How should I manage a patient who accidentally received 400 mg amiodarone instead of 20 mg lisinopril, who is hypertensive but has no cardiac arrhythmia?

Management of Accidental Amiodarone 400 mg Administration

Observe the patient closely with continuous cardiac monitoring for at least 24 hours, checking vital signs every 1-2 hours, with particular attention to heart rate, blood pressure, and ECG changes, but do not administer activated charcoal or attempt gastric lavage as amiodarone is poorly absorbed and these interventions are ineffective. 1, 2

Immediate Assessment and Monitoring

Vital Sign Surveillance

• Monitor heart rate continuously for bradycardia, which occurs in 4.9% of patients receiving amiodarone and represents the most common acute cardiovascular complication 3, 4
• Check blood pressure every 1-2 hours for the first 24 hours, as hypotension occurs in 16% of patients and can progress to cardiogenic shock in overdose situations 3, 1
• Obtain a 12-lead ECG immediately and repeat every 4-6 hours for the first 24 hours to assess PR interval, QRS duration, and QT interval prolongation 5

Critical Parameters to Monitor

• PR interval prolongation indicating AV nodal conduction delay 5
• QT interval for excessive prolongation (>500 ms warrants intervention) 5
• Development of second- or third-degree heart block, which represents an absolute indication for temporary pacing 3, 4

Expected Clinical Course

Pharmacokinetic Considerations

• Amiodarone is poorly absorbed from the gastrointestinal tract, so a single 400 mg oral dose will result in significantly lower systemic exposure than 400 mg IV 2, 6
• Peak plasma levels occur 3-7 hours after oral administration, so the patient should be monitored most intensively during this window 2, 6
• The extremely long half-life (58 days average, range 15-100 days) means that while acute toxicity from a single dose is unlikely, cumulative effects could theoretically occur if the error is repeated 3, 7

Anticipated Effects from Single 400 mg Dose

• Minimal hemodynamic impact is expected from a single 400 mg oral dose in a patient without pre-existing cardiac disease, as therapeutic loading doses are 600-800 mg daily for weeks 5, 7
• Bradycardia risk is low with a single dose but increases with repeated administration, particularly in women 5, 4
• Hypotension is more common with IV formulation (16%) than oral administration 3, 1

Management of Complications

If Bradycardia Develops (HR <50 bpm)

• Administer atropine 0.5-1 mg IV if the patient becomes symptomatic with dizziness, syncope, or hypotension 1
• Prepare for temporary transcutaneous pacing if bradycardia is severe (<40 bpm) or associated with hemodynamic compromise 3, 1
• Do not administer additional rate-controlling medications (beta-blockers, calcium channel blockers, or digoxin) as these have additive effects with amiodarone 3, 4

If Hypotension Develops (SBP <90 mmHg)

• Administer IV crystalloid bolus (500-1000 mL normal saline) as first-line therapy 1
• Consider vasopressor support (norepinephrine or dopamine) if hypotension persists despite fluid resuscitation 1
• Avoid beta-agonists with pure chronotropic effects as they may worsen any underlying arrhythmia 1

If AV Block Develops

• Initiate temporary transcutaneous pacing immediately for second- or third-degree heart block 3, 4, 1
• Consult cardiology for consideration of temporary transvenous pacing if transcutaneous pacing is required for >2 hours 3, 4

Blood Pressure Management

Addressing the Missed Lisinopril Dose

• Administer the scheduled lisinopril 20 mg once the patient is confirmed stable (typically after 4-6 hours of observation) 3
• Monitor blood pressure closely after lisinopril administration, as amiodarone can potentiate hypotensive effects through its calcium channel blocking properties 5, 3
• Target blood pressure <140/90 mmHg for most hypertensive patients, but accept slightly higher readings (up to 150/95 mmHg) in the first 24 hours if the patient is otherwise stable 3

Laboratory Monitoring

Baseline Assessment

• Check serum electrolytes immediately, particularly potassium and magnesium, as deficiencies increase proarrhythmic risk 5
• Obtain baseline liver function tests (AST, ALT, alkaline phosphatase) as amiodarone can cause hepatotoxicity even with acute overdose 1, 8
• Measure thyroid function (TSH, free T4) as a baseline, though acute thyroid effects are unlikely from a single dose 5, 2, 6

Electrolyte Correction

• Maintain potassium >4.0 mEq/L and magnesium >2.0 mg/dL to minimize proarrhythmic risk 5
• Replete aggressively if levels are below these thresholds 5

Drug Interactions to Consider

Warfarin Interaction

• If the patient is taking warfarin, check INR within 3-5 days as amiodarone significantly increases anticoagulation effects, though a single dose is unlikely to cause clinically significant changes 5, 3, 7
• Do not adjust warfarin dose based on a single accidental amiodarone dose 5, 3

Digoxin Interaction

• If the patient is taking digoxin, monitor for signs of toxicity (nausea, visual changes, arrhythmias) as amiodarone doubles digoxin levels, though this effect takes days to weeks to develop 5, 3
• Check digoxin level in 5-7 days if the patient is on chronic digoxin therapy 5

Statin Interaction

• If the patient is taking simvastatin or atorvastatin, counsel about myopathy risk and advise reporting any muscle pain or weakness, though acute rhabdomyolysis from a single dose is extremely unlikely 6

Disposition and Follow-Up

Discharge Criteria (After 24 Hours of Observation)

• Heart rate remains >50 bpm without symptomatic bradycardia 3, 4
• Blood pressure stable (SBP >90 mmHg) without vasopressor support 3, 1
• No evidence of AV block on serial ECGs 3, 4, 1
• QT interval <500 ms on most recent ECG 5, 3
• Patient asymptomatic without dizziness, syncope, or chest pain 3, 1

Outpatient Follow-Up

• Schedule follow-up visit in 3-5 days to reassess vital signs and repeat ECG 3, 2
• Instruct patient to report immediately any symptoms of bradycardia (dizziness, lightheadedness, syncope) or other concerning symptoms 3, 4
• Resume normal antihypertensive regimen (lisinopril 20 mg daily) once stable 3
• No long-term monitoring is required beyond the initial 3-5 day follow-up if the patient remains asymptomatic, as a single 400 mg dose does not produce sustained therapeutic levels 2, 6

Common Pitfalls to Avoid

• Do not induce vomiting or administer activated charcoal, as amiodarone is poorly absorbed and these interventions are ineffective and potentially harmful 2, 6
• Do not administer additional rate-controlling agents (beta-blockers, calcium channel blockers) prophylactically, as this increases bradycardia risk 3, 4
• Do not withhold the patient's scheduled lisinopril indefinitely, as undertreated hypertension poses greater risk than the minimal interaction with a single amiodarone dose 3
• Do not discharge the patient before 24 hours of observation, as peak effects occur 3-7 hours post-ingestion and delayed complications can occur 2, 6
• Do not order routine chest X-ray or pulmonary function tests, as pulmonary toxicity requires weeks to months of cumulative exposure and does not occur from a single dose 5, 2, 8, 6