A 68-year-old woman presents to the cardiology clinic for a routine follow-up. She has a h… — USMLE Step 3

A $68$ -year-old woman presents to the cardiology clinic for a routine follow-up. She has a history of heart failure with reduced ejection fraction (left ventricular ejection fraction of $30\%$ ). Her current daily medication regimen includes lisinopril, carvedilol, spironolactone, and digoxin. She has been stable on these medications for the past two years, with her most recent serum digoxin concentration measuring $0.8 \text{ ng/mL}$ (therapeutic range for heart failure: $0.5 - 0.9 \text{ ng/mL}$ ). During today's visit, an electrocardiogram reveals new-onset paroxysmal atrial fibrillation with a ventricular rate of $110/\text{min}$ . The physician decides to initiate amiodarone therapy for rhythm control and suppression of the atrial fibrillation. Which of the following is the most appropriate next step regarding her current medication regimen?

Reduce the digoxin dosage by $50\%$ and recheck the serum digoxin concentration in $1$ to $2$ weeksCevap
B
Maintain the current medication dosages and schedule a routine follow-up in $3$ months
C
Substitute carvedilol with oral diltiazem to optimize atrioventricular nodal blockade
D
Discontinue digoxin immediately and obtain an echocardiogram to assess for worsening systolic function
E
Order a baseline prothrombin time and international normalized ratio to monitor for amiodarone-induced coagulopathy

Cevap

Reduce the digoxin dosage by

50\%

and recheck the serum digoxin concentration in

1

2

weeks

Amiodarone is a potent inhibitor of P-glycoprotein, which is the primary efflux transporter responsible for the renal and biliary clearance of digoxin. When amiodarone is initiated in a patient already taking digoxin, the clearance of digoxin decreases significantly, causing serum concentrations to rise by approximately

70\%

100\%

. To prevent severe and potentially fatal digoxin toxicity (e.g., life-threatening arrhythmias, visual disturbances, nausea), the standard of care requires preemptively reducing the digoxin dose by

30\%

50\%

when amiodarone is started. Close clinical monitoring and a repeat serum digoxin level should be evaluated within

1

2

weeks after the adjustment.

Adım Adım Çözüm

Identify the critical drug-drug interaction between the newly prescribed amiodarone and the patient's baseline digoxin.

Amiodarone inhibits P-glycoprotein, reducing the clearance of digoxin and significantly elevating its serum levels.

Recognizing pharmacokinetic interactions is a mandatory safety check prior to finalizing treatment modifications.

Determine the appropriate dose adjustment and monitoring interval for digoxin.

The digoxin dose must be empirically reduced by

30\%

50\%

, followed by a repeat serum level check in

1

2

weeks.

Preemptive dose reduction prevents life-threatening digoxin toxicity, such as fatal arrhythmias, while maintaining therapeutic efficacy.

Anahtar Kavram

When initiating amiodarone in a patient currently taking digoxin, the digoxin dose must be reduced by

50\%

to prevent toxicity due to amiodarone's inhibition of P-glycoprotein-mediated digoxin clearance.

Tahmini Süre:1m 15s

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Adım Adım Çözüm

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