About This Topic

The Dofetilide Initiation Dose Calculator is designed to assist healthcare professionals in determining the appropriate starting dose of dofetilide for patients with atrial fibrillation or atrial flutter. Dosing is critically dependent on two factors: the patient's renal function, estimated by creatinine clearance (CrCl), and their baseline corrected QT interval (QTc). Accurate initial dosing is essential to balance efficacy with the risk of proarrhythmia, particularly Torsades de Pointes.

Outputs Explained

The tool provides one of two primary outputs:

• Recommended Starting Dose: If the patient's CrCl and QTc are within acceptable limits, the calculator will display the specific initial dose (e.g., 500 mcg, 250 mcg, or 125 mcg) to be administered twice daily.
• Contraindication Alert: If the patient's calculated CrCl is below 20 mL/min or their baseline QTc exceeds 440 msec, the tool will indicate that dofetilide is contraindicated, as the risk of adverse events is unacceptably high.

How to Use This Information

To use the underlying logic of this calculation for clinical decision-making, follow these steps:

1. Determine Creatinine Clearance (CrCl): Either calculate CrCl using a standard formula like Cockcroft-Gault (requiring age, weight, sex, and serum creatinine) or use a directly measured value.
2. Measure Baseline QTc: Obtain a baseline 12-lead ECG to determine the corrected QT interval. If a significant intraventricular conduction delay (e.g., bundle branch block) is present, use the absolute QT interval instead of the QTc.
3. Apply Dosing Algorithm: Use the determined CrCl and QTc values to select the appropriate starting dose according to the approved prescribing information.

Dosing Overview

The initial dosing of dofetilide is based on the following algorithm, as specified in the FDA-approved label:

Dofetilide is also contraindicated in patients with a baseline QTc or QT interval >440 msec. Dose adjustments are made 2-3 hours after each subsequent dose based on QTc monitoring during the mandatory in-hospital initiation period.

Switching to Dofetilide

When switching from other antiarrhythmic drugs to dofetilide, a sufficient washout period is necessary to minimize the risk of additive pharmacodynamic effects, such as QTc prolongation. For amiodarone, a minimum of three months is required before initiating dofetilide, or until plasma levels are below 0.3 mcg/mL. For other Class I or Class III agents, a washout period of at least three plasma half-lives is recommended before starting dofetilide.

Missed Dose

If a patient misses a dose of dofetilide, they should be instructed to take the next dose at its regularly scheduled time. They must not double the dose to make up for the missed one, as this increases the risk of proarrhythmia.

Safety Alerts

Frequently Asked Questions (FAQ)

Why is creatinine clearance (CrCl) essential for dofetilide dosing?

Dofetilide is primarily eliminated by the kidneys. Impaired renal function leads to drug accumulation, increasing the risk of QTc prolongation and Torsades de Pointes. Therefore, the dose must be adjusted based on CrCl.

What happens if the baseline QTc is above 440 msec?

A baseline QTc interval greater than 440 msec (or 500 msec in patients with ventricular conduction abnormalities) is an absolute contraindication for dofetilide therapy due to the high risk of life-threatening arrhythmias.

Can this calculator be used for dose adjustments after initiation?

No, this tool is only for determining the *initial* starting dose. Subsequent dose adjustments during the first three days of therapy are based on QTc measurements taken 2-3 hours after each dose.

Why must dofetilide be started in a hospital?

The risk of proarrhythmia is highest during the initial dosing period. In-hospital initiation allows for continuous ECG monitoring for at least 72 hours to detect excessive QTc prolongation and manage any potential arrhythmias promptly.

What is the difference between the Cockcroft-Gault formula and directly entering CrCl?

The Cockcroft-Gault formula is an equation used to *estimate* creatinine clearance from serum creatinine, age, weight, and sex. Directly entering a CrCl value is appropriate when it has been determined by other methods, such as a 24-hour urine collection.

What if a patient's renal function changes during therapy?

Renal function should be monitored regularly in patients on dofetilide. If a significant decline in CrCl occurs, the dofetilide dose may need to be reduced or discontinued according to the established dosing algorithm.

Are there any drugs that cannot be taken with dofetilide?

Yes, several drugs are contraindicated with dofetilide because they inhibit its metabolism or clearance, increasing arrhythmia risk. These include verapamil, ketoconazole, cimetidine, trimethoprim, prochlorperazine, megestrol, and hydrochlorothiazide.

Is dofetilide safe in patients with heart failure?

Dofetilide has been shown to be safe in patients with congestive heart failure (CHF) and reduced left ventricular ejection fraction. Unlike many other antiarrhythmics, it has a neutral effect on mortality in this population.

References

• TIKOSYN® (dofetilide) Prescribing Information. U.S. Food and Drug Administration. Pfizer Inc. Revised: 01/2022.
• Pfizer Medical Information - Tikosyn (dofetilide). Pfizer Inc.
• Tikosyn (dofetilide) REMS Program. U.S. Food and Drug Administration.
• January CT, Wann LS, Alpert JS, et al. 2014 AHA/ACC/HRS Guideline for the Management of Patients With Atrial Fibrillation. J Am Coll Cardiol. 2014;64(21):e1-e76.