What is the cause of bradycardia with a heart rate of 45 beats per minute while taking Atenolol (Metoprolol)?

From the Guidelines

Bradycardia with a heart rate of 45 beats per minute while taking atenolol is a medical emergency that requires immediate attention, and the first line of treatment should be atropine 0.5-1 mg IV, which may be repeated every 3-5 minutes to a maximum dose of 3 mg, as recommended by the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.

Causes and Mechanism

Atenolol, a beta-blocker, can cause bradycardia by blocking beta-1 receptors in the heart, reducing the influence of adrenaline and noradrenaline, thereby slowing heart rate and reducing the force of contractions. This is normally therapeutic for conditions like hypertension and angina, but excessive slowing below 50 beats per minute can compromise cardiac output and tissue perfusion, especially in elderly patients or those with underlying heart conditions.

Management and Treatment

The management of bradycardia attributable to sinus node dysfunction (SND) or atrioventricular block involves the use of medications such as:

• Atropine: 0.5-1 mg IV, which may be repeated every 3-5 minutes to a maximum dose of 3 mg 1
• Dopamine: 5 to 20 mcg/kg/min IV, starting at 5 mcg/kg/min and increasing by 5 mcg/kg/min every 2 min 1
• Isoproterenol: 20-60 mcg IV bolus followed by doses of 10-20 mcg, or infusion of 1-20 mcg/min based on heart rate response 1
• Epinephrine: 2-10 mcg/min IV or 0.1-0.5 mcg/kg/min IV titrated to desired effect 1

Important Considerations

It is crucial not to stop taking atenolol suddenly without medical supervision as this can cause rebound hypertension or worsen underlying heart conditions. While awaiting medical advice, it is essential to monitor for symptoms like dizziness, fatigue, confusion, or fainting, which may indicate inadequate blood flow to the brain.

Key Points to Remember

• Bradycardia with atenolol requires immediate medical attention
• Atropine is the first line of treatment for symptomatic sinus bradycardia or atrioventricular block
• Do not stop taking atenolol suddenly without medical supervision
• Monitor for symptoms of inadequate blood flow to the brain while awaiting medical advice

From the FDA Drug Label

Bradycardia 3 0 3 0 In a series of investigations in the treatment of acute myocardial infarction, bradycardia and hypotension occurred more commonly, as expected for any beta-blocker, in atenolol-treated patients than in control patients. Conventional Therapy Plus Atenolol (n = 244) Conventional Therapy Alone (n = 233) Bradycardia 43 (18%) 24 (10%) OVERDOSAGE ... The predominant symptoms reported following atenolol overdose are lethargy, disorder of respiratory drive, wheezing, sinus pause and bradycardia BRADYCARDIA: Atropine intravenously. If there is no response to vagal blockade, give isoproterenol cautiously.

Bradycardia is a known adverse effect of atenolol. The drug label reports that bradycardia occurred in 3% of patients in controlled studies, and in 18% of patients in a study of acute myocardial infarction.

• Treatment of bradycardia caused by atenolol overdose includes atropine intravenously, and isoproterenol if there is no response to vagal blockade 2 2. In the case of a patient with a heart rate of 45/min taking atenolol, it is likely that the bradycardia is caused by the medication.
• The patient should be closely monitored and atropine or isoproterenol should be administered if necessary, as indicated in the drug label 2.

From the Research

Bradycardia with Atenolol

• Atenolol is a beta-selective adrenoceptor blocking drug that can cause bradycardia as a side effect 3.
• In a study of patients with drug-induced bradycardia, three patients were taking only beta-blockers, including atenolol, and experienced heart rates ranging from 20-49 beats per minute 4.
• The study found that the clinical presentation of patients taking beta-blockers and sodium channel blockers was worse than those taking only beta-blockers, and included cardiogenic shock and heart failure 4.
• Atenolol has been shown to be effective in reducing heart rate and improving symptoms in patients with advanced left ventricular dysfunction 5.
• Beta-blockers, including atenolol, are commonly used to control ventricular rate in patients with atrial fibrillation and other cardiac arrhythmias 6.

Mechanism of Action

• Atenolol works by blocking beta-adrenergic receptors, which reduces the influence of the sympathetic nervous system on the heart 3.
• This results in a negative inotropic, chronotropic, bathmotropic, and dromotropic effect, which can lead to bradycardia 6.

Clinical Implications

• Patients taking atenolol should be monitored for signs of bradycardia, including dizziness, fatigue, and shortness of breath 4.
• The dose of atenolol may need to be adjusted in patients with renal impairment or other underlying medical conditions 3.
• Beta-blockers, including atenolol, are an important treatment option for patients with cardiac arrhythmias, but should be used with caution and under close medical supervision 6.