Health A-Z

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What Is It?

A cardiac arrhythmia is any abnormal heart rate or rhythm.

In normal adults, the heart beats regularly at a rate of 60 to 100 times per minute, and the pulse (felt at the wrist, neck or elsewhere) matches the contractions of the heart's two powerful lower chambers, called the ventricles. The heart's two upper chambers, called the atria, also contract to help fill the ventricles, but this milder contraction occurs just before the ventricles contract, and it is not felt in the pulse. Under normal circumstances, the signal for a heartbeat comes from the heart's sinus node, the natural pacemaker located in the upper portion of the right atrium. From the sinus node, the heartbeat signal travels to the atrioventricular node, or "A-V node," (located between the atria) and through the bundle of His (pronounced HISS - a series of modified heart muscle fibers located between the ventricles) to the muscles of the ventricles. This causes the ventricles to contract and produces a heartbeat.

Cardiac arrhythmias sometimes are classified according to their origin as either ventricular arrhythmias (originating in the ventricles) or supraventricular arrhythmias (originating in heart areas above the ventricles, typically the atria). They also can be classified according to their effect on the heart rate, with bradycardia indicating a heart rate of less than 60 beats per minute and tachycardia indicating a heart rate of more than 100 beats per minute.

Some common types of cardiac arrhythmias include:

  • Sinus node dysfunction - This usually causes a slow heart rate (bradycardia), with a heart rate of 50 beats per minute or less. The most common cause is scar tissue that develops and eventually replaces the sinus node. Why this happens is not known. Sinus node dysfunction also can be caused by coronary artery disease, hypothyroidism, severe liver disease, hypothermia, typhoid fever or other conditions. It also can be the result of vasovagal hypertonia, an unusually active vagus nerve.

  • Supraventricular tachyarrhythmias - This diverse family of cardiac arrhythmias causes rapid heartbeats (tachycardias) that start in parts of the heart above the ventricles. In most cases, the problem is either an abnormality in the A-V node or an abnormal pathway that bypasses the typical route for heartbeat signals.

  • Atrial fibrillation - This is a supraventricular arrhythmia that causes a rapid and irregular heartbeat, during which the atria quiver or "fibrillate" instead of beating normally. During atrial fibrillation, heartbeat signals begin in many different locations in the atria rather than in the sinus node. Although these abnormal signals manage to trigger 300 to 500 contractions per minute within the atria, the extraordinarily high number of heartbeat signals overwhelms the A-V node. As a result, the A-V node sends sporadic, irregular signals to the ventricles, causing an irregular and rapid heartbeat of 80 to 160 beats per minute. The disordered heartbeat of atrial fibrillation cannot pump blood out of the heart efficiently. This causes blood to pool in the heart chambers and increases the risk of a blood clot forming inside the heart. The major risk factors for atrial fibrillation are age, coronary artery disease, rheumatic heart disease (caused by rheumatic fever), hypertension, diabetes and thyrotoxicosis (an excess of thyroid hormones).

  • A-V block or heart block - In this family of arrhythmias, there is some problem conducting the heartbeat signal from the sinus node to the ventricles. There are three degrees of A-V block:

    • First-degree A-V block, where the signal gets through, but may take longer than normal to travel from the sinus node to the ventricles

    • Second-degree A-V block, in which some heartbeat signals are lost between the atria and ventricles

    • Third-degree A-V block, in which no signals reach the ventricles, so the ventricles beat slowly on their own with no direction from above

    Some common causes of A-V block include coronary artery disease, heart attack or an overdose of the heart medication digitalis.

  • Ventricular tachycardia (VT) - This is an abnormal heart rhythm that begins in either the right or left ventricle. It may last for a few seconds (non-sustained VT) or for many minutes or even hours (sustained VT). Sustained VT is a dangerous rhythm and if it is not treated, it often progresses to ventricular fibrillation.

  • Ventricular fibrillation - In this arrhythmia, the ventricles quiver ineffectively, producing no real heartbeat. The result is unconsciousness, with brain damage and death within minutes. Ventricular fibrillation is a cardiac emergency. Ventricular fibrillation can be caused by a heart attack, an electrical accident, a lightning strike or drowning.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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