Types of Arrhythmias

Tachycardia is a term meaning rapid (fast) heart beat. Some tachycardias originate in the atrium (the upper chambers of the heart) and are called atrial tachycardias. These include atrial flutter or atrial fibrillation. Other tachycardias originate in the ventricle (the lower chambers of the heart) and are called ventricular tachycardia. Still others occur in both the atrium and ventricle. Your physician will be able to obtain clues as to the type of tachycardia that you might have from your medical history, your electrocardiogram (ECG or EKG) when your heart rate is normal and when it's fast, and from additional testing. Following your evaluation, your physician will be able to tell you more details concerning the type of tachycardia you have. Common types of tachycardias are discussed below.

Supraventricular Tachycardia
Supraventricular tachycardias occur in the upper chambers. The heart rate or pulse is determined by how many of the atrial tachycardia signals are allowed to communicate to the ventricles. For example, atrial flutter is a type of tachycardia that occurs in a specific electrical circuit (or pathway) in the atrium that goes around the tricuspid valve at a rate of approximately 300 beats per minute (bpm.) Because there is normally a delay from the atrium to the ventricle through the AV node, the ventricular rate during atrial flutter will typically be 100-150 bpm- or even less. In atrial flutter, symptoms from the tachycardia may be due to either rapid ventricular rate of 150 bpm continuously or the loss of the normal electrical relationship between the atrium (upper chambers) and ventricle (lower chambers).

Another type of atrial tachycardia is atrial fibrillation. With this condition, the atria are disorganized and are beating at such a rapid rate--as fast as 400-600 bpm--that they are quivering. Fortunately, the AV node usually prevents the ventricular rate from beating so fast. During atrial fibrillation, symptoms may be caused by the rapid ventricular rate, loss of normal atrial-ventricular relationship, or simply by the occurrence of a very irregular heart rhythm.

There are at least two additional, but less common types of atrial tachycardias. The first is called crista terminalis tachycardia and occurs in an area in the right atrium near the normal sinus node. This type of tachycardia is frequently confused with sinus tachycardia. The heart rate during this type of tachycardia, however, is faster than it should be for a patient's given level of activity. The second type of atrial tachycardia called reentry atrial tachycardia can originate from either the right or left atria and can start with extra heartbeats. This feature determines the ease with which some atrial tachycardias can be effectively treated with definitive therapy. A particular type of atrial tachycardia can then be diagnosed by electrocardiograms and/or during diagnostic electrophysiology study (this is explained below.) 

Ventricular Tachycardia
Ventricular tachycardia is confined to the lower heart chambers, or ventricles only. In some patients, ventricular tachycardia is associated with a history of coronary artery disease or structural heart problems. If you have ventricular tachycardia, your physician may perform additional tests to evaluate your heart function. In some patients with ventricular tachycardia, there is no additional heart problem.

Long QT interval syndrome is a congenital or acquired abnormality in the way the heart recovers electrically following a heart beat. This abnormality predisposes the individual to a variety of very rapid ventricular tachycardia-called Torsades de pointe.

Tachycardias in both Atria and Ventricles
There are two common types of tachycardias that involve both the atria and the ventricles. These tachycardias usually occur in otherwise healthy young people without structural heart problems, but because they are so common, they will occasionally occur in people with structural heart problems as well.

Tachycardia due to reentry in the AV Node can occur in patients of any age. The electrical circuit responsible for this tachycardia uses tissue in the lower part of the right atrium, near the AV node. For each beat of tachycardia, there is both atrial and ventricular activity occurring simultaneously. The regular tachycardia usually starts and stops suddenly and may occur in relationship to physical activity.

Orthodromic Reciprocating Tachycardia (ORT) is the other common type of tachycardia that occurs in patients without structural heart problems. In this tachycardia, the electrical circuit uses the AV node and His-Purkinje system to activate the ventricle. Electrical activity communicates from the ventricle to the atrium using an extra or accessory connection (pathways). These accessory connections (pathways) are congenital abnormalities and consist of microscopic strands of heart muscle that did not disappear during normal heart development.

The most common ages for the first episode of tachycardia are infancy and adolescence, although the first episode of tachycardia may not occur until age 20 or even older. The accessory connections occur more frequently between the left atrium and left ventricle, but they can occur at any location around the mitral or tricuspid valve. Up to 10% of people with an accessory connection may have two accessory connections. In addition, 10% of patients with tachycardia using an accessory connection also have tachycardia due to reentry in the AV node.

In some patients with ORT, the accessory connection can send electrical signals not only from the ventricle to the atrium, but from the atrium to the ventricle as well. That is, in a normal person, when the heart rate is normal, and the sinus node is controlling the heart rate, electrical signals go through the atrial muscle to the AV node. When an accessory connection conducts electricity from the atrium to the ventricle, the AV node is bypassed, and there is no delay between the electrical signals in the atrium and those in the ventricle. Although this does not affect your heartÕs pumping action, it does change the appearance of the electrocardiogram (ECG) because the signals from the atria are very close to those from the ventricles. Since the normal electrical system is also being bypassed, the signal from the ventricle takes on an abnormal shape. In a patient with symptoms of fast heart beat, this electrocardiographic finding is termed Wolff-Parkinson-White (WPW) syndrome. Named after the three physicians' who- in 1930- first described the abnormal ECG appearance of young patients with episodes of tachycardia. We now know that both the tachycardia and ECG abnormality are due to the presence of extra connections between the atrium and ventricle. Since these accessory connections have no delay in them, we know that the occurrence of atrial fibrillation in some patients with WPW can result in potentially dangerous ventricular rates. If you have WPW, your physician will discuss with you whether your accessory connection is capable of very rapid rates.

Bradycardia, a term describing an abnormally slow heart beat, which can be due to either an abnormal sinus node (the normal heart pacemaker) or failure of the sinus node to communicate with the ventricles because of damage or abnormal function in the AV node or His-Purkinje system.

The precise definition of an abnormally slow heart rhythm depends on the age of the patient and activity level. For example, a healthy and athletic teenager might have a normal resting heart rate of 40-50 bpm (beats per minute), while an adult patient who has had cardiac surgery might have a sinus node that is functioning too slowly.

Sinus Node Dysfunction
Bradycardia can occur when the sinus node (the normal heart pacemaker) is injured during cardiac surgery or affected by underlying cardiac disease. With exercise, the heart rate should increase gradually with normal peak heart rates predicted primarily by the patient age and effort. If the heart rate does not increase to a normal rate with exercise, this would then be another indication of abnormal sinus node function.

Sick sinus syndrome, also called bradycardia-tachycardia syndrome, is a common occurrence in patients with sinus node dysfunction. These patients, who have bradycardia, may also have frequent episodes of atrial tachycardia. In the presence of bradycardia, the atria have lots of opportunities to work abnormally and tachycardia occurs.

AV Node and His-Purkinje Dysfunction: The Heart Blocks
Damage to the AV node or His-Purkinje system, whether congenital or acquired through damage from disease or surgery, limits the ability of the atria to communicate with the ventricles. This may cause abnormally slow ventricular rates, both at rest and with exercise. In order to compensate for the slower-than-normal rates, the heart can enlarge. In some patients, the secondary heart pacemaker controlling the heart rate in the ventricles may also be unreliable. As a result, an artificial pacemaker may be implanted.

The term heart block describes a delay or interruption in electrical communication from the atrium to the ventricle. (In this context, it does not refer in any way to a problem or blockage in the coronary arteries.) There are degrees of heart block:

• First degree, or mild heart block - slightly slower delay between the atrium and the ventricles
• Second degree heart block - intermittent or lack of communication between the atrium and ventricle
• Third degree heart block- the presence of complete block, where there is little communication between the atrium and the ventricles.
  

Bradycardia can occur when the sinus node (the normal heart pacemaker) is injured during cardiac surgery or affected by underlying cardiac disease. With exercise, the heart rate should increase gradually with normal peak heart rates predicted primarily by the patient age and effort. If the heart rate does not increase to a normal rate with exercise, this would then be another indication of abnormal sinus node function.Sick sinus syndrome, also called bradycardia-tachycardia syndrome, is a common occurrence in patients with sinus node dysfunction. These patients, who have bradycardia, may also have frequent episodes of atrial tachycardia. In the presence of bradycardia, the atria have lots of opportunities to work abnormally and tachycardia occurs.

Many factors determine how and if these blocks are treated, and your physician will discuss your individual situation with you in greater detail.

Nearly everyone will feel an occasional skipped heart beat, a premature heart beat or an extra heart beat. This extra beat is called an extrasystole, either PAC (Premature Atrial Contraction) or PVC (Premature Ventricular Contraction), that originates in either the atria or the ventricles. These occur normally in children and adults. What may be felt is either the pause ("I thought my heart might stop") or the stronger beat ("It felt like my heart was going to pop out of my chest.") These extra beats require no treatment, and a cause is usually not found. Most people experience them every day but only notice the extra beats occasionally.

Another type of irregular heart rhythm is noted during breathing in healthy individuals. The heart rate slows as a person breathes in and increases when breathing out. This, too, is a perfectly normal.