Drugs that block the transmission of certain nerve impulses and are used to treat disorders of the heart and circulation
Beta-blocker drugs, or beta-adrenergic blocking agents, are widely prescribed to treat disorders of the heart and circulation and also to treat other conditions. Cardioselective beta-blockers act mainly on the heart. Other beta-blockers act on blood vessels throughout the body and on other target tissues such as the cells that secrete fluid in the eye.
Why are they used?
Beta-blockers are prescribed principally to treat heart and circulatory disorders such as angina, a disabling chest pain caused by too little oxygenated blood reaching the heart muscle, some irregularities of heart rhythm (see Arrhythmias), and high blood pressure (see Hypertension). They are also sometimes given after a heart attack (see Myocardial infarction) to reduce the likelihood of further damage to the heart muscle.
Beta-blockers are also used to treat other conditions, such as migraine and glaucoma, in which the pressure in the eye is increased by a build-up of excess fluid. Some beta-blockers are used to reduce the physical symptoms of anxiety, such as tremor. The drugs also help in the treatment of an overactive thyroid gland (see Hyperthyrodism) by controlling the symptoms caused by excess production of thyroid hormone, such as sweating, tremor, and rapid heart rate.
How do they work?
Beta-blockers work by blocking the action of epinephrine (adrenaline) and norepinephrine (noradrenaline), two chemicals produced by the body that increase heart rate and raise the blood pressure. Cardioselective beta-blockers are especially effective at slowing heart rate and reducing the force of the heartbeat, thereby reducing the workload of the heart. These actions make beta-blockers effective in the treatment of angina, hypertension, and some types of arrhythmia. Other beta-blockers have a more widespread action and block many of the effects that norepinephrine has throughout the body, such as narrowing blood vessels and causing tremor and sweating. As a result, they may be effective in treating some physical symptoms caused by this chemical, such as tremor. Beta-blockers do not treat the underlying causes of these conditions.
Beta-blockers are used for migraine because they block the effects of norepinephrine on blood vessels in the head. They also reduce fluid formation, and thus pressure, in the eyeball, and so are used to treat glaucoma.
How are they used?
If you have angina, high blood pressure, or recurrent migraine, you may be given long-term treatment with oral beta-blockers. Your doctor may initially prescribe a low dose and increase it until the desired effect is reached. If you have glaucoma, you may be prescribed eyedrops that contain the beta-blocker timolol. A beta-blocker may be given intravenously in hospital if you experience sudden severe angina or if you have a heart attack.
Beta-blockers are also used along with other types of drug, especially in the treatment of angina, high blood pressure, and some types of heart failure.
What are the side effects?
Beta-blockers may precipitate asthma, which can be life-threatening. They should therefore not be used by anybody who has, or has ever had, asthma or similar breathing problems, except in exceptional circumstances and under specialist supervision, when a cardioselective beta-blocker may be given.
Beta-blockers can also mask some of the symptoms of hypoglycaemia (low blood glucose), which is a serious condition that must be recognized and treated immediately, and the drugs may therefore not be suitable for some people who need to take insulin for diabetes mellitus.
If you are taking a beta-blocker, your sleeping pattern may be disrupted, and your hands and feet may feel cold because the blood circulation in the extremities is reduced. Men sometimes experience erectile dysfunction, but normal sexual function usually returns when the drug is stopped. Rarely, beta-blockers cause rashes and dry eyes. All these side effects tend to be more common and more severe in elderly people.
If you need to stop taking a beta-blocker, your doctor will advise you on how to reduce the dose gradually to avoid rebound high blood pressure, a recurrence of angina, or a heart attack.
Do not suddenly stop taking a beta-blocker drug without consulting your doctor. Abrupt withdrawal of the drug could cause a rise in blood pressure, worsening of angina, or an increased risk of a heart attack.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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