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Difficulty: Very hardMonitoring and Treatment Evaluation

A 68-year-old man is admitted to the telemetry unit for initiation of antiarrhythmic therapy to manage highly symptomatic paroxysmal atrial fibrillation. His medical history is significant for hypertension and benign prostatic hyperplasia. He has no history of structural heart disease, and a recent echocardiogram demonstrated a left ventricular ejection fraction of 60%.

Vital signs are within normal limits. Laboratory results on admission are as follows:

TestResult
Serum sodium140 mEq/L
Serum potassium4.6 mEq/L
Serum magnesium2.1 mg/dL
Serum creatinine0.9 mg/dL

His estimated creatinine clearance is 82 mL/min. A baseline 12-lead electrocardiogram demonstrates normal sinus rhythm with a heart rate of 72/min, a QRS duration of 90 ms, and a QTc interval of 420 ms.

The physician initiates dofetilide therapy at 500 mcg twice daily. Two and a half hours after the administration of the first dose, a repeat 12-lead electrocardiogram shows normal sinus rhythm with a heart rate of 68/min and a QTc interval of 488 ms.

Which of the following is the most appropriate next step in the management of this patient's antiarrhythmic therapy?

  1. A
    Continue dofetilide at 500 mcg twice daily
  2. Decrease the dofetilide dose to 250 mcg twice dailyAnswer
  3. C
    Discontinue dofetilide and monitor on telemetry
  4. D
    Administer intravenous magnesium sulfate
  5. E
    Withhold the next dose and measure a serum dofetilide level

Answer

The correct answer is to decrease the dofetilide dose to 250 mcg twice daily.
The correct answer is to decrease the dofetilide dose to 250 mcg twice daily. Dofetilide is a class III antiarrhythmic agent associated with a significant risk of dose-related QT interval prolongation and Torsades de Pointes. Initiation requires continuous ECG monitoring in a facility. The standard dosing protocol dictates that a baseline 12-lead ECG must be obtained to ensure the QTc is < 440 ms (or < 500 ms with bundle branch block). Two to three hours after the first dose, a repeat ECG must be obtained. If the QTc interval increases by more than 15% from baseline OR if the absolute QTc exceeds 500 ms, the dose must be reduced by 50% (in this case, from 500 mcg to 250 mcg twice daily). For this patient, a 15% increase from his baseline QTc of 420 ms is calculated as 420×1.15=483420 \times 1.15 = 483 ms. His post-dose QTc of 488 ms exceeds this 15% threshold, mandating a dose reduction.

Step-by-Step Solution

1
Identify the critical monitoring window and parameters for dofetilide initiation.
Dofetilide requires continuous telemetry and a 12-lead ECG 2 to 3 hours after the first dose to evaluate for potentially fatal QTc prolongation.
To mitigate the high risk of Torsades de Pointes during the loading phase, standard protocols strictly dictate ECG timing.
2
Calculate the threshold for a clinically significant QTc increase based on the baseline ECG.
The baseline QTc is 420 ms. A >15% increase is calculated as 420×1.15=483420 \times 1.15 = 483 ms. The protocol also checks if the absolute QTc > 500 ms.
The FDA-mandated REMS protocol for dofetilide requires immediate dose adjustment if either threshold (>15% relative increase OR >500 ms absolute value) is breached.
3
Compare the post-dose QTc to the calculated thresholds and determine the appropriate protocol-driven intervention.
The post-dose QTc of 488 ms is greater than the 483 ms relative threshold. Because this occurred after the first dose, the protocol dictates reducing the subsequent doses by 50% (to 250 mcg twice daily), rather than permanently discontinuing.
Adhering to strict dose-reduction algorithms prevents fatal arrhythmias while allowing the patient the opportunity to safely remain on a therapeutic antiarrhythmic at a lower dose.

Key Concept

Dofetilide monitoring protocol requires a 50% dose reduction if the QTc increases by >15% from baseline or exceeds 500 ms 2-3 hours after the first dose.
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Topics

Monitoring and Treatment Evaluation

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