What is the role of beta blockers in the management of unstable angina?

Role of Beta Blockers in Unstable Angina

Beta blockers should be administered early in the management of unstable angina in the absence of contraindications to reduce myocardial oxygen demand and prevent progression to myocardial infarction. 1

Mechanism of Action and Benefits

Beta blockers provide several important benefits in unstable angina:

• Competitively block catecholamine effects on beta receptors in the myocardium
• Reduce myocardial contractility, heart rate, and AV node conduction velocity
• Decrease systolic blood pressure and myocardial oxygen demand
• Increase diastolic filling time, improving coronary perfusion
• Blunt heart rate and contractility responses to pain and exertion 1

These effects collectively reduce myocardial oxygen consumption while potentially improving oxygen supply, addressing the fundamental imbalance in unstable angina.

Timing and Administration

• Oral administration: Should be started early (within first 24 hours) in stable patients without contraindications
• Intravenous administration: May be warranted for patients with ongoing rest pain, especially with tachycardia or hypertension 1
• Caution: Greater caution is now suggested with early IV beta blockers, which should be targeted to specific indications and avoided with heart failure, hypotension, and hemodynamic instability 1

Dosing Recommendations

For intravenous administration in appropriate patients:

• Metoprolol: 5 mg increments by slow IV administration (over 1-2 min), repeated every 5 min for total initial dose of 15 mg
• If tolerated, oral therapy can begin 15 min after last IV dose at 25-50 mg every 6 hours for 48 hours
• Maintenance dose: up to 100 mg twice daily 1

For oral administration:

• Metoprolol: 50-200 mg twice daily
• Atenolol: 50-200 mg per day
• Propranolol: 20-80 mg twice daily 1

Contraindications and Precautions

Beta blockers should be avoided in patients with:

• Marked first-degree AV block (PR interval >0.24 sec)
• Second or third-degree AV block without functioning pacemaker
• History of asthma or severe bronchospastic disease
• Severe LV dysfunction or heart failure (rales or S3 gallop)
• High risk for cardiogenic shock
• Hypotension (systolic BP <90 mmHg)
• Significant sinus bradycardia (heart rate <50 bpm)
• Evidence of low-output state (e.g., oliguria) 1, 2

Agent Selection

• Beta blockers without intrinsic sympathomimetic activity are preferred
• Cardioselective agents (metoprolol, atenolol) are preferred in patients with reactive airway disease
• For patients with concerns about intolerance, short-acting cardioselective agents (metoprolol, esmolol) are recommended 1
• For patients with mild wheezing or history of COPD, use reduced doses of cardioselective agents (e.g., 12.5 mg metoprolol) rather than completely avoiding beta blockers 1

Monitoring

During beta blocker therapy, especially with IV administration:

• Frequent checks of heart rate and blood pressure
• Continuous ECG monitoring
• Auscultation for rales and bronchospasm 1

Evidence of Benefit

Beta blockers have demonstrated several important benefits in unstable angina:

• Reduction in progression to myocardial infarction by approximately 13% 1
• Lower short-term mortality in patients undergoing PCI for unstable angina (0.6% vs 2.0% at 30 days; 1.7% vs 3.7% at 6 months) 1

Important Cautions

• Never abruptly discontinue beta blockers in patients with coronary artery disease due to risk of severe angina exacerbation, myocardial infarction, and ventricular arrhythmias 2
• When discontinuation is necessary, gradually reduce dosage over 1-2 weeks with careful monitoring 2
• Monitor for masking of hypoglycemia symptoms in diabetic patients 2
• Use with caution in patients with bronchospastic disease - if necessary, use cardioselective agents at lowest effective dose 2

Beta blockers remain a cornerstone of unstable angina management when used appropriately, with careful attention to contraindications and proper monitoring.