Abnormal rates and/or rhythms of the heartbeat
The normal resting adult heart rate is 60–80 beats per minute. Both heart rate and rhythm may be affected in arrhythmias, which may involve both the atria (upper heart chambers) and the ventricles (lower heart chambers). There are two types of arrhythmias: tachycardias, in which the heart rate is too high, and bradycardias, in which the rate is too low. Tachycardias may arise in the atria or the ventricles and can be regular or irregular. When tachycardias occur in the ventricles, they can deteriorate into ventricular fibrillation, a serious arrhythmia that leads to cardiac arrest. Bradycardias include sick sinus syndrome and complete heart block. Most arrhythmias are caused by disorders of the heart and its blood vessels. A heart rate outside the usual range is not always a cause for concern. An elevated heart rate is normal during exercise and pregnancy, and exceptionally fit people have a resting heart rate that is lower than normal.
Arrhythmias may reduce the pumping efficiency of the heart, causing too little blood to reach the brain. Although arrhythmias may cause alarming symptoms, such as a thumping heartbeat, the different types vary in seriousness.
Most arrhythmias are caused by heart disease. The most common underlying cause is coronary artery disease. Other common causes include heart failure and disorders of the electrical system that controls heart rate. Less common causes include various heart valve disorders and inflammation of the heart muscle (see Myocarditis). Some types of arrhythmia present from birth are due to a defect of the heart, such as an abnormal electrical pathway between the atria and the ventricles. However, these arrhythmias typically cause symptoms later in life.
Some arrhythmias occur in people with otherwise healthy hearts. Causes include an imbalance of thyroid hormones (see Hyperthyroidism), or of blood chemistry, such as an excess of potassium. Some drugs, such as bronchodilators and digitalis drugs may cause arrhythmias, as may caffeine and tobacco, but sometimes their cause is unknown.
Symptoms do not always develop, but, if they do, their onset is usually sudden. The symptoms may include:
Palpitations (awareness of an irregular, abnormally rapid, or heavy heartbeat).
Light-headedness, sometimes leading to loss of consciousness.
Shortness of breath.
Pain in the chest or neck.
Your doctor may suspect an arrhythmia from your symptoms and by checking your pulse. He or she may arrange for you to have electrocardiography (see ECG) to monitor electrical activity in the heart. Since some arrhythmias occur only intermittently, you may need to have a continuous ECG over 24 hours or be fitted with an event monitor (see Ambulatory ECG). You may also have tests to detect abnormalities that affect electrical pathways of the heart (see Cardiac electrophysiological studies).
In some cases, antiarrhythmic drugs can be used to treat arrhythmias. In other cases, electric shock treatment may be given to restore a normal heartbeat (see Cardioversion). Abnormal electrical pathways in the heart can be destroyed using a technique called radiofrequency ablation, which is carried out at the same time as electrophysiological studies. If the heart rate is too low, a cardiac pacemaker may be fitted to stimulate an increase in the heart rate.
The outlook for an arrhythmia depends on the type. Supraventricular tachycardia usually is not serious and does not affect life expectancy, whereas ventricular arrhythmias are potentially fatal and need emergency medical treatment.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.