From the Guidelines
The patient should hold the metoprolol until their heart rate returns to an acceptable range, typically above 60 beats per minute, which may take approximately 24-48 hours given metoprolol's half-life of 3-7 hours 1. After the heart rate normalizes, they should resume the correct dose of 12.5 mg twice daily as prescribed for atrial fibrillation. During this time, the patient should monitor their heart rate regularly and contact their healthcare provider immediately if they experience dizziness, lightheadedness, confusion, or fainting, which could indicate poor perfusion due to bradycardia. The patient should also seek medical attention if their heart rate doesn't improve within 48 hours. Metoprolol causes bradycardia by blocking beta-1 receptors in the heart, reducing the influence of the sympathetic nervous system and thereby decreasing heart rate and contractility. The doubled dose (25 mg versus 12.5 mg) has resulted in excessive beta-blockade, causing the dangerous bradycardia of 40 beats per minute. Some key points to consider in this scenario include:
- The patient's heart rate is currently 40 beats per minute, which is considered bradycardia and may lead to poor perfusion and other complications.
- The patient was taking a doubled dose of metoprolol, which has a half-life of 3-7 hours 1.
- The correct dose of metoprolol for atrial fibrillation is typically 25-200 mg twice daily, with a target heart rate of 60-80 beats per minute 1.
- The patient should be closely monitored for signs of poor perfusion, such as dizziness, lightheadedness, confusion, or fainting, and should seek medical attention immediately if they experience any of these symptoms. It's also important to note that the 2023 ACC/AHA/ACCp/HRS guideline for the diagnosis and management of atrial fibrillation recommends titrating the dose of beta blockers to achieve a heart rate of 60-80 beats per minute, while minimizing the risk of hypotension and bradycardia 1. In this case, holding the metoprolol until the patient's heart rate returns to an acceptable range and then resuming the correct dose is the best course of action to minimize the risk of complications and ensure the patient's safety.
From the Research
Metoprolol Overdose and Heart Rate Reduction
- The patient has taken an overdose of metoprolol, 25 mg bid instead of 12.5 mg bid, for a fib, resulting in a heart rate of 40 after 3 days 2.
- Beta-blockers, such as metoprolol, are commonly used to control heart rate in patients with atrial fibrillation, but the optimal heart rate target is not well established 3, 4.
- A heart rate of 40 is considered too low, and the patient may be at risk of adverse effects such as dizziness, fatigue, and shortness of breath 2.
Recommended Course of Action
- The patient should stop taking metoprolol until their heart rate returns to a normal range, typically between 60-100 beats per minute 2.
- The duration of metoprolol withdrawal will depend on the patient's individual response and the severity of the overdose.
- It is recommended to monitor the patient's heart rate and blood pressure closely during the withdrawal period to prevent rebound effects, such as a sudden increase in heart rate and blood pressure 5.
Considerations for Restarting Metoprolol
- Once the patient's heart rate has returned to a normal range, metoprolol can be restarted at a lower dose, typically 12.5 mg bid, and gradually increased as needed to achieve optimal heart rate control 4, 6.
- The patient's heart rate and blood pressure should be closely monitored during the titration period to prevent adverse effects.
- Alternative rate-control strategies, such as pacing or pulmonary vein ablation, may be considered if the patient is unable to tolerate beta-blockers or if their heart rate remains uncontrolled despite optimal medical therapy 4.