The normal electrical system of the heart
The heart has its own electrical conduction system. This conduction system sends electrical signals (impulses) throughout the upper (atria) and lower (ventricles) chambers of the heart to make it beat in a regular, coordinated rhythm. The conduction system is made up of two nodes that contain conduction cells and special pathways that transmit the impulse.
- The sinus node is responsible for setting the rate and rhythm of the heart and is therefore referred to as the heart’s “pacemaker”. The electrical impulse fired from this node spreads throughout the atria, causing them to contract and squeeze blood into the ventricles.
- The electrical impulse then reaches the atrioventricular node (AV node or junction box), which acts as a gateway, slowing and regulating the impulses travelling between the atria and the ventricles. As the impulse travels down the pathways into the ventricles they contract and pump blood to the lungs and around the body.
The normal adult heart beats in a regular pattern 60-100 times a minute; this is called sinus rhythm.
What cause a heart rhythm disorder?
Sometimes, if the conduction pathway is damaged or becomes blocked, or if an extra pathway exists, the heart’s rhythm changes. The heart may beat too quickly (tachycardia), too slowly (bradycardia) or irregularly which may affect the heart’s ability to pump blood around the body. These abnormal heartbeats are known as arrhythmias. Arrhythmias can occur in the upper chambers of the heart, the atria, or in the lower chambers of the heart, the ventricles.
Bradycardia is a term that describes a slow rate defined as less than 50 beats per minute.
Sinus bradycardia is an unusually slow heartbeat and commonly occurs in athletes or during a state of deep relaxation. This is perfectly normal and should not usually cause any difficulties.
Sinus bradycardia may also be caused by age–related degeneration of the heart’s electrical conduction system, coronary heart disease or by medications prescribed to treat arrhythmias or high blood pressure.
Bradycardia can also be caused by delay or block on other parts of the electrical conduction system. Heart block (Atrioventricular block or AV block) occurs when electrical impulses are slowed or blocked as they travel from the top chamber of the heart (atria) through the atrioventricular node (AV node) into the bottom chambers (ventricles). Bradycardia is treated by discontinuing any medication that slows the heartbeat, and treating any underlying conditions and/or by implanting a permanent pacemaker.
Tachycardia is a term that describes a fast heart rate, defined as more than 100 beats per minute.
Types of Tachycardia
Arrhythmias that occur in the atria (the top chambers of the heart) are either atrial or supraventricular (above the ventricles) in origin whereas ventricular arrhythmias start in the ventricles (the lower chambers of the heart). While some arrhythmias are merely a nuisance, others can be life-threatening. In general, ventricular arrhythmias caused by heart disease are the most serious kind, and require prompt medical attention. The location of the problematic electrical circuit helps define the arrhythmia. For instance a rhythm is called supraventricular if it originates above the ventricles (lower chambers).
Supraventricular Tachycardia (SVT)
This type of arrhythmia commonly occurs in young, healthy people. Doctors often refer to supraventricular tachycardia (SVT) as re-entry tachycardias as the electrical impulse does not fade out as with the normal heartbeat but continues to move in a rapid circle within the conduction system. This is due to an extra electrical pathway which can form a short circuit within the heart’s conduction system. SVT is usually a rapid, regular rhythm. The two most common types of SVT are AV-node re-entry tachycardia (AVNRT) and AV Nodal Re-entry Tachycardia (AVNRT)
This is one of the most common types of arrhythmia. Atrial Fibrillation occurs in the atria, in the upper chambers of the heart. In Atrial Fibrillation, many electrical impulses are fired rapidly and at chaotically throughout the atria down to the ventricles. The resulting heartbeat is irregular and usually fast. The main risk associated with AF is stroke. This occurs because the atria are fibrillating and not beating in a co-ordinated way. As a result, the blood in the atria can become stagnant and then does not flow through the heart smoothly. This causes blood cells to stick together and form a clot which can travel to the brain and result in a stroke (an embolism). To minimise the risk of blood clots, drugs such as Aspirin or Warfarin may be recommended.
Atrial Flutter also occurs in the atria (the upper chambers of the heart). In Atrial Flutter the electrical impulses fire rapidly but the resulting rhythm is regular and organised. The rhythm is due to a re-entry circuit within the atria, whereby the electrical impulse travels in circles leaving and arriving back at the same point. There are several types of Atrial Flutter, the most common being typical right Atrial Flutter.
Ventricular Tachycardia (VT)
Ventricular Tachycardia (VT) occurs when the electrical impulses arise in the ventricles, the bottom chambers of the heart. The ventricles star t beating at an abnormally fast, regular rate. Because the ventricles are beating rapidly the heart does not work as efficiently. This can cause symptoms of weakness, dizziness, chest pain, shortness of breath or even collapse. There are several different types of Ventricular Tachycardia (VT) and the seriousness of the condition can vary. However, Ventricular Tachycardia (VT) can be a potentially life threatening heart rhythm as it can progress to Ventricular Fibrillation
and cause the heart to stop beating (cardiac arrest). There are a number of reasons why people may develop Ventricular Tachycardia (VT). For example, in people who have had a previous myocardial infarction (heart attack), the area of the heart muscle damaged by the heart attack forms scar tissue and this can make the heart susceptible to abnormal heart rhythms. Other people who may experience Ventricular Tachycardia (VT) are patients with
cardiomyopathy, previous corrective congenital heart surgery or inherited arrhythmias. There is also a small group of people who have VT with a structurally normal heart where the VT may be well tolerated.
Ventricular Fibrillation occurs in the ventricles, in the bottom chambers of the heart. In ventricular Fibrillation, the electrical impulses are fired from multiple sites in the ventricles in a very fast and irregular way, causing the heart to quiver rather than to beat and pump blood. Ventricular Fibrillation is an extremely dangerous heart rhythm and prompt emergency care must be provided to get the heart pumping again, or death can occur.
What symptoms may I experience with a heart rhythm disorder?
- Palpitations, flutterings or a skipped beats
- Slow Heartbeat – Bradycardias
- Blurred or narrowed (tunnel) vision
- Nausea and/or vomiting
- Stomachache/abdominal discomfort
- Shortness of breath
- Chest pain
- Confusion and/or difficulty speaking clearly or coherently
- Fainting or near fainting
- Sudden loss of consciousness