From Wikipedia, the free encyclopedia - View original article
|Use||Hypertension, arrhythmia, etc.|
|Biological target||beta receptors|
|Use||Hypertension, arrhythmia, etc.|
|Biological target||beta receptors|
Beta blockers (β-blockers, beta-adrenergic blocking agents, beta antagonists, beta-adrenergic antagonists, beta-adrenoreceptor antagonists, or beta adrenergic receptor antagonists) are a class of drugs that target the beta receptor. Beta receptors are found on cells of the heart muscles, smooth muscles, airways, arteries, kidneys, and other tissues that are part of the sympathetic nervous system and lead to stress responses, especially when they are stimulated by epinephrine (adrenaline). Beta blockers interfere with the binding to the receptor of epinephrine and other stress hormones, and weaken the effects of stress hormones.
They are particularly used for the management of cardiac arrhythmias, protecting the heart from a second heart attack (myocardial infarction) after a first heart attack (secondary prevention), and hypertension.
In 1962, Sir James W. Black  found the first clinically significant beta blockers—propranolol and pronethalol; it revolutionized the medical management of angina pectoris and is considered by many to be one of the most important contributions to clinical medicine and pharmacology of the 20th century.
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. Three types of beta receptors are known, designated β1, β2 and β3 receptors. 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-adrenergic receptors are located in fat cells.
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:
Beta blockers have also been used for:
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 efficacy at reducing morbidity and mortality. Bisoprolol, carvedilol and sustained-release metoprolol are specifically indicated as adjuncts to standard ACE inhibitor and diuretic therapy in congestive heart failure.
Beta blockers are known primarily for their reductive effect on heart rate, although this is not the only mechanism of action of importance in congestive heart failure. Beta blockers, in addition to their sympatholytic B1 activity in the heart, influence the renin–angiotensin system at the kidneys. Beta blockers cause a decrease in renin secretion, which in turn reduces the heart oxygen demand by lowering extracellular volume and increasing the oxygen-carrying capacity of blood. Beta blockers' sympatholytic activities reduce heart rate, thereby increasing the ejection fraction of the heart despite an initial reduction in ejection fraction.
Trials have shown beta blockers reduce the absolute risk of death by 4.5% over a 13-month period. In addition to reducing the risk of mortality, the number of hospital visits and hospitalizations were also reduced in the trials.
Officially, beta blockers are not approved for anxiolytic use by the U.S. Food and Drug Administration. However, many controlled trials in the past 25 years indicate beta blockers are effective in anxiety disorders, though the mechanism of action is not known. The physiological symptoms of the fight-or-flight response (pounding heart, cold/clammy hands, increased respiration, sweating, etc.) are significantly reduced, thus enabling anxious individuals to concentrate on the task at hand.
Musicians, public speakers, actors, and professional dancers have been known to use beta blockers to avoid performance anxiety, stage fright and tremor during both auditions and public performances. The application to stage fright was first recognized in The Lancet in 1976, and by 1987, a survey conducted by the International Conference of Symphony Orchestra Musicians, representing the 51 largest orchestras in the United States, revealed 27% of its musicians had used beta blockers and 70% obtained them from friends, not physicians. Beta blockers are inexpensive, said to be relatively safe and, on one hand, seem to improve musicians' performances on a technical level, while some, like Barry Green, the author of "The Inner Game of Music," and Don Greene, a former Olympic diving coach who teaches Juilliard students to overcome their stage fright naturally, say the performances may be perceived as "soulless and inauthentic."
Since they promote lower heart rates and reduce tremors, beta blockers have been used in professional sports where high accuracy is required, including archery, shooting, golf and snooker. Beta blockers are banned by the International Olympic Committee. A recent, high-profile transgression took place in the 2008 Summer Olympics, where 50 metre pistol silver medallist and 10 metre air pistol bronze medallist Kim Jong-su tested positive for propranolol and was stripped of his medals.
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, alopecia (hair loss), abnormal vision, hallucinations, insomnia, nightmares, 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. Due to the high penetration across the blood–brain barrier, lipophilic beta blockers, such as propranolol and metoprolol, are more likely than other, less lipophilic, beta blockers to cause sleep disturbances, such as insomnia and vivid dreams and nightmares.
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.
Hypoglycemia can occur with beta blockade because β2-adrenoceptors normally stimulate hepatic glycogen breakdown (glycogenolysis) and pancreatic release of glucagon, which work together to increase plasma glucose. Therefore, blocking β2-adrenoceptors lowers plasma glucose. β1-blockers have fewer metabolic side effects in diabetic patients; however, the tachycardia that serves as a warning sign for insulin-induced hypoglycemia may be masked. Therefore, beta blockers are to be used cautiously in diabetics.
A 2007 study revealed diuretics and beta blockers used for hypertension increase a patient's risk of developing diabetes, while ACE inhibitors and angiotensin II receptor antagonists (angiotensin receptor blockers) actually decrease the risk of diabetes. 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.
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. The appropriate antihypertensive drugs to administer during hypertensive crisis resulting from stimulant abuse are vasodilators such as nitroglycerin, diuretics such as furosemide and alpha blockers such as phentolamine.
Beta blockers are contraindicated in patients with asthma as stated in the British National Formulary 2011. They should also be avoided in patients with a history of cocaine use or in cocaine-induced tachycardia.
Glucagon, used in the treatment of overdose, increases the strength of heart contractions, increases intracellular cAMP, and decreases renal vascular resistance. It is therefore useful in patients with beta-blocker cardiotoxicity. Cardiac pacing is usually reserved for patients unresponsive to pharmacological therapy.
Patients experiencing bronchospasm due to the β2 receptor-blocking effects of non-selective beta blockers may be treated with anticholinergic drugs, such as ipratropium, which are safer than beta agonists in patients with cardiovascular disease. Other antidotes for beta-blocker poisoning are salbutamol and isoprenaline.
Stimulation of β1 receptors by epinephrine and norepinephrine 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, 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 and norepinephrine-mediated sympathetic actions, but have minimal effect on resting subjects. That is, they reduce excitement/physical exertion on heart rate and force of contraction, and also tremor and breakdown of glycogen, but increase dilation of blood vessels and constriction of bronchi.
Therefore, nonselective beta blockers are expected to have antihypertensive effects. The primary antihypertensive mechanism of beta blockers is unclear, but may involve reduction in cardiac output (due to negative chronotropic and inotropic effects). It may also be due to reduction in renin release from the kidneys, and a central nervous system effect to reduce sympathetic activity (for those beta blockers that do cross the blood–brain barrier, e.g. propranolol).
Antianginal effects result from negative chronotropic and inotropic effects, which decrease cardiac workload and oxygen demand. Negative chronotropic properties of beta blockers allow the lifesaving property of heart rate control. Beta blockers are readily titrated to optimal rate control in many pathologic states.
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.
Also referred to as intrinsic sympathomimetic effect, this term is used particularly with beta blockers that can show both agonism and antagonism at a given beta receptor, depending on the concentration of the agent (beta blocker) and the concentration of the antagonized agent (usually an endogenous compound, such as norepinephrine). See partial agonist for a more general description.
Some beta blockers (e.g. oxprenolol, pindolol, penbutolol and acebutolol) 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 after myocardial infarctions, 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.
Also known as cardioselective