure within days to weeks 1, 4, 3
• Visual symptoms: Perform ophthalmologic examination to evaluate for optic neuropathy or optic neuritis, which can progress to permanent blindness 1, 4, 3
• Respiratory distress: Monitor FiO₂, SaO₂, and PaO₂ closely, as postoperative ARDS has been reported in patients on amiodarone 3

Critical Thresholds for Action

Discontinue amiodarone if:

• Liver enzymes exceed 3 times normal, unless treating life-threatening arrhythmias where benefit outweighs risk 1
• Progressive hepatic injury develops despite dose reduction 3
• Acute pulmonary toxicity with respiratory compromise occurs 3

Amiodarone-induced hypothyroidism does NOT require discontinuation; continue amiodarone while initiating levothyroxine replacement. 1 However, hyperthyroidism requires aggressive treatment and possible amiodarone withdrawal due to increased mortality risk. 2

Common Monitoring Pitfalls

• Do not overlook pulmonary function testing when respiratory symptoms develop, as pulmonary toxicity can be fatal 1
• Avoid grapefruit juice, which inhibits amiodarone metabolism 1
• Remember amiodarone's 58-day half-life complicates management of adverse effects, as stopping the drug provides little short-term benefit 1, 5
• Monitor infusion rate closely for IV amiodarone, as hypotension and bradycardia are rate-related rather than dose-related 3