Arrhythmias are abnormal heart rhythms which are produced when the natural electrical conduction system of the heart malfunctions. Not all arrhythmias pose health risks. For instance, a person may have a persistent minor arrhythmia which is in fact normal for his heart. Similarly, a temporary arrhythmia caused by alcohol, caffeine, or insomnia is not usually a cause for concern.

Arrhythmias are generally classified into two groups: 
1. Bradycardia, an abnormally slow heart rhythm
2. Tachycardia, an abnormally fast heart rhythm

Patients with bradycardia generally have a heart rate of only about 40 to 60 beats per minute. They tend to experience fatigue, dizziness, and episodes of fainting as their hearts do not pump enough blood to supply their brains and other vital organs with sufficient oxygen.
In contrast, patients with tachycardia have abnormally rapid heart rhythms, usually of more than 100 beats per minute. The main symptoms of this condition are similar to those associated with bradycardia.
Tachycardia may arise in either the atria or the ventricles. The former occurs when the atria become enlarged by hypertension, inflamed, or damaged by coronary heart disease. The resultant rapid irregular beating of the atria is known as atrial fibrillation or atrial flutter. If atrial fibrillation remains untreated, there is an increased risk of stroke. Ventricular tachycardia is much more dangerous in comparison. When the ventricles beat too rapidly, the heart does not have enough time to fill with blood in between beats, causing blood pressure to drop. Ventricular tachycardia may worsen and turn into ventricular fibrillation, a potentially fatal condition in which the ventricles merely quiver and do not pump any blood at all. If untreated, ventricular fibrillation will lead to immediate loss of consciousness and death.
What is Atrial Fibrillation (AF)?
Atrial Fibrillation (AF) is the commonest heart rhythm disorder and affects millions of people around the world. The condition increases with age - it is very unusual in people below the age of 30 but affects as many as 1 in 20 (i.e. 5%) people over the age of 65.
The heart beat in normal people is controlled by a small area of specialized heart tissue in the upper right chamber (right atrium), called the sino-atrial (SA) node (Figure 1). The electrical activity generated from the SA node spreads through specialized conducting tissue to activate the rest of the heart muscle, resulting in coordinated contraction of the heart and the pumping out of blood to the rest of the body by the ventricles. In AF the SA node loses control of the heart and a chaotic, irregular rhythm develops instead. Consequently, the atria do not squeeze blood out into the ventricles - this both decreases the efficiency of the heart and increases the risk of blood clots forming in the atria. 
What are the symptoms of AF?
Most people with AF will experience some symptoms, although a small proportion of patients may have no symptoms and the AF may be picked up by the doctor during a medical check-up. Symptoms include:
Palpitations (abnormal awareness of one’s heartbeat) 
Dizzy spells or fainting 
Chest pain 
Symptoms of stroke (e.g. weakness of part of the body or slurred speech)
AF can be present all the time (persistent or permanent AF) or may be intermittent (known as paroxysmal AF). Patients with both types of AF may experience the symptoms described above. In addition, patients with paroxysmal AF are often very aware of the moments when their heart rhythm abruptly changes to AF. These sudden and unpredictable changes can be both unpleasant and distressing to the patient as well as lead to the other symptoms of AF during the episodes.
Some patients may notice that they are in AF because they cannot do as much as they could when they were in a normal rhythm.
What causes AF?
AF can result from a variety of causes and has a strong association with other cardiovascular conditions such as high blood pressure, heart failure, heart valve diseases and coronary artery disease. AF is also associated with other conditions outside the heart, such as excessive alcohol intake and an overactive thyroid gland. In a minority of individuals, there is no obvious cause for their AF and their hearts otherwise appear to be normal - this type of AF is called “lone AF”.