Your heart pumps oxygen-rich blood all around your body through a network of blood vessels (the arteries) to tissues including organs, muscles and nerves.
The normal electrical pattern of a heartbeat starts when an electrical impulse is produced by part of your heart called the sinus node. The impulse is conducted to the top chambers of your heart (the left atrium and right atrium). This causes the atria to contract, pushing your blood into two lower, larger, chambers (the left ventricle and right ventricle). The electrical impulse is then transmitted to the ventricles causing them to contract. This pushes the blood out of your heart to your lungs and the rest of your body.
Illustration:The chambers and electrical conducting system of the heart
Your heart will normally beat between 60 and 100 times a minute when you’re resting. It may beat faster or slower than this, but won’t usually cause any problems if you have any of the following.
- Sinus tachycardia. This is when your heartbeat is still regular, but faster than usual (more than 100 beats per minute). It’s normal in certain situations, such as during exercise or if you have a fever, but may also occur at other times for no obvious reason. Conditions such as an overactive thyroid gland or anaemia may result in sinus tachycardia.
- Sinus bradycardia. This is when your heartbeat is still regular, but slower than usual (fewer than 60 beats per minute). Bradycardia is common in athletes but can occur if you’re exposed to the cold and have a low body temperature or are resting or sleeping.
- Ectopic beats. These are extra heartbeats and are very common. You may feel an ectopic beat as a missed beat or a thump from your heart. They are most noticeable when you’re resting.
Types of arrhythmia
There are a number of different types of arrhythmia. Most arrhythmias that come from the top of your heart (supraventricular or atrial) can cause symptoms but aren’t life threatening. Arrhythmias that arise from your ventricles (ventricular) are more serious and can sometimes be fatal.
Atrial fibrillation (AF) is the most common type of arrhythmia and happens when the electrical impulses in your atria become disorganised, which overrides your heart’s normal rate and rhythm. This causes the atria to contract in an irregular manner or ‘fibrillate’. You may notice that your heartbeat is irregular or feels uneven. Attacks of AF can last from a few seconds to a number of days. You may feel tired or less able to exercise because AF can cause your heart to be less efficient at pumping blood around your body.
AF can potentially lead to a blood clot forming in your heart – this is because your blood isn’t able to flow through properly. If a clot forms, it may travel to your brain and cause a stroke.
Most supraventricular tachycardias (SVTs) are caused by one or more extra electrical pathways in your heart, between the atria and the ventricles. This allows electrical impulses to ‘short-circuit’ and re-enter the atria. The impulses end up travelling around your heart in a circle.
SVT can make your heart beat very quickly; usually between 130 and 250 beats per minute. Attacks of SVT may only last for a few seconds but can last for several hours or, rarely, even days.
In ventricular tachycardia, the electrical impulses fire too quickly from the ventricles, causing blood to be pumped out quicker than normal. The ventricles may not have enough time to fill up properly with blood.
If the attack lasts for 30 seconds or more it is called sustained ventricular tachycardia. In some cases ventricular tachycardia can cause cardiac arrest (when the heart stops pumping blood around or body) or it can progress to a condition called ventricular fibrillation.
In ventricular fibrillation, electrical impulses start firing from multiple sites in the ventricles, very rapidly and in an irregular rhythm. This makes your heart quiver and unable to beat properly. If prompt treatment isn’t given, you may have a cardiac arrest, which can be fatal. If this happens, you will lose your pulse, and will quickly become unconscious and stop breathing.
If you collapse and stop breathing, a bystander must seek emergency help immediately. If you’re unconscious and aren’t breathing normally, they should perform emergency resuscitation (cardiopulmonary resuscitation or CPR).
If you have heart block, it means there is a problem affecting how the electrical impulses are transmitted through your heart. Heart block can occur in the atrioventricular (AV) node or in the muscle fibres that lead into your ventricles.
In tachybrady syndrome (also called sick sinus syndrome) there is a disease of the sinus node that causes your heart to beat slowly and then fast and abnormally. This can cause you to feel dizzy or collapse.
Causes of arrhythmia
An arrhythmia can be caused by conditions including:
- an underlying heart disease
- heart valve problems
- thyroid disease
- high blood pressure
- Wolff-Parkinson-White syndrome – an electrical abnormality in the heart that can cause SVT and AF
- a heart attack
- coronary artery disease
- inflammation of your heart
Some types of arrhythmia may be caused by certain triggers, such as certain medicines, alcohol, caffeine and smoking tobacco or cannabis.
Sometimes there may be no known cause for an arrhythmia.