What is it and how does it work?
Beta-blockers have a long track record in treating hypertension and are also effective in preventing recurrent heart attacks.

Beta-blockers are drugs that block two hormones from interacting with their specific beta receptors in the heart. These hormones, norepinephrine and epinephrine, are responsible for heart rate and pumping action. Blood pressure is lowered because of the decrease in heart rate and pumping action. Beta-blockers also suppress the production of renin, an enzyme in the kidney. This prevents the formation of angiotensin II, a powerful constrictor of arteries. In this way, blood pressure decreases.

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There are two types of beta-receptors. Beta 1 receptors are located mainly in the heart muscle and beta 2 receptors are in the airways and blood vessels. Cardioselective drugs act mainly on beta 1 receptors. Non-cardioselective drugs act on both types.

A few beta-blockers

• Cardioselective: Atenolol in Tenormin, Hexa-blok, Adco-Atenolol.
• Non-cardioselective: Propanolol in Pur-Bloka, Cardiblok, Inderal.

Who benefits from beta-blockers?
These drugs slow the heart rate and decreases the force of the heart beat. This reduces the workload of the heart and its oxygen consumption, and makes it beneficial to patients that suffer from angina. This effect, and the fact that it helps to control a rapid heartbeat, may protect a patient against a second heart attack. However, these actions may worsen heart failure. Beta-blockers can be helpful in patients with heart failure, but must be introduced at a very low dosage and used with caution. Beta-blockers also help to prevent attacks of uncontrollable heartbeats.

Compelling indications for use as antihypertensive drug:

• Patients with angina – chest pain due to decreased oxygen supply to the heart muscle.
• Secondary prevention – after myocardial infarction, beta-blockers protects against a second heart attack.
• Rapid heart rhythm – called tachyarrhythmias.

Possible indications in some hypertensives:

• Patients with heart failure – very low starting dose.
• Pregnancy, only in some cases – ask your specialist.

How to take beta-blockers
Beta-blockers are usually taken once or twice daily, before or after meals. The types that need to be taken once a day help to improve the patient’s compliance as it is more convenient.

These drugs should never be stopped suddenly after being used for a long time. This could cause a sudden and severe recurrence of the disorder being treated. If beta-blockers need to be stopped, it should be withdrawn gradually under medical supervision.

Most beta-blockers need to be stopped before general anaesthetic. Always discuss this with the anaethetist.

Possible side effects
The receptors blocked by beta-blockers in the heart tissue are also found in the lung. Blocking these lung receptors can cause the airways to constrict and precipitate or aggravate asthma attacks. Although this is more likely when using non-cardioselective drugs, both types may affect the respiratory system.

Some patients experience fatigue and decreased exercise tolerance. Beta-blockers can constrict the blood vessels and cause cold hands and feet. Impotence and a lower sex drive are potential, but not common, side effects. Beta-blockers can cause a slight increase in blood fat abnormalities. The symptoms of low blood sugar can be masked in insulin dependent diabetics taking beta-blockers. In particular, symptoms like palpitations and tremor may be suppressed.

Different types of these drugs are available. Some mainly affect the heart, while others can reach the brain and may cause depression.

Who should not take beta-blockers?

Compelling contraindications:

• Asthma
• Chronic obstructive airways disease – like emphysema.
• Heart block or very slow heart rate (under 50 beats per minute)
• Peripheral vascular disease – blood vessel disease in the legs.

Possible contraindications/caution or limited value:

• Abnormal blood fats – dyslipidaemia.
• Insulin dependent diabetes – diabetes type I.
• Heart failure – caution and low dosage.
• Athletes and physically active patients.
• As monotherapy in African patients – limited value.

Inform your doctor if:

• You have any lung disorders like asthma or chronic bronchitis.
• You have poor circulation.
• You have heart problems like heart failure or heart block.
• You have any kidney problems.
• You have diabetes.
• You are taking other medication or self-medication.
• You are planning to have any surgery or dental procedures under general anaesthetic.

Can it be taken with other medication? – Possible drug interactions
Beta-blockers are often used in combination with other antihypertensive drugs. In this way, lower dosages of each can be used, and fewer side effects are experienced.

Beta-blockers and certain drugs can lead to dangerous interactions. These include some anti-psychotics and drugs that alter heart rhythm like some calcium channel blockers, especially verapamil.

Antacids may reduce the absorption of some beta-blockers.

Written by Dr Kathleen Coetzee, MBChB.

Read more:
Risk factors you can control
Alcohol and hypertension

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