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Beta-adrenoceptor blocking agents

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Beta-adrenoceptor blocking agents, or beta-blockers as they are more commonly called, are used predominantly in the treatment of such conditions as hypertension, angina and cardiac arrhythmias. They work by preventing catecholamines from accessing the beta-adrenergic receptor sites in the heart, bronchi and other organs (Hopkins, 1999).

There are two types of beta receptor:

- Beta 1 receptors located mainly in the myocardium

- Beta 2 receptors located in the airways and blood vessels.

The action of beta-blockers has the effect of inhibiting and thereby reducing the response of the body to sympathetic stimulation.

The demand for oxygen made by the heart muscle when exposed to excessive stimulation, such as that created by the stress of exercise or emotional situations, is reduced by the use of beta-blockers. This action makes them an ideal choice for the treatment of such conditions as angina. By blocking the action of the sympathetic nervous system they are also particularly useful in the treatment of chronic hypertension.

Blood pressure

Three main factors, cardiac output, peripheral resistance and total blood volume, affect blood pressure. A change in any of these three factors will result in a change in the blood pressure. Blood pressure is regulated by the autonomous nervous system.

The amount of tension in the vessel walls is measured by the baro-receptors in the aortic arch and the carotid vessels. This measurement of tension is relayed to the cardiac and vasomotor centres of the medulla.

If the measurement of tension indicates that the blood pressure is low, noradrenaline is released by the sympathetic nerves, stimulating the alpha-receptors, which in turn leads to vasoconstriction of the vessels. Simultaneously, the beta-receptors in the heart activate causing an increase in heart rate and an increase in the contractile forces of the myocardium. These actions combine to result in an increase in blood pressure. If the blood pressure level rises the activity of the parasympathetic nerves increases as that of the sympathetic nerves decreases, leading to a reduction in tension in the vessels, a slower heart rate and thus a drop in blood pressure.

Hypertension occurs when the blood pressure is persistently raised and can be both symptomatic and asymptomatic. Persistent hypertension can cause cardiovascular disease, myocardial infarction and stroke. Hypertension is more commonly seen in elderly patients because, with age, there is an increase in peripheral resistance within the blood vessels, along with a decrease in their elasticity. Reducing either the peripheral resistance or the cardiac output can treat this hypertension. Beta-blockers work by reducing the force and rate of the heart beat, thus reducing hypertension.

Angina

Angina pectoris occurs when there is insufficient oxygenated blood reaching the myocardium. This situation can be caused by a narrowing in the blood vessels reducing the flow of blood to the myocardium or by the response to an increase in cardiac output caused by the previously mentioned stress factors. Once again treatment with beta-blockers is aimed at reducing the force of the output.

Groups of beta-blockers

Beta-blockers are generally divided into two key groups.

- Group I. These act on both the beta 1 and beta 2 receptors and are non-cardioselective

- Group II. These are cardioselective and act on the beta 1 receptors.

Group I beta-blockers

- Propanolol

- Solatol

- Oxprenolol

- Timolol

- Nadolol.

Group II beta-blockers

- Atenolol

- Metoprolol

- Bisoprolol

- Celiprolol.

Potential side-effects of beta-blockers

- Sleep disturbances (more typically with lipid-soluble drugs)

- Bronchoconstriction (more common with Group I)

- Bradycardia

- Reduction in peripheral blood flow

- Gastrointestinal disturbances

- Postural hypotension - usually on awakening.

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