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Cardiac Arrhythmias (Irregular Heartbeat): Tachycardia, Bradycardia and Atrial Fibrillation

Arrhythmias are abnormal heartbeats — any heartbeat less than 60 beats per minute or more than 100 beats per minute.

Arrhythmias happen when electrical impulses that control the heartbeat do not fire as they normally should. The heart might beat too quickly, too slowly or in an irregular way.

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Many arrhythmias are harmless — such as a fluttering sensation in the heart. However, when an arrhythmia interferes with daily living — causing fatigue, dizziness or chest pain — this abnormal heartbeat can be a sign of a life-threatening condition. The heart may pump less efficiently and the normal quantity of blood — which contains oxygen and essential nutrients — does not reach the rest of the body. Damage to the brain, heart and other organs can result.

How the Heart Beats: An Overview

The heart has a natural pacemaker. A group of heart cells, called the sinoatrial node (also "SA node" or "sinus node"), sends electrical impulses to other areas of the heart, telling the heart when to contract, or beat. The sinus node responds to changes in physical activity, mood and hormones by increasing the heart rate. It slows the heart rate during sleep.

For the heart to pump blood in a well-coordinated manner, these electrical signals must travel in the correct sequence. Impulses sent by the sinus node fan out through the atria (upper chambers of the heart) to the atrioventricular node (or "AV node"), which connects the atria to the ventricles (the heart's pumping chambers). Signals continue onward to a network of specialized fibers, called the His-Purkinje system, to all parts of the ventricles.

Causes of Arrhythmias

Arrhythmias can be caused by a wide range of factors — some are external (medications, alcohol, tobacco and stress, for example) and others are internal (chemicals in the blood). In some cases, they have no apparent cause. A patient can be born with an arrhythmia, as well.

An arrhythmia can occur when:

  • The sinus node (the heart's natural pacemaker) triggers electrical impulses at an abnormal rate or rhythm.
  • The normal pathway of the electrical impulses becomes interrupted.
  • Another area of the heart takes over the sinus node's role as pacemaker.

If the sinus node weakens, other heart tissue can assume the role of natural pacemaker, firing electrical impulses that trigger a heartbeat. However, this "back-up" pacemaker's signals will not travel along the correct sequence of a normal heartbeat.

Types of Arrhythmias

There are two major types of arrhythmias. In tachycardia, the heart beats too quickly: over 100 beats per minute. In bradycardia, the heart beats too slowly: less than 60 beats per minute.

Tachycardias are classified by their location:

  • Superventricular tachycardia (SVT) is the most common type of arrhythmia. It can affect the top chambers of the heart, including the atria (chambers that fill with blood) and the AV node. Electrical signals in the atria fire abnormally, interfering with signals coming from the SA node. A rapid, regular heartbeat of 150 to 200 beats per minute may result.
  • In ventricular tachycardia, electrical signals originate in the ventricles, and do not travel through the heart as they normally should. In effect, the SA node no longer controls how the ventricles contract and pump blood. The heart beats so quickly that it cannot effectively pump blood. A serious, but uncommon condition, ventricular tachycardia may be a warning sign of underlying heart disease.
  • Bradycardia most commonly happens when the sinus node transmits electrical impulses more slowly, or fails to fire impulse. Likewise, nearby tissue may block the sinus node's signals before they reach the atria and ventricles through the proper channels. This is called heart block (also "atrioventricular block"). Bradycardia has many possible causes. Frequently, the sinus node is damaged, due to heart disease, aging, inherited or congenital defects, or medications, including those used to control arrhythmias and high blood pressure. Bradycardia commonly affects the elderly.

Other Common Arrhythmias

Atrial fibrillation (or "a-fib") is very similar to superventricular tachycardia (SVT) — both conditions affect the atria of the heart. The difference rests in the regularity of the heartbeat. SVT is a fast, regular heartbeat; a-fib is a fast, irregular heartbeat. In a-Fib, electrical signals fire in a very fast or uncontrolled fashion, causing the atrial chambers of the heart to twitch, quiver or contract. Blood pools up within the atrium, rather than flowing to the ventricle. Blood clots can form in the atrium. If a blood clot travels to the brain, stroke can occur.

Wolff-Parkinson-White Syndrome occurs when electrical signals reach the ventricles early and are relayed back to the atria. This is caused by additional muscle pathways forming between the atria and the ventricles. A fast heart rate results, leading to dizziness, palpitations and fainting.

Diagnosis and Treatment

Diagnosis for arrhythmia starts with a health history and physical examination. At John C. Lincoln, patients who may have arrhythmia are commonly given electrophysiology (EP) studies. A thin, flexible catheter is inserted at the groin and guided along vessels to the heart. At the end of the catheter are electrodes. These will map electrical impulses at several precise locations around the heart. The electrodes also can stimulate the heart, to trigger the irregular heartbeat. Learn more about diagnostic heart procedures at John C. Lincoln.

An EP study may be followed immediately by a procedure called radiofrequency ablation. In this minimally invasive procedure, a special machine delivers focused energy through the catheter to the areas of the heart causing the irregular heartbeat. The energy essentially "zaps" the abnormal heart tissue, disconnecting the electrical pathway of the irregular rhythm.

Another treatment for arrhythmia is defrillator implant surgery, in which an implantable cardioverter defibrillator (ICD) is inserted in the chest. This compact device sends an electrical shock to the heart when it beats dangerously fast.