CHAPTER 9 ยท ANTIANGINAL DRUGS

Introduction to Medical Pharmacology

Antianginal Drugs

Beta-Blockers in Angina

Module 3 of 7
Section 1
Receptor Pharmacology
Beta-blockers are first-line antianginal therapy, and their selectivity for different adrenergic receptors determines both their benefits and their hazards.

Beta-adrenergic receptor blockers remain first-line pharmacotherapy for stable exertional angina. Their anti-ischemic mechanism follows directly from which adrenergic receptors they block and where those receptors are located in the body.

Beta-1, Beta-2, and Alpha-1 Receptors
Cardiac-Predominant
Beta-1 Receptors
  • Sinoatrial and atrioventricular nodes, ventricular muscle
  • Increased heart rate and contractility
  • Blockade is the primary anti-ischemic mechanism
Peripheral and Pulmonary
Beta-2 Receptors
  • Bronchial and vascular smooth muscle
  • Bronchodilation, peripheral and coronary vasodilation
  • Blockade causes bronchoconstriction
Vascular Smooth Muscle
Alpha-1 Receptors
  • Vasoconstriction
  • Not blocked by most beta-blockers
  • Becomes unopposed when beta-2 is blocked
Cardioselective vs. Non-Selective Agents

Cardioselective beta-blockers, such as metoprolol and bisoprolol, preferentially block beta-1 receptors over beta-2 receptors at standard doses, producing less bronchoconstriction and less interference with peripheral vasodilation. Non-selective agents, such as propranolol, block beta-1 and beta-2 receptors equally. Selectivity is relative rather than absolute: at high enough doses, cardioselective agents lose their selectivity and begin to block beta-2 receptors as well.

High-Yield Contraindication, Revisited

This is the same mechanism introduced in Module 1: blocking beta-2 receptors removes a normal coronary vasodilatory influence, leaving alpha-1-mediated vasoconstriction unopposed. In vasospastic angina this can precipitate or worsen spasm, and the effect applies to all beta-blockers, including cardioselective agents at typical doses.

A three-panel diagram comparing beta-1 receptors (heart rate and contractility), beta-2 receptors (bronchodilation and vasodilation), and alpha-1 receptors (vasoconstriction), showing their locations and effects.
Beta-1, beta-2, and alpha-1 receptors: locations and effects.
Section 2
Hemodynamic Mechanisms in Angina
Beta-blockers act on two of the four pharmacological levers introduced in Module 1: heart rate and, secondarily, contractility.

Beta-blockers are uniquely effective antianginal agents because beta-1 blockade simultaneously reduces oxygen demand and improves oxygen supply, addressing both sides of the supply-demand equation introduced in Module 1.

Heart Rate Reduction: The Primary Effect

Beta-1 blockade in the sinoatrial node slows the heart's intrinsic pacemaker activity, lowering both resting and exertional heart rate. Heart rate is the single largest determinant of myocardial oxygen demand, so slowing it lowers demand at rest and prevents the sympathetically driven heart rate surge that would otherwise push a patient above their ischemic threshold during exertion.

Slowing the heart rate also improves oxygen supply. Coronary blood flow to the subendocardium occurs predominantly during diastole, since systolic compression of the coronary microvasculature limits flow during contraction. A slower heart rate lengthens the diastolic interval per cycle, increasing the time available for coronary filling.

Contractility Reduction

Beta-1 blockade in the ventricular myocardium also reduces the force of each contraction. Lower contractility means less oxygen consumed per cardiac cycle, contributing a secondary reduction in myocardial oxygen demand alongside the dominant heart rate effect.

A flow diagram showing how beta-1 blockade reduces heart rate and contractility, lowering myocardial oxygen demand while increasing coronary filling time, together reducing myocardial ischemia.
How beta-1 blockade reduces myocardial ischemia.
Blunting Reflex Tachycardia from Other Agents

Both nitrates and dihydropyridine calcium channel blockers lower blood pressure, which triggers a baroreceptor-mediated reflex increase in heart rate that partially offsets their anti-ischemic benefit. Beta-blockers prevent this reflex from raising heart rate, which is the central rationale for combining a beta-blocker with either a nitrate or a dihydropyridine calcium channel blocker.

Section 3
Distinguishing Features Among Agents
Beyond cardioselectivity, a few additional properties separate individual beta-blockers from one another.

While all beta-blockers share the core mechanism described above, a handful of agents have additional pharmacological properties that distinguish them clinically.

Property Representative Agent Clinical Relevance
Added alpha-1 blockadeCarvedilolAdditional peripheral vasodilation and afterload reduction
Nitric oxide-mediated vasodilationNebivololLess peripheral vasoconstriction than typical beta-blockers
High lipid solubilityPropranolol, metoprololGreater central nervous system penetration; more fatigue and sleep disturbance

For routine stable angina, a cardioselective agent without these additional properties is typically sufficient. Carvedilol and nebivolol become more relevant when angina coexists with heart failure or when minimizing peripheral vasoconstriction is a priority.

Section 4
Contraindications
Most beta-blocker contraindications follow directly from the mechanisms already covered.
Unopposed Alpha-1 Vasoconstriction
Vasospastic Angina
  • Class effect, all beta-blockers
  • Can precipitate or worsen spasm
  • Use calcium channel blockers instead
Conduction System Already Impaired
High-Degree Atrioventricular Block
  • Without a pacemaker in place
  • Beta-blockade further depresses conduction
  • Risk of complete heart block
Acute Negative Inotropy Is Dangerous
Decompensated Heart Failure, Cardiogenic Shock
  • Acute initiation worsens hemodynamics
  • Wait until euvolemic and stable
  • Chronic, stable heart failure is different
Beta-2 Blockade Effect
Active Bronchospastic Disease
  • Severe asthma, active bronchospasm
  • Even cardioselective agents carry some risk
  • Relative contraindication
A reference table listing four contraindications to beta-blocker therapy: vasospastic angina, high-degree atrioventricular block, decompensated heart failure or cardiogenic shock, and active bronchospastic disease, with the mechanism explaining each.
Major contraindications to beta-blocker therapy.

Beta-blockers are not contraindicated in stable, well-compensated heart failure with reduced ejection fraction. In that setting, carefully initiated and slowly uptitrated beta-blockade is part of standard therapy and provides a survival benefit. The contraindication applies specifically to acute decompensation, not to chronic stable disease.

Section 5
Beta-Blocker Withdrawal Syndrome
Stopping a beta-blocker suddenly can be more dangerous than the disease it was treating.

Beta-blockers should never be discontinued abruptly in a patient with coronary artery disease or angina. Stopping suddenly can produce a withdrawal syndrome that is sometimes more severe than the original symptoms the drug was prescribed to control.

Why Withdrawal Happens

Chronic beta-receptor blockade causes the body to compensate by increasing the number and sensitivity of beta-adrenergic receptors on cardiac tissue, a process called receptor upregulation. If the beta-blocker is stopped abruptly, this enlarged, more sensitive receptor population is suddenly exposed to the patient's normal circulating catecholamines.

The result is an exaggerated sympathetic response: rebound tachycardia and hypertension, a sharp rise in myocardial oxygen demand, and an increased risk of rebound angina that can be more severe than the patient's baseline symptoms, along with an elevated risk of myocardial infarction and dangerous arrhythmias.

Clinical Principle

Beta-blockers are always tapered, never stopped abruptly, in patients with known coronary artery disease. A gradual dose reduction over one to two weeks allows the upregulated receptor population time to return to its normal baseline sensitivity before the protective blockade is fully withdrawn.

Suggested References
Author / Organization Title Source
Freemantle N, Cleland J, Young P, Mason J, Harrison J.Beta blockade after myocardial infarction: systematic review and meta regression analysisBMJ, 1999
MERIT-HF Study Group.Effect of metoprolol CR/XL in chronic heart failure (MERIT-HF)Lancet, 1999
CIBIS-II Investigators and Committees.The Cardiac Insufficiency Bisoprolol Study II (CIBIS-II)Lancet, 1999
Packer M, Coats AJ, Fowler MB, et al.Effect of carvedilol on survival in severe chronic heart failure (COPERNICUS)New England Journal of Medicine, 2001
Knuuti J, Wijns W, Saraste A, et al.2019 ESC Guidelines for the diagnosis and management of chronic coronary syndromesEuropean Heart Journal, 2020
Fihn SD, Gardin JM, Abrams J, et al.2012 ACCF/AHA Guideline for the diagnosis and management of patients with stable ischemic heart diseaseJournal of the American College of Cardiology, 2012
Bangalore S, Steg G, Deedwania P, et al.Beta-blocker use and clinical outcomes in stable outpatients with and without coronary artery diseaseJAMA, 2012
Salpeter SR, Ormiston TM, Salpeter EE.Cardioselective beta-blockers for reversible airway diseaseCochrane Database of Systematic Reviews, 2002
Lopez-Sendon J, Swedberg K, McMurray J, et al.Expert consensus document on beta-adrenergic receptor blockersEuropean Heart Journal, 2004
Poole-Wilson PA, Swedberg K, Cleland JGF, et al.Comparison of carvedilol and metoprolol on clinical outcomes in patients with chronic heart failure (COMET)Lancet, 2003
Dargie HJ.Effect of carvedilol on outcome after myocardial infarction in patients with left-ventricular dysfunction (CAPRICORN)Lancet, 2001
Flather MD, Shibata MC, Coats AJS, et al.Randomized trial to determine the effect of nebivolol on mortality and cardiovascular hospital admission in elderly patients with heart failure (SENIORS)European Heart Journal, 2005
Devereaux PJ, Yang H, Yusuf S, et al.Effects of extended-release metoprolol succinate in patients undergoing non-cardiac surgery (POISE trial)Lancet, 2008
Introduction to Medical Pharmacology · Chapter 9 · Module 3 of 7
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