Beta blocker

  Beta blockers (sometimes written as β-blockers) are a class of drugs used for various indications, but particularly for the management of cardiac arrhythmias and cardioprotection after myocardial infarction. Whilst once first-line treatment for hypertension, their role was downgraded in June 2006 in the United Kingdom to fourth-line as they do not perform as well as other drugs, particularly in the elderly, and there is increasing evidence that the most frequently used beta-blockers especially in combination with thiazide-type diuretics carry an unacceptable risk of provoking type 2 diabetes.^[1]

Propranolol was the first clinically useful beta adrenergic receptor antagonist. Invented by Sir James W. Black, it revolutionized the medical management of angina pectoris and is considered to be one of the most important contributions to clinical medicine and pharmacology of the 20th century.^[2]

Beta blockers may also be referred to as beta-adrenergic blocking agents, beta-adrenergic antagonists, or beta antagonists.

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Pharmacology

Beta blockers block the action of endogenous catecholamines (epinephrine (adrenaline) and norepinephrine (noradrenaline) in particular), on β-adrenergic receptors, part of the sympathetic nervous system which mediates the "fight or flight" response.

There are three known types of beta receptor, designated β₁, β₂ and β₃. β₁-Adrenergic receptors are located mainly in the heart and in the kidneys. β₂-Adrenergic receptors are located mainly in the lungs, gastrointestinal tract, liver, uterus, vascular smooth muscle, and skeletal muscle. β₃-receptors are located in fat cells.

β-Receptor antagonism

Stimulation of β₁ receptors by epinephrine induces a positive chronotropic and inotropic effect on the heart and increases cardiac conduction velocity and automaticity. Stimulation of β₁ receptors on the kidney causes renin release. Stimulation of β₂ receptors induces smooth muscle relaxation (resulting in vasodilation and bronchodilation amongst other actions), induces tremor in skeletal muscle, and increases glycogenolysis in the liver and skeletal muscle. Stimulation of β₃ receptors induces lipolysis.

Beta blockers inhibit these normal epinephrine-mediated sympathetic actions, but have minimal effect on resting subjects. That is, they reduce the effect of excitement/physical exertion on heart rate and force of contraction, dilation of blood vessels and opening of bronchi, and also reduce tremor and breakdown of glycogen.

It is therefore expected that non-selective beta blockers have an antihypertensive effect. The antihypertensive mechanism appears to involve: reduction in cardiac output (due to negative chronotropic and inotropic effects), reduction in renin release from the kidneys, and a central nervous system effect to reduce sympathetic activity.

Antianginal effects result from negative chronotropic and inotropic effects, which decrease cardiac workload and oxygen demand.

The antiarrhythmic effects of beta blockers arise from sympathetic nervous system blockade – resulting in depression of sinus node function and atrioventricular node conduction, and prolonged atrial refractory periods. Sotalol, in particular, has additional antiarrhythmic properties and prolongs action potential duration through potassium channel blockade.

Blockade of the sympathetic nervous system on renin release leads to reduced aldosterone via the renin angiotensin aldosterone system with a resultant decrease in blood pressure due to decreased sodium and water retention.

Intrinsic sympathomimetic activity

Some beta blockers (e.g. oxprenolol and pindolol) exhibit intrinsic sympathomimetic activity (ISA). These agents are capable of exerting low level agonist activity at the β-adrenergic receptor while simultaneously acting as a receptor site antagonist. These agents, therefore, may be useful in individuals exhibiting excessive bradycardia with sustained beta blocker therapy.

Agents with ISA are not used in post-myocardial infarction as they have not been demonstrated to be beneficial. They may also be less effective than other beta blockers in the management of angina and tachyarrhythmia.^[3]

α₁-Receptor antagonism

Some beta blockers (e.g. labetalol and carvedilol) exhibit mixed antagonism of both β- and α₁-adrenergic receptors, which provides additional arteriolar vasodilating action.

Other effects

Beta blockers decrease nocturnal melatonin release, perhaps partly accounting for sleep disturbance caused by some agents.^[4] Beta blockers protect against social anxiety: "Improvement of physical symptoms has been demonstrated with beta-blockers such as propranolol; however, these effects are limited to the social anxiety experienced in performance situations." ^[5] Beta blockers can impair the relaxation of bronchial muscle (mediated by beta-2) and so should be avoided by asthmatics.

Clinical use

Large differences exist in the pharmacology of agents within the class, thus not all beta blockers are used for all indications listed below.

Indications for beta blockers include:

• Hypertension
• Angina
• Mitral valve prolapse
• Cardiac arrhythmia
• Congestive heart failure
• Myocardial infarction
• Glaucoma
• Migraine prophylaxis
• Symptomatic control (tachycardia, tremor) in anxiety and hyperthyroidism
• Essential tremor
• Phaeochromocytoma, in conjunction with α-blocker

Beta blockers have also been used in the following conditions:

• Hypertrophic obstructive cardiomyopathy
• Acute dissecting aortic aneurysm
• Marfan syndrome (chronic treatment with propranolol slows progression of aortic dilation and its complications)
• Prevention of variceal bleeding in portal hypertension
• Possible mitigation of hyperhidrosis

Congestive heart failure

Although beta blockers were once contraindicated in congestive heart failure, as they have the potential to worsen the condition, studies in the late 1990s showed their positive effects on morbidity and mortality in congestive heart failure.^{[6] [7] [8]} Bisoprolol, carvedilol and sustained-release metoprolol are specifically indicated as adjuncts to standard ACE inhibitor and diuretic therapy in congestive heart failure.

The beta blockers are a benefit due to the reduction of the heart rate which will lower the myocardial energy expenditure. This is turns prolongs the diastolic filling and lengthens coronary perfusion.^[9] Beta blockers have also been a benefit to improving the ejection fraction of the heart despite an initial reduction in it.

Trials have shown that Beta blockers reduce the absolute risk of death by 4.5% over a 13 month period. As well as reducing the risk of mortality, the number of hospital visits and hospitalizations were also reduced in the trials.^[9]

Anxiety and performance enhancement

Some people, particularly musicians, use beta blockers to avoid stage fright and tremor during public performance and auditions. The physiological symptoms of the fight/flight response associated with performance anxiety and panic (pounding heart, cold/clammy hands, increased respiration, sweating, etc.) are significantly reduced, thus enabling anxious individuals to concentrate on the task at hand. Officially, beta blockers are not approved for anxiolytic use by the U.S. Food and Drug Administration. ^[10]

Since they lower heart rate and reduce tremor, beta blockers have been used by some Olympic marksmen to enhance performance, though beta blockers are banned by the International Olympic Committee (IOC).^[11] Although they have no recognisable benefit to most sports, it is acknowledged that they are beneficial to sports such as archery and shooting.

Adverse effects

Adverse drug reactions (ADRs) associated with the use of beta blockers include: nausea, diarrhea, bronchospasm, dyspnea, cold extremities, exacerbation of Raynaud's syndrome, bradycardia, hypotension, heart failure, heart block, fatigue, dizziness, abnormal vision, decreased concentration, hallucinations, insomnia, nightmares, clinical depression, sexual dysfunction, erectile dysfunction and/or alteration of glucose and lipid metabolism. Mixed α₁/β-antagonist therapy is also commonly associated with orthostatic hypotension. Carvedilol therapy is commonly associated with edema.^[3]

Central nervous system (CNS) adverse effects (hallucinations, insomnia, nightmares, depression) are more common in agents with greater lipid solubility, which are able to cross the blood-brain barrier into the CNS. Similarly, CNS adverse effects are less common in agents with greater aqueous solubility (listed below).

Adverse effects associated with β₂-adrenergic receptor antagonist activity (bronchospasm, peripheral vasoconstriction, alteration of glucose and lipid metabolism) are less common with β₁-selective (often termed "cardioselective") agents, however receptor selectivity diminishes at higher doses. Beta blockade, especially of the beta-1 receptor at the macula densa inhibits renin release, thus decreasing the release of aldosterone. This causes hyponatremia and hyperkalemia.

A 2007 study revealed that diuretics and beta-blockers used for hypertension increase a patient's risk of developing diabetes whilst ACE inhibitors and Angiotensin II receptor antagonists (Angiotensin Receptor Blockers) actually decrease the risk of diabetes.^[12] Clinical guidelines in Great Britain, but not in the United States, call for avoiding diuretics and beta-blockers as first-line treatment of hypertension due to the risk of diabetes.^[13]

Beta blockers must not be used in the treatment of cocaine, amphetamine, or other alpha adrenergic stimulant overdose. The blockade of only beta receptors increases hypertension, reduces coronary blood flow, left ventricular function, and cardiac output and tissue perfusion by means of leaving the alpha adrenergic system stimulation unopposed. ^[14] The appropriate antihypertensive drugs to administer during hypertensive crisis resulting from stimulant abuse are vasodilators like nitroglycerin, diuretics like furosemide and alpha blockers like phentolamine. ^[15]

Examples of beta blockers

Non-selective agents

• Alprenolol
• Carteolol
• Levobunolol
• Mepindolol
• Metipranolol
• Nadolol
• Oxprenolol
• Penbutolol
• Pindolol
• Propranolol
• Sotalol
• Timolol

β₁-Selective agents

• Acebutolol
• Atenolol
• Betaxolol
• Bisoprolol^[16]
• Esmolol
• Metoprolol
• Nebivolol

Mixed α₁/β-adrenergic antagonists

• Carvedilol
• Celiprolol
• Labetalol

β₂-Selective agents

• Butaxamine (weak α-adrenergic agonist activity)

Side Effects / Health Consequences

• Low Blood Pressure
• Slow Heart Rate
• Impaired Circulation
• Loss of Sleep
• Heart Failure
• Asthma
• Depression
• Sexual Dysfunction
• Nausea
• Headaches
• Dizziness
• Muscle Cramps

Comparative information

Pharmacological differences

• Agents with intrinsic sympathomimetic action (ISA)
  • Acebutolol, carteolol, celiprolol, mepindolol, oxprenolol, pindolol
• Agents with greater aqueous solubility
  • Atenolol, celiprolol, nadolol, sotalol
• Agents with membrane stabilising activity
  • Acebutolol, betaxolol, pindolol, propranolol
• Agents with antioxidant effect
  • Carvedilol
  • Nebivolol

Indication differences

• Agents specifically indicated for cardiac arrhythmia
  • Esmolol, sotalol
• Agents specifically indicated for congestive heart failure
  • Bisoprolol, carvedilol, sustained-release metoprolol, nebivolol
• Agents specifically indicated for glaucoma
  • Betaxolol, carteolol, levobunolol, metipranolol, timolol
• Agents specifically indicated for myocardial infarction
  • Atenolol, metoprolol, propranolol
• Agents specifically indicated for migraine prophylaxis
  • Timolol, propranolol

Propranolol is the only agent indicated for control of tremor, portal hypertension and esophageal variceal bleeding, and used in conjunction with α-blocker therapy in phaeochromocytoma.^[3]

References

• Katzung, Bertram G. (2004). Basic & clinical pharmacology, 9th edition, New York: Lange Medical Books/McGraw-Hill, 130, table 9-3. ISBN 0-07-144097-6.