Betabloqueadores

How do beta-blockers work?

Betabloqueadores work by blocking the transmission of certain nerve impulses. The ends of some nerves release a chemical (neurotransmitter) called noradrenaline when the nerve is stimulated. This chemical then stimulates beta-adrenergic receptors.

These receptors are tiny structures which occur on cells in various parts of the body, including the heart, brain, and blood vessels. When these receptors are stimulated, they cause various effects. For example, nerve impulses to the heart can stimulate beta-adrenergic receptors on heart cells. This causes an increase in the force and rate of the heartbeat. This can cause a fast heartbeat and higher blood pressure.

The beta-adrenergic receptors are also stimulated by adrenaline (epinephrine), a hormone which circulates in the bloodstream. Adrenaline is made in the adrenal gland. The blood level of adrenaline can vary. For example, you may release a lot of adrenaline into the bloodstream when you are frightened or anxious which can cause an increase in your heart rate, and other effects.

The beta-blocker medicine 'sits' on beta-adrenergic receptors and stops (blocks) the receptor from being stimulated. So, for example, if beta-adrenergic receptors in the heart are blocked, the force and rate of the heartbeat are reduced.

There are several types of beta-blockers - these include:

• Acebutolol.

• Atenolol.

• Bisoprolol.

• Carvedilol.

• Celiprolol.

• Labetalol.

• Metoprolol.

• Nadolol.

• Nebivolol.

• Oxprenolol.

• Pindolol.

• Sotalol.

• Propranolol.

• Timolol.

Each type has one or more brand names.

Heart and blood vessel conditions

Because of their effect on the heart cells, beta-blockers may be used to:

• Prevent angina pains.

• Reduce the risk of a further heart attack (myocardial infarction) if you have already had one.

• Control certain abnormal heart rhythms (arrhythmias), particularly those where the heartbeat is too fast. A common arrhythmia is called atrial fibrillation. Beta-blockers are often used to control the fast heart rate in atrial fibrillation. Calcium-channel blockers are an alternative medicine that can be used for this problem.

• Help to treat insuficiência cardíaca.

• Lower your blood pressure if you have high blood pressure.

• Treat postural tachycardia syndrome (PoTS).

Outras condições

Because beta-adrenergic receptors are found in other parts of the body, beta-blockers are also used for various other conditions. These include:

• Glaucoma. Beta-blocker eye drops reduce the fluid that you make in the front chamber of the eye. This reduces the pressure in the eye.

• Ansiedade. Beta-blockers do not reduce anxiety itself but can reduce some of the symptoms. For example, they can reduce shaking (tremor) and a fast heart rate.

• Glândula tireoide hiperativa. Beta-blockers can help to reduce symptoms such as tremor, and slow down a fast heart rate.

• Enxaqueca. Beta-blockers can reduce the number of migraine attacks if the attacks occur frequently.

• Tremores - beta-blockers can be used in benign hand tremors which have no other underlying cause and are causing significant problems in daily activities.

Most people who take beta-blockers have no side-effects, or only minor ones. However, because of their action in various parts of the body, some people have unwanted side-effects. The side-effects of beta-blockers include::

• Sometimes the heart rate can go too slowly. This can make you dizzy or feel faint.

• If you have diabetes you need to be aware that beta-blockers may dull the warning signs of low blood sugar (hipoglicemia - often called a hypo). For example, you may not develop the sensation of rapid, forceful or irregular heartbeats (palpitations) or tremor, which tend to occur as the blood sugar is going too low.

• Some people develop cool hands and feet when taking beta-blockers. This is because they can narrow (constrict) small blood vessels and reduce the circulation to the skin of the hands and feet.

• Tiredness, depressão, inability to achieve a proper erection (impotence), vivid dreams, nightmares and other sleeping problems occur in some people.

• There is some evidence to suggest that beta-blockers may provoke type 2 diabetes to develop in some people.

Some people with asthma are advised not to take a beta-blocker. If you have asthma or something similar, discuss this with your GP.

The above is not a full list of possible side-effects but mentions the main ones that may occur. Read the information leaflet that comes with your particular brand for a full list of possible side-effects and cautions.

The length of treatment depends on why you are taking a beta-blocker. Some people only need to take beta-blockers for a few weeks, or months - for example, if you have an overactive thyroid. Some people need to take beta-blockers for the rest of their lives - for example, after a ataque cardíaco (infarto do miocárdio).

You cannot take a beta-blocker if you have certain conditions, including:

• Asma.

• Uncontrolled heart failure.

• Very slow heart rate (bradicardia).

• Low blood pressure (hypotension).

• Certain problems with the rhythm of your heart - eg, sick sinus syndrome.