A beta blocker is any of a class of drugs that block impulses to certain nerve receptors (beta receptors) in various tissues throughout the body, including the heart, airways, and peripheral arteries. Beta blockers, also known as beta-adrenergic blocking agents, are generally prescribed to regulate the heart rate, reduce blood pressure, relieve angina, and improve survival following a heart attack. However, they are also being used in an increasingly wide range of other conditions, including glaucoma, liver disease, thyrotoxicosis (overactive thyroid gland), migraine, and anxiety states. Beta blockers are not suitable for all patients and are not used for patients with asthma or severe lung disease.
How beta blockers work
Beta blockers block beta-receptors – specific sites on body tissues where neurotransmitters (chemicals released from nerve endings) bind. There are two types of beta-receptor: beta1-receptors found in heart tissue and beta2-receptors found in the lungs, blood vessels, and other tissues. At these receptors, two chemicals, adrenaline and noradrenaline, are released from nerve endings in the sympathetic nervous system, the part of the involuntary nervous system that enables the body to deal with stress, anxiety, and exercise. These neurotransmitters bind to beta-receptors to increase the force and speed of the heartbeat, to dilate the airways to increase air flow to the lungs, and to dilate blood vessels.
Cardioselective beta blockers combine predominantly with beta1-receptors; noncardioselective beta blockers combine with both types. Beta blockers slow heart rate and reduce the force of contraction of heart muscle. These effects can be used to slow a fast heart rate and regulate abnormal rhythms.
Beta blockers prevent angina attacks by reducing the work performed by the heart muscle and so the heart's oxygen requirement. High blood pressure is reduced because the rate and force at which the heart pumps blood into the circulation system is lowered.
The effect of blocking beta-receptors on muscles elsewhere in the body is to reduce the muscle tremor of anxiety and an overactive thyroid gland. Beta blockers can help to reduce the frequency of migraine attacks by preventing the dilation of blood vessel surrounding the brain, which is responsible for the headache. In glaucoma they lower pressure in the eye by reducing fluid production in the eyeball.
Possible adverse effects
By reducing heart rate and air flow to the lungs, beta blockers may reduce an individual's capacity for strenuous exercise, although this may not be noticed if physical activity is already limited by heart problems.
Beta blockers may worsen the symptoms of asthma, bronchitis, or other forms of lung disease. They may also reduce the flow of blood to the limbs, and thus aggravate peripheral vascular disease.
If long-term treatment with beta blockers is abruptly withdrawn, there may a sudden severe reoccurrence of the patient's symptoms and a significant rise in blood pressure. These problems can be avoided by gradually decreasing the dose.